The Astronomy BookDK
An essential guide to milestone developments in astronomy, telling the story of our ideas about space, time, and the physics of the cosmos—from ancient times to the present day.
From planets and stars to black holes and the Big Bang, take a journey through the wonders of the universe. Featuring topics from the Copernican Revolution to the mind-boggling theories of recent science, The Astronomy Book uses flowcharts, graphics, and illustrations to help clarify hard-to-grasp concepts and explain almost 100 big astronomical ideas. Covering the biographies of key astronomers through the ages such as Ptolemy, Galileo, Newton, Hubble, and Hawking, The Astronomy Book details their theories and discoveries in a user-friendly format to make the information accessible and easy to follow.
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COSMIC EXPANSION IS ACCELERATING GRAVITY EXPLAINS THE MOTIONS OF THE PLANETS THE UNMOVING STARS GO UNIFORMLY WESTWARD THE UNIVERSE IS EXPANDING IN ALL DIRECTIONS THE SEARCH FOR EXTRATERRESTRIAL INTELLIGENCE IS A SEARCH FOR OURSELVES RIPPLES THROUGH SPACETIME THE ASTRONOMY BOOK THE WAY TO I FOUND THE THAT IT IS A COMET, FOR STARS IS IT HAS CHANGED BIG IDEAS SIMPLY EXPLAINED OPEN ITS PLACE A SLOW PROCESS OF ANNIHILATION OF MATTER FINALLY WE SHALL PLACE THE SUN HIMSELF AT THE CENTER OF THE UNIVERSE THE MOST TRUE PATH OF THE PLANET IS AN ELLIPSE AN EXACT SOLUTION TO RELATIVITY PREDICTS BLACK HOLES STARS ARE FACTORIES FOR THE CHEMICAL ELEMENTS DK LONDON SENIOR EDITOR Victoria Heyworth-Dunne US EDITOR Margaret Parrish PRE-PRODUCTION PRODUCER Jacqueline Street-Elkayam SENIOR PRODUCER Mandy Inness DK DELHI SENIOR ART EDITORS Gillian Andrews, Nicola Rodway JACKET DESIGNER Suhita Dharamjit MANAGING EDITOR Gareth Jones EDITORIAL COORDINATOR Priyanka Sharma SENIOR MANAGING ART EDITOR Lee Griffiths SENIOR DTP DESIGNER Harish Aggarwal ART DIRECTOR Karen Self MANAGING JACKETS EDITOR Saloni Singh ASSOCIATE PUBLISHING DIRECTOR Liz Wheeler produced for DK by TALL TREE LTD. PUBLISHING DIRECTOR Jonathan Metcalf EDITORS Rob Colson, David John SENIOR JACKET DESIGNER Mark Cavanagh DESIGN Ben Ruocco JACKET EDITOR Claire Gell ILLUSTRATIONS James Graham JACKETS DESIGN DEVELOPMENT MANAGER Sophia MTT original styling by STUDIO 8 First American Edition, 2017 Published in the United States by DK Publishing, 345 Hudson Street, New York, New York 10014 Copyright © 2017 Dorling Kindersley Limited DK, a Division of Penguin Random House LLC 17 18 19 20 21 10 9 8 7 6 5 4 3 2 1 001—283974—Sep/2017 All rights reserved. Without limiting the rights under the copyright reserved above, no part of this publication may be reproduced, stored in or introduced into a retrieval system, or transmitted, in any form, or by any means (electronic, mechanical, photocopying, recording, or otherwise), without the prior written permission of the copyright owner. Published in Great Britain by Dorling Kindersley Limited. A catalog record for this book is available from the Library of Congress. ISBN: 978-1-4654-6418-7 DK books are available at special discounts when purchased in bulk for sales promotions, premiums, fund-raising, or educational use. For details, contact: DK Publishing Special Markets, 345 Hudson Street, New York, New York 10014 SpecialSales@dk.com Printed in China A WORLD OF IDEAS: SEE ALL THERE IS TO KNOW www.dk.com CONTRIBUTORS JACQUELINE MITTON, CONSULTANT EDITOR ROBERT DINWIDDIE Jacqueline Mitton is the author of more than 20 books on astronomy, including books for children. She has been a contributor, editor, and consultant for many other books. Becoming an astronomer was Jacqueline’s childhood ambition. She studied physics at Oxford University and then earned her Ph.D. at Cambridge, where she still lives. Robert Dinwiddie is a science writer specializing in educational illustrated books on astronomy, cosmology, earth science, and the history of science. He has written or contributed to more than 50 books, including the DK titles Universe, Space, The Stars, Science, Ocean, Earth, and Violent Earth. He lives in southwest London and enjoys travel, sailing, and stargazing. DAVID W. HUGHES David W. Hughes is Emeritus Professor of Astronomy at the University of Sheffield, UK. He is an international authority on comets, asteroids, and the history of astronomy. He has spent more than 40 years explaining the joys of astronomy and physics to his students, and has published well over 200 research papers, as well as books on the moon, the solar system, the universe, and the Star of Bethlehem. He was a co-investigator on the European Space Agency’s GIOTTO space mission to Halley's Comet and also on ESA’s Smart 1 mission to the moon. David has served on a host of space and astronomy committees, and has been a vice president of both the Royal Astronomical Society and the British Astronomical Association. PENNY JOHNSON Penny Johnson started out as an aeronautical engineer, working on military aircraft for 10 years, before becoming a science teacher, and then a publisher producing science courses for schools. Penny has been a full-time educational writer for the last 15 years. TOM JACKSON Tom Jackson is a science writer based in Bristol, UK. He has written about 150 books and contributed to many others, covering all kinds of subjects from fish to religion. Tom writes for adults and children, mostly about science and technology, with a focus on the histories of the sciences. He has worked on several astronomy books, including collaborations with Brian May and Patrick Moore. 6 CONTENTS 10 INTRODUCTION 26 FROM MYTH TO SCIENCE The unmoving stars go uniformly westward Earth’s rotation 27 A little cloud in the night sky Mapping the galaxies 28 A new calendar for China The solar year 30 We have re-observed all of the stars in Ptolemy’s catalog Improved instruments 32 Finally we shall place the sun himself at the center of the universe The Copernican model 600 BCE–1550 CE 20 It is clear that Earth does not move The geocentric model 21 Earth revolves around the sun on the circumference of a circle Early heliocentric model 22 The equinoxes move over time Shifting stars 23 The moon’s brightness is produced by the radiance of the sun Theories about the moon 24 All matters useful to the theory of heavenly things Consolidating knowledge THE TELESCOPE REVOLUTION 64 A perfectly circular spot centered on the sun The transit of Venus 65 New moons around Saturn Observing Saturn’s rings 66 Gravity explains the motions of the planets Gravitational theory 74 I dare venture to foretell that the comet will return again in the year 1758 Halley’s comet 78 These discoveries are the most brilliant and useful of the century Stellar aberration 79 A catalog of the southern sky Mapping southern stars 1550–1750 44 I noticed a new and unusual star The Tychonic model 48 Mira Ceti is a variable star A new kind of star 50 The most true path of the planet is an ellipse Elliptical orbits 56 Our own eyes show us four stars traveling around Jupiter Galileo’s telescope 7 URANUS TO NEPTUNE 1750–1850 84 I found that it is a comet, for it has changed its place Observing Uranus 100 A survey of the whole surface of the heavens The southern hemisphere 102 An apparent movement of the stars Stellar parallax 103 Sunspots appear in cycles The surface of the sun 86 The brightness of the star was altered Variable stars 104 A spiral form of 87 Our Milky Way is the dwelling, the nebulae are the cities Messier objects 106 The planet whose position 88 On the construction of the heavens The Milky Way 90 Rocks fall from space Asteroids and meteorites 92 The mechanism of the heavens Gravitational disturbances 94 I surmise that it could be something better than a comet The discovery of Ceres arrangement was detected Examining nebulae you have pointed out actually exists The discovery of Neptune THE RISE OF ASTROPHYSICS 1850–1915 112 Sodium is to be found in the solar atmosphere The sun’s spectrum 113 Stars can be grouped by their spectra Analyzing starlight 114 Enormous masses of luminous gas Properties of nebulae 116 The sun’s yellow prominence differs from any terrestrial flame The sun’s emissions 117 Mars is traversed by a dense network of channels Mapping Mars’s surface 118 Photographing the stars Astrophotography 120 A precise measurement of the stars The star catalog 122 Classifying the stars according to their spectra reveals their age and size The characteristics of stars 128 There are two kinds of red star Analyzing absorption lines 129 Sunspots are magnetic The properties of sunspots 130 The key to a distance scale of the universe Measuring the universe 138 Stars are giants or dwarfs Refining star classification 140 Penetrating radiation is coming from space Cosmic rays 141 A white hot star that is too faint Discovering white dwarfs 8 178 White dwarfs have 196 It took less than an hour 179 The radio universe 198 Stars are factories for a maximum mass The life cycles of stars Radio astronomy 180 An explosive transition ATOMS, STARS, AND GALAXIES 1915–1950 146 Time and space and gravitation have no separate existence from matter The theory of relativity 154 An exact solution to relativity predicts black holes Curves in spacetime 156 The spiral nebulae are stellar systems Spiral galaxies 162 Stars are dominated by hydrogen and helium Stellar composition 164 Our galaxy is rotating The shape of the Milky Way 166 A slow process of annihilation of matter Nuclear fusion within stars to a neutron star Supernovae 182 The source of energy in to make the atomic nuclei The primeval atom the chemical elements Nucleosynthesis 200 Sites of star formation Dense molecular clouds 184 A reservoir of comets NEW WINDOWS ON THE UNIVERSE 185 Some galaxies have active 206 A vast cloud surrounds 186 The match of lunar and 207 Comets are dirty snowballs stars is nuclear fusion Energy generation exists beyond the planets The Kuiper belt regions at their centers Nuclei and radiation Earth material is too perfect The origin of the moon 188 Important new discoveries will be made with flying telescopes Space telescopes 1950–1975 the solar system The Oort cloud The composition of comets 208 The way to the stars is open The launch of Sputnik 210 The search for interstellar communications Radio telescopes 212 Meteorites can vaporize on impact Investigating craters 213 The sun rings like a bell The sun’s vibrations 214 The data can best be The birth of the universe explained as X-rays from sources outside the solar system Cosmic radiation 172 The universe is expanding 218 Brighter than a galaxy, 168 A day without yesterday in all directions Beyond the Milky Way but it looks like a star Quasars and black holes 9 222 An ocean of whispers 298 Cosmic expansion left over from our eruptive creations Searching for the Big Bang is accelerating Dark energy 304 Peering back over 228 The search for 13.5 billion years Studying distant stars extraterrestrial intelligence is a search for ourselves Life on other planets 306 Our mission is to land on a comet Understanding comets 236 It has to be some new kind of star Quasars and pulsars 240 Galaxies change over time Understanding stellar evolution 242 We choose to go to the moon The Space Race 250 The planets formed from a disk of gas and dust The nebular hypothesis 252 Solar neutrinos can only be seen with a very large detector The Homestake experiment 254 A star that we couldn’t see Discovering black holes 255 Black holes emit radiation Hawking radiation THE TRIUMPH OF TECHNOLOGY 1975–PRESENT 260 A grand tour of the giant planets Exploring the solar system 312 The violent birth of 268 Most of the universe is missing Dark matter 272 Negative pressures produce repulsive gravity Cosmic inflation 274 Galaxies appear to be on the surfaces of bubblelike structures Redshift surveys 276 Stars form from the inside out Inside giant molecular clouds 280 Wrinkles in time Observing the CMB 286 The Kuiper belt is real Exploring beyond Neptune 288 Most stars are orbited by planets Exoplanets 296 The most ambitious map of the universe ever A digital view of the skies 297 Our galaxy harbors a massive central black hole The heart of the Milky Way the solar system The Nice model 314 A close-up view of an oddball of the solar system Studying Pluto 318 A laboratory on Mars Exploring Mars 326 The biggest eye on the sky Looking farther into space 328 Ripples through spacetime Gravitational waves 332 DIRECTORY 340 GLOSSARY 344 INDEX 352 ACKNOWLEDGMENTS INTRODU CTION 12 INTRODUCTION T hroughout history, the aim of astronomy has been to make sense of the universe. In the ancient world, astronomers puzzled over how and why the planets moved against the backdrop of the starry sky, the meaning of the mysterious apparition of comets, and the seeming remoteness of the sun and stars. Today, the emphasis has changed to new questions concerning how the universe began, what it is made of, and how it has changed. The way in which its constituents, such as galaxies, stars, and planets, fit into the larger picture and whether there is life beyond Earth are some of the questions humans still endeavor to answer. Understanding astronomy The baffling cosmic questions of the day have always inspired big ideas to answer them. They have stimulated curious and creative minds for millennia, resulting in pioneering advances in philosophy, mathematics, technology, and observation techniques. Just when one fresh breakthrough seems to explain gravitational waves, another discovery throws up a new conundrum. For all we have learned about the universe’s familiar constituents, as seen through telescopes and detectors of various kinds, one of our biggest discoveries is what we do not understand at all: more than 95 percent of the substance of the universe is in the form of “dark matter” and “dark energy.” The origins of astronomy In many of the world’s most populated areas today, many of us are barely aware of the night sky. We cannot see it because the blaze of artificial lighting overwhelms the faint and delicate light of the stars. Light pollution on this scale has exploded since the mid-20th century. In past times, the starry patterns of the sky, the phases of the moon, and the meanderings of the planets were a familiar part of daily experience and a perpetual source of wonder. Few people fail to be moved the first time they experience a clear sky on a truly dark night, in which the magnificent sweep of the Milky Way arches across the sky. Our ancestors were driven by a mixture of curiosity and awe in their search for order and meaning in the great vault of the sky above their heads. The mystery and grandeur of the heavens were explained by the spiritual and divine. At the same time, however, the orderliness and predictability of repetitive cycles had vital practical applications in marking the passage of time. Archaeology provides abundant evidence that, even in prehistoric times, astronomical phenomena were a cultural resource for societies around the world. Where there is no written record, we can only speculate as to the knowledge and beliefs early societies held. The oldest astronomical records to survive in written form come from Mesopotamia, the region that was between and around the valleys of the Tigris and Euphrates rivers, in present-day Iraq and neighboring countries. Clay tablets inscribed with astronomical information date back to about Philosophy is written in this grand book, the universe, which stands continually open to our gaze. Galileo Galilei INTRODUCTION 13 1600 bce. Some of the constellations (groupings of stars) we know today have come from Mesopotamian mythology going back even earlier, to before 2000 bce. Astronomy and astrology The Babylonians of Mesopotamia were greatly concerned with divination. To them, planets were manifestations of the gods. The mysterious comings and goings of the planets and unusual happenings in the sky were omens from the gods. The Babylonians interpreted them by relating them to past experience. To their way of thinking, detailed records over long periods were essential to establish connections between the celestial and the terrestrial, and the practice of interpreting horoscopes began in the 6th century bce. Charts showed where the sun, moon, and planets appeared against the backdrop of the zodiac at some critical time, such as a person’s birth. For some 2,000 years, there was little distinction between astrology, which used the relative positions of celestial bodies to track the course of human lives and history, and the astronomy on which it relied. The needs of astrology, rather than pure curiosity, justified observation of the heavens. From the mid-17th century onward, however, astronomy as a scientific activity diverged from traditional astrology. Today, astronomers reject astrology, because it is unfounded in scientific evidence, but they have good reason to be grateful to the astrologers of the past for leaving an invaluable historical record. Time and tide The systematic astronomical observations once used for astrology started to become increasingly important as a means of both timekeeping and navigation. Countries had highly practical reasons—civil, as well as military — to establish national observatories, as the world industrialized and international trade grew. For many centuries, only astronomers had the skills and equipment to preside over the world’s timekeeping. This remained the case until the development of atomic clocks in the mid-20th century. Human society regulates itself around three natural astronomical clocks: Earth’s rotation, detectable by the apparent daily march of the stars around the celestial sphere to give us the day; the time our planet takes to make a circuit around the sun, otherwise known as a year; and the monthly cycle of the moon’s phases. The combined motion in space of Earth, the sun, and the moon also determines the timing and magnitudes of the oceanic tides, which are of crucial importance to coastal communities and seafarers. Astronomy played an equally important role in navigation, the stars acting as a framework of reference points visible from anywhere at sea (cloud permitting). In 1675, British King Charles II commissioned an observatory, the Royal Observatory at Greenwich, near London. The instruction to its director, the first Astronomer Royal, John Flamsteed, was to apply himself diligently to making the observations needed “for the perfecting of the art of navigation.” ❯❯ You have to have the imagination to recognize a discovery when you make one. Clyde Tombaugh 14 INTRODUCTION Astronomy was largely discarded as the foundation of navigation in the 1970s, and replaced by artificial satellites, which created a global positioning system. The purpose of astronomy The practical reasons for pursuing astronomy and space science may have changed, but they still exist. For example, astronomy is needed to assess the risks our planet faces from space. Nothing illustrated Earth’s apparent fragility more powerfully than the iconic images, such as “Earthrise” and “Blue Marble,” taken from space by Apollo astronauts in the 1960s. These images reminded us that Earth is a small planet adrift in space. As What a wonderful and amazing scheme have we here of the magnificent vastness of the universe. Christiaan Huygens surface inhabitants, the protection afforded by the atmosphere and Earth’s magnetic field may make us feel secure, but in reality we are at the mercy of a harsh space environment, blasted by energetic particles and radiation, and at risk of colliding with rocks. The more we know about that environment, the better equipped we are to deal with the potential threats it presents. A universal laboratory There is another very important reason for doing astronomy. The universe is a vast laboratory in which to explore the fundamental nature of matter, and of time and space. The unimaginably grand scales of time, size, and distance, and the extremes of density, pressure, and temperature go far beyond the conditions we can readily simulate on Earth. It would be impossible to test the predicted properties of a black hole or watch what happens when a star explodes in an Earth-bound experiment. Astronomical observations have spectacularly confirmed the predictions of Albert Einstein’s general theory of relativity. As Einstein himself pointed out, his theory explained apparent anomalies in Mercury’s orbit, where Newton’s theory of gravity failed. In 1919, Arthur Eddington took advantage of a total solar eclipse to observe how the paths of starlight deviated from a straight line when the light passed through the gravitational field of the sun, just as relativity predicted. Then, in 1979, the first example of a gravitational lens was identified, when the image of a quasar was seen to be double due to the presence of a galaxy along the line of sight, again as relativity had predicted. The most recent triumphant justification of Einstein’s theory came in 2015 with the first detection of gravitational waves, which are ripples in the fabric of spacetime, generated by the merging of two black holes. When to observe One of the main methods scientists use to test ideas and search for new phenomena is to design experiments and carry them out in controlled laboratory conditions. For the most part, however, with the exception of the solar system—which is close enough for experiments to be carried out by robots—astronomers have to settle for a role as passive collectors of the radiation and elementary particles that happen to arrive on Earth. The key skill astronomers have mastered is that of making informed choices about INTRODUCTION 15 what, how, and when to observe. For instance, it was through the gathering and analysis of telescopic data that the rotation of galaxies could be measured. This, in turn, quite unexpectedly led to the discovery that invisible “dark matter” must exist. In this way, astronomy’s contribution to fundamental physics has been immense. Astronomy’s scope Up to the 19th century, astronomers could only chart the positions and movements of heavenly bodies. This led the French philosopher Auguste Comte to state in 1842 that it would never be possible to determine the compositions of planets or stars. Then, some two decades later, new techniques for the spectrum analysis of light began to open up the possibility of investigating the physical nature of stars and planets. A new word was invented to distinguish this new field from traditional astronomy: astrophysics. Astrophysics became just one of many specialisms in the study of the universe in the 20th century. Astrochemistry and astrobiology are more recent branches. They join cosmology—the study of the origin and evolution of the universe as a whole—and celestial mechanics, which is the branch of astronomy concerned with the movement of bodies, especially in the solar system. The term “planetary science” encompasses every aspect of the study of planets, including Earth. Solar physics is another important discipline. Technology and innovation With the spawning of so many branches of enquiry connected with everything in space, including Earth as a planet, the meaning of the word “astronomy” has evolved once again to become the collective name encompassing the whole of the study of the universe. However, one closely related subject does not come under astronomy: “space If astronomy teaches anything, it teaches that man is but a detail in the evolution of the universe. Percival Lowell science.” This is the combination of technology and practical applications that blossomed with the establishment of the “space age” in the mid-20th century. Collaboration of science Every space telescope and mission to explore the worlds of the solar system makes use of space science, so sometimes it is hard to separate it from astronomy. This is just one example of how developments in other fields, especially technology and mathematics, have been crucial in propelling astronomy forward. Astronomers were quick to take advantage of the invention of telescopes, photography, novel ways of detecting radiation, and digital computing and data handling, to mention but a few technological advances. Astronomy is the epitome of “big science”—a large-scale scientific collaboration. Understanding our place in the universe goes to the heart of our understanding of ourselves: the formation of Earth as a lifesupporting planet; the creation of the chemical building blocks from which the solar system formed; and the origin of the universe as a whole. Astronomy is the means by which we tackle these big ideas. ■ FROM M TO SCIE 600 –1550 BCE CE YTH NCE 18 INTRODUCTION Anaximander of Miletus produces one of the earliest attempts at a scientific explanation of the universe. In his On the Heavens, Aristotle outlines an Earth-centered model of the universe. Many of his ideas will dominate thinking for 2,000 years. In Alexandria, Eratosthenes measures the circumference of Earth and estimates the distance to the sun. C.550 BCE 350 BCE C.200 BCE T C.530 BCE C.220 BCE C.150 CE Pythagoras establishes a school in Croton, where he promotes the idea of a cosmos in which bodies move in perfect circles. Aristarchus of Samos proposes a sun-centered model of the universe, but his idea does not gain wide acceptance. Ptolemy writes the Almagest, which sets out an Earth-centered model of the universe that becomes widely accepted. he traditions on which modern astronomy is built began in ancient Greece and its colonies. In nearby Mesopotamia, although the Babylonians had become highly proficient at celestial forecasting using complicated arithmetic, their astronomy was rooted in mythology, and their preoccupation was with divining the future. To them, the heavens were the realm of the gods, outside the scope of rational investigation by humans. By contrast, the Greeks tried to explain what they observed happening in the sky. Thales of Miletus (c.624–c.546 bce) is regarded as the first in a line of philosophers who thought that immutable principles in nature could be revealed by logical reasoning. The theoretical ideas put forward two centuries later by Aristotle (384–322 bce) were to underpin the whole of astronomy until the 16th century. Aristotle’s beliefs Aristotle was a pupil of Plato, and both were influenced by the thinking of Pythagoras and his followers, who believed that the natural world was a “cosmos” as opposed to “chaos.” This meant that it is ordered in a rational way rather than incomprehensible. Aristotle stated that the heavenly realms are unchanging and perfect, unlike the world of human experience, but he promoted ideas that were consistent with “common sense.” Among other things, this meant Earth was stationary and at the center of the universe. Although it contained inconsistencies, his philosophy was adopted as the most acceptable overall framework of ideas for science and was later incorporated into Christian theology. Geometrical order Mathematically, much of Greek astronomy was based on geometry, particularly motion in circles, which were considered to be the most perfect shapes. Elaborate geometrical schemes were created for predicting the positions of the planets, in which circular motions were combined. In 150 ce, the Graeco–Egyptian astronomer Ptolemy, working in Alexandria, put together the ultimate compendium of Greek astronomy. However, by 500 ce, the Greek approach to astronomy had lost momentum. In effect, after Ptolemy, there were FROM MYTH TO SCIENCE 19 In the Aryabhatiya, Indian astronomer Aryabhata suggests that the stars move across the sky because Earth is rotating. Italian scholar Gerard of Cremona makes Arabic texts, including Ptolemy’s Almagest, accessible in Europe by translating them into Latin. 499 CE Mongol ruler Ulugh Beg corrects many of the postions of stars found in the Almagest. C.1180 1025 Arab scholar Ibn al-Haytham produces a work that criticizes the Ptolomaic model of the universe for its complexity. no significant new ideas in astronomy in this tradition for nearly 1,400 years. Independently, great cultures in China, India, and the Islamic world developed their own traditions through the centuries when astronomy in Europe made little 1437 1279 Chinese astronomer Guo Shoujing produces an accurate measurement of the length of the solar year. progress. Chinese, Arab, and Japanese astronomers recorded the 1054 supernova in the constellation Taurus, which made the famous Crab nebula. Although it was much brighter than Venus, there is no record of its appearance being noted in Europe. The spread of learning It is the duty of an astronomer to compose the history of the celestial motions through careful and expert study. Nicolaus Copernicus 1543 Ultimately, Greek science returned to Europe via a roundabout route. From 740 ce, Baghdad became a great center of learning for the Islamic world. Ptolemy’s great compendium was translated into Arabic, and became known as the Almagest, from its Arabic title. In the 12th century, many texts in Arabic were translated into Latin, so the legacy of the Greek philosophers, as well as the writings of the Islamic scholars, reached Western Europe. Nicolaus Copernicus’s book De revolutionibus orbium coelestium is published, outlining a sun-centered cosmos. The invention of the printing press in the mid-15th century widened access to books. Nicolaus Copernicus, who was born in 1473, collected books throughout his life, including the works of Ptolemy. To Copernicus, Ptolemy’s geometrical constructions failed to do what the original Greek philosophers saw as their objective: describe nature by finding simple underlying principles. Copernicus intuitively understood that a sun-centered method could produce a much simpler system, but in the end his reluctance to abandon circular motion meant that real success eluded him. Nevertheless, his message that physical reality should underpin astronomical thinking arrived at a pivotal moment to set the scene for the telescopic revolution. ■ 20 IT IS CLEAR THAT EARTH DOES NOT MOVE THE GEOCENTRIC MODEL IN CONTEXT KEY ASTRONOMER Aristotle (384–322 bce) BEFORE 465 bce Greek philosopher Empedocles thinks that there are four elements: earth, water, air, and fire. Aristotle contends that the stars and planets are made of a fifth element, aether. 387 bce Plato’s student Eudoxus suggests that the planets are set in transparent rotating spheres. AFTER 355 bce Greek thinker Heraclides claims that the sky is stationary and Earth spins. O ne of the most influential of all Western philosophers, Aristotle, from Macedonia in northern Greece, believed that the universe was governed by physical laws. He attempted to explain these through deduction, philosophy, and logic. Aristotle observed that the positions of the stars appeared to be fixed in relation to each other, and that their brightness never changed. The constellations always Earth casts a circular shadow on the moon during a lunar eclipse. This convinced Aristotle that Earth was a sphere. Earth’s shadow 12th century Italian Catholic priest Thomas Aquinas begins teaching Aristotle’s theories. moon 1577 Tycho Brahe shows that the Great Comet is farther from Earth than the moon. 1687 Isaac Newton explains force in his Philosophiae Naturalis Principia Mathematica. sun’s rays Earth stayed the same, and spun daily around Earth. The moon, sun, and planets, too, appeared to move in unchanging orbits around Earth. Their motion, he believed, was circular and their speed constant. His observations of the shadow cast by Earth on the moon’s surface during a lunar eclipse convinced him that Earth was a sphere. His conclusion was that a spherical Earth remained stationary in space, never spinning or changing its position, while the cosmos spun eternally around it. Earth was an unmoving object at the center of the universe. Aristotle believed that Earth’s atmosphere, too, was stationary. At the top of the atmosphere, friction occurred between the atmospheric gases and the rotating sky above. Episodic emanations of gases from volcanoes rose to the top of the atmosphere. Ignited by friction, these gases produced comets, and, if ignited quickly, they produced shooting stars. His reasoning remained widely accepted until the 16th century. ■ See also: Consolidating knowledge 24–25 ■ The Copernican model 32–39 The Tychonic model 44–47 ■ Gravitational theory 66–73 ■ FROM MYTH TO SCIENCE 21 EARTH REVOLVES AROUND THE SUN ON THE CIRCUMFERENCE OF A CIRCLE EARLY HELIOCENTRIC MODEL IN CONTEXT KEY ASTRONOMER Aristarchus (310–230 bce) BEFORE 430 bce Philolalus of Craton proposes that there is a huge fire at the center of the universe, around which the sun, moon, Earth, five planets, and stars revolve. 350 bce Aristotle states that Earth is at the center of the universe and everything else moves around it. AFTER 150 ce Ptolemy publishes his Almagest, describing an Earth-centered (geocentric) model of the universe. 1453 Nicolaus Copernicus proposes a heliocentric (sun-centered) universe. 1838 German astronomer Friedrich Bessel is the first to obtain an accurate measurement of the distance to a star, using a method known as parallax. A n astronomer and mathematician from the Greek island of Samos, Aristarchus is the first person known to have proposed that the sun, not Earth, is at the center of the universe, and that Earth revolves around the sun. Aristarchus’s thoughts on this matter are mentioned in a book by another Greek mathematician, Archimedes, who states in The Sand Reckoner that Aristarchus had formulated a hypothesis that “the fixed stars and sun remain unmoved” and “Earth revolves about the sun.” Unfashionable idea Aristarchus persuaded at least one later astronomer—Seleucus of Seleucia, who lived in the second century bce—of the truth of his heliocentric (sun-centered) view of the universe, but otherwise it seems his ideas did not gain wide acceptance. By the time of Ptolemy, in about 150 ce, the prevailing view was still a geocentric (Earthcentered) one, and this remained Aristarchus was the real originator of the Copernican hypothesis. Sir Thomas Heath Mathematician and classical scholar the case until the 15th century, when the heliocentric viewpoint was revived by Nicolaus Copernicus. Aristarchus also believed that the stars were much farther away than had previously been imagined. He made estimates of the distances to the sun and moon, and their sizes relative to Earth. His estimates regarding the moon were reasonably accurate, but he underestimated the distance to the sun, mainly because of an inaccuracy in one of his measurements. ■ See also: The geocentric model 20 ■ Consolidating knowledge 24–25 The Copernican model 32–39 ■ Stellar parallax 102 ■ 22 THE EOUINOXES MOVE OVER TIME SHIFTING STARS IN CONTEXT KEY ASTRONOMER Hipparchus (190–120 bce) BEFORE 280 bce Greek astronomer Timocharis records that the star Spica is 8° west of the fall equinox. AFTER 4th century ce Chinese astronomer Yu Xi notices and measures precession. 1543 Nicolaus Copernicus explains precession as a motion of Earth’s axis. 1687 Isaac Newton demonstrates precession to be a consequence of gravity. 1718 Edmond Halley discovers that, except for the relative motion between stars and reference points on the celestial sphere, stars have a gradual motion relative to each other. This is because they are moving in different directions and at different speeds. I n about 130 bce, the Greek astronomer and mathematician Hipparchus of Nicaea noticed that a star named Spica had moved 2o east of a point on the celestial sphere, called the fall equinox point, compared to its position recorded 150 years earlier. Further research showed him that the positions of all stars had shifted. This shift became known as “precession of the equinoxes.” The celestial sphere is an imaginary sphere surrounding Earth, in which stars are found at specific points. Astronomers use Industrious, and a great lover of the truth. Ptolemy describing Hipparchus See also: Gravitational theory 66–73 ■ exactly defined points and curves on the surface of this sphere as references for describing the positions of stars and other celestial objects. The sphere has north and south poles, and a celestial equator, which is a circle lying above Earth’s equator. The ecliptic is another important circle on the sphere, which traces the apparent path of the sun against the background of stars over the course of the year. The ecliptic intersects the celestial equator at two points: the spring and fall equinox points. These mark the positions on the celestial sphere that the sun reaches on the equinoxes in March and September. The precession of the equinoxes refers to the gradual drift of these two points relative to star positions. Hipparchus put this precession down to a “wobble” in the movement of the celestial sphere, which he believed to be real and to rotate around Earth. It is now known that the wobble is actually in the orientation of Earth’s spin axis, caused by the gravitational influence of the sun and the moon. ■ Halley’s comet 74–77 FROM MYTH TO SCIENCE 23 THE MOON’S BRIGHTNESS IS PRODUCED BY THE RADIANCE OF THE SUN THEORIES ABOUT THE MOON IN CONTEXT KEY ASTRONOMER Zhang Heng (78–139 ce) BEFORE 140 bce Hipparchus discovers how to predict eclipses. 1st century bce Jing Fang advances the “radiating influence” theory, stating that the light of the moon is the reflected light of the sun. AFTER 150 ce Ptolemy produces tables for calculating the positions of celestial bodies. 11th century Shen Kuo’s Dream Pool Essays explains that heavenly bodies are round like balls rather than flat. 1543 Nicolaus Copernicus’s On the Revolutions of the Celestial Spheres describes a heliocentric system. 1609 Johannes Kepler explains the movements of the planets as free-floating bodies, describing ellipses. T he Chief Astrologer at the court of Chinese emperor An-ti, Zhang Heng was a skilled mathematician and a careful observer. He cataloged 2,500 “brightly shining” stars and estimated that there were a further 11,520 “very small” ones. Also a distinguished poet, Zhang expressed his astronomical ideas through simile and metaphor. In his treatise Ling Xian, or The Spiritual Constitution of the Universe, he placed Earth at the center of the cosmos, stating that “the sky is like a hen’s egg, and is as round as a crossbow pellet, and Earth is the yolk of the egg, lying alone at the center.” The sun is like fire and the moon like water. The fire gives out light and the water reflects it. Zhang Heng sun is fully lit, and the side that is away from it is dark.” He also described a lunar eclipse, during which the sun’s light cannot reach the moon because Earth is in the way. He recognized that the planets were similarly subject to eclipses. Zhang’s work was developed further in the 11th century by another Chinese astronomer, Shen Kuo. Shen demonstrated that the waxing and waning of the moon proved that the moon and sun were spherical. ■ Shape but no light Zhang concluded that the moon had no light of its own, but rather reflected the sun “like water.” In this, he embraced the theories of his compatriot Jing Fang who, a century earlier, had declared that “the moon and the planets are Yin; they have shape but no light.” Zhang saw that “the side that faces the See also: The Copernican model 32–39 ■ Elliptical orbits 50–55 24 ALL MATTERS USEFUL TO THE THEORY OF HEAVENLY THINGS CONSOLIDATING KNOWLEDGE IN CONTEXT KEY ASTRONOMER Ptolemy (85–165 ce) BEFORE 12th century bce The Babylonians organize the stars into constellations. 350 bce Aristotle asserts that the stars are fixed in place and Earth is stationary. 135 bce Hipparchus produces a catalog of over 850 star positions and brightnesses. AFTER 964 ce Persian astronomer al-Sufi updates Ptolemy’s star catalog. 1252 The Alfonsine Tables are published in Toledo, Spain. These list the positions of the sun, moon, and planets based on Ptolemy’s theories. 1543 Copernicus shows that it is far easier to predict the movement of the planets if the sun is placed at the center of the cosmos rather than Earth. I n his greatest known work, the Almagest, the Graeco-Egyptian astronomer Ptolemy produced a summary of all the astronomical knowledge of his time. Rather than producing radical new ideas of his own, Ptolemy mostly consolidated and built upon previous knowledge, particularly the works of the Greek astronomer Hipparchus, whose star catalog formed the basis of most of the calculations in the Almagest. Ptolemy also detailed the mathematics required to calculate the future positions of the planets. His system would be used by generations of astrologers. The constellations devised by Ptolemy are used in this 17th-century star map. The number of stars per constellation ranges from two (Canis Minor) to 42 (Aquarius). Ptolemy’s model of the solar system had a stationary Earth at its center, with the heavens spinning daily around it. His model required complicated additions to make it match the data and allow it to be used to calculate the positions of the planets; nonetheless, it was largely unchallenged until Copernicus placed the sun at the center of the cosmos in the 16th century. FROM MYTH TO SCIENCE 25 See also: The geocentric model 20 ■ Shifting stars 22 ■ The Copernican model 32–39 ■ The Tychonic model 44–47 ■ Elliptical orbits 50–55 Ptolemy produced a catalog of 1,022 star positions and listed 48 constellations in the part of the celestial sphere known to the Greeks—everything that could be seen from a northern latitude of about 32o. Ptolemy’s constellations are still used today. Many of their names can be traced even further back to the ancient Babylonians, including Gemini (twins), Cancer (crab), Leo (lion), Scorpio (scorpion), and Taurus (bull). The Babylonian constellations are named on a cuneiform tablet called the Mul Apin, which dates back to the 7th century bce, however, they are thought to have been compiled about 300 years earlier. Early quadrant To improve his measurements, Ptolemy built a plinth. One of the earliest examples of a quadrant, his plinth was a huge rectangular block of stone, one of whose vertical sides accurately aligned in the north–south plane. A horizontal bar protruded from the top of the stone, and its shadow gave a precise indication of the height of the sun at noon. Ptolemy took daily measurements to obtain accurate estimates of the time of the solstices and equinoxes, which confirmed previous measurements showing that the seasons were different lengths. He believed that the orbit of the sun around Earth was circular, but his calculations led him to the conclusion that Earth could not be at the exact center of that orbit. Ptolemy the astrologist Like most thinkers of his day, Ptolemy believed that the movements of the heavenly bodies profoundly affected events on Earth. His book on astrology, Tetrabiblos, rivaled the Almagest in popularity over the following 1,000 years. Ptolemy had not only provided a means to calculate planetary positions, but he had also produced a comprehensive interpretation of the ways those movements affected humans. ■ Sun Sun’s height Horizontal bar Stone plinth 0o Sun’s shadow Claudius Ptolemy Ptolemy was a polymath and produced works on a wide range of topics, including astronomy, astrology, geography, music, optics, and mathematics. Very little is known about him, but he probably spent all his life in Alexandria, the Egyptian seaport with a reputation for scholarship and a great library, where he was taught by the renowned mathematician Theon of Smyrna. Many of his prolific writings have survived. They were translated into Arabic and Latin, disseminating his ideas across the medieval world. Geography listed the locations of most of the places in the known world, and was carried by Christopher Columbus on his voyages of discovery in the 15th century. The Almagest remained in continual use in academia until about 1643, a century after Ptolemy’s model of the universe had been challenged by Copernicus. Key works Ptolemy describes the design of his stone plinth in the Almagest. It was a quadrant, an instrument that measures angles between 0° and 90°. 90o c.150 ce Geography c.150 ce Almagest c.150 ce Tetrabiblos 26 THE UNMOVING STARS GO UNIFORMLY W ESTWARD EARTH’S ROTATION IN CONTEXT KEY ASTRONOMER Aryabhata (476–550 ce) BEFORE 350 bce Heraclides Ponticus, a pupil of Plato, proposes that Earth rotates once a day on its axis. The idea does not become widespread because it contradicts Aristotle, who is considered more authoritative. 4th century bce Aristotle states that Earth is stationary in space. AFTER 950 ce Iranian astronomer al-Sijzi supports the idea that Earth rotates. 1543 Nicolaus Copernicus states that Earth rotates as part of his heliocentric (sun-centered) model of the universe. 1851 The first demonstration of Léon Foucault’s pendulum in Paris provides the final scientific proof that Earth is rotating. F rom the 4th century bce to the 16th century ce, the prevailing view throughout the Western world was that Earth is stationary and located at the center of the universe. Suggestions that Earth might be rotating were dismissed on the grounds that this would cause objects on Earth’s surface to fly off into space. In India, however, an astronomer named Aryabhata was convinced that the movement of stars across the night sky was due not to the stars revolving in a distant sphere around Earth, but to Earth itself rotating. An illusory movement According to Aryabhata, the stars were stationary and their apparent movement toward the west was an illusion. His notion of a spinning Earth was not widely accepted until the mid-17th century—a century after Nicolaus Copernicus had endorsed the idea. Aryabhata’s achievements were considerable. His book Aryabhatiya was the most important work of astronomy in the 6th century. He was the father of the Indian cyclic astronomy … that determines more accurately the true positions and distances of the planets. Helaine Selin Historian of astronomy Essentially a compendium of the fundamentals of astronomy and relevant mathematics, it greatly influenced Arabic astronomy. Among other achievements, Aryabhata calculated the length of the sidereal day (the time it takes Earth to rotate once in relation to the stars) to a high degree of accuracy, and devised original and accurate ways of compiling astronomical tables. ■ See also: The geocentric model 20 ■ The Copernican model 32–39 The Tychonic model 44–47 ■ Elliptical orbits 50–55 ■ FROM MYTH TO SCIENCE 27 A LITTLE CLOUD IN THE NIGHT SKY MAPPING THE GALAXIES IN CONTEXT KEY ASTRONOMER Abd al-Rahman al-Sufi (903–986 ce) BEFORE 400 bce Democritus suggests that the Milky Way is made of a dense mass of stars. 150 ce Ptolemy records several nebulae (or cloudy objects) in the Almagest. A bd al-Rahman al-Sufi, once better known in the West as Azophi, was a Persian astronomer who made the first record of what are now understood to be galaxies. To al-Sufi, these fuzzy, nebulous objects looked like clouds in the night’s sky. Al-Sufi made most of his observations in Isfahan and Shiraz, in what is now central Iran, but he AFTER 1610 Galileo sees stars in the Milky Way using a telescope, confirming Democritus’s theory. 1845 Lord Rosse makes the first clear observation of a spiral nebula, now known as the Whirlpool Galaxy. 1917 Vesto Slipher discovers that spiral nebulae are rotating independently of the Milky Way. 1929 Edwin Hubble shows that many spiral nebulae are far beyond the Milky Way and are galaxies themselves. The Large Magellanic Cloud, seen here above the ESO’s Paranal observatory in Chile, can be easily observed with the naked eye from the southern hemisphere. also consulted Arab merchants who traveled to the south and east, and who saw more of the sky. His work centered on translating Ptolemy’s Almagest into Arabic. In the process, al-Sufi tried to merge the Hellenistic constellations (which dominate star maps today) with their Arab counterparts, most of which were totally different. The fruit of this labor was Kitab suwar al-kawakib, or the Book of Fixed Stars, published in 964 ce. The work contained an illustration of “a little cloud,” which is now know to be the Andromeda Galaxy. This object was probably known to earlier Persian astronomers, but al-Sufi’s mention is the earliest record. Similarly, The Book of Fixed Stars includes the White Ox, another cloudy object. This is now named the Large Magellanic Cloud and is a small galaxy that orbits the Milky Way. Al-Sufi would not have been able to observe this object himself, but would have received reports of it from astronomers in Yemen and sailors who crossed the Arabian Sea. ■ See also: Consolidating knowledge 24–25 ■ Examining nebulae 104–05 Spiral galaxies 156–61 ■ Beyond the Milky Way 172–77 ■ 28 A NEW CALENDAR FOR CHINA THE SOLAR YEAR IN CONTEXT KEY ASTRONOMER Guo Shoujing (1231–1314) BEFORE 100 bce Emperor Wu of the Han Dynasty establishes the Chinese calendar based on a solar year. T he traditional Chinese calendar is a complex blend of lunar and solar cycles, with 12 or 13 lunar months matched up to the solar-derived seasons. It had first been formalized in the 1st century bce during the Han Dynasty, and used a solar year of 365.25 days (365 days and 6 hours). 46 bce Julius Caesar reforms the Roman calendar using a year-length of 365 days and 6 hours, and adds a leap day every four years. AFTER 1437 The Timurid astronomer Ulugh Beg measures the solar year as 365 days, 5 hours, 49 minutes, and 15 seconds using a 164-ft (50-m) gnomon (the central column of a sundial). 1582 Pope Gregory adopts the Gregorian calendar as a reform of the ancient Julian calendar by using a 365.25-day year, the same year as Guo’s Shoushi calendar. China’s calculations were ahead of the West’s: 50 years later, this same period was used by Julius Caesar to create the Roman Empire’s Julian system. By the time the Mongol leader Kublai Khan conquered most of China in 1276, a variant of the original calendar, the Daming calendar, was in use, but was centuries old and in need of correction. The khan decided to impose his authority with a new, more accurate calendar, which became known as the Shoushi (“well-ordered”) calendar. The task of creating it was entrusted to Guo Shoujing, the khan’s brilliant Chinese chief astronomer. Measuring the year A trained engineer, Guo Shoujing invented a water-powered version of an armillary sphere, which is an instrument used to model the positions of celestial bodies. Guo’s job was to measure the length of the solar year, and to this end he set up an observatory in Khanbaliq (the “City of the Khan”), a new imperial capital that would one day become known as Beijing. The observatory may have been the largest anywhere in the world at the time. Working with mathematician Wang Chun, Guo began a series of observations tracking the motion of the sun throughout the year. FROM MYTH TO SCIENCE 29 See also: Shifting stars 22 ■ Improved instruments 30–31 The two men traveled widely, setting up another 26 observatories across China. In 1279, the pair announced that there were 29.530593 days to a month, and that the true solar year was 365.2524 days long (365 days, 5 hours, 49 minutes, and 12 seconds). This is just 26 seconds longer than the current accepted measurement. Again, China was ahead of the West. The same figure was not independently measured and adopted for the universal Gregorian calendar in Europe until 300 years later. Enduring calendar A great technological innovator, Guo invented several new observational devices and made enhancements to the Persian equipment that had begun to arrive in China under Kublai Khan’s rule. Most importantly, he built a giant gnomon to a height of 44 ft (13.3 m), which was five times taller than the previous Persian design and featured a horizontal crossbar marked with Guo Shoujing ■ Zu Chongzhi (Directory) 334 The calendar has 365 days and 6 hours in the year, but does not match the motion of the sun through the year. To measure the length of the year, better instruments must be created. There is a need to create a new calendar that matches the solar year. The solar year is found to be 365 days, 5 hours, 49 minutes, and 12 seconds. There is a new calendar for China. measurements. This allowed Guo to measure the angle of the sun with far greater accuracy. The Shoushi calendar was widely regarded as the most accurate calendar in the world at the time. As a testament to its success, it continued to be used for 363 years, making it the longest- serving official calendar in Chinese history. China officially adopted the Gregorian calendar in 1912, but the traditional calendar, today known as the rural or former calendar, still plays a role in Chinese culture, determining the most propitious dates to hold weddings, family celebrations, and public holidays. ■ Guo Shoujing was born into a poor family in the north of China, in the years when the Mongols were consolidating their control over the region. A child prodigy who had built a highly advanced water clock by the age of 14, Guo was taught mathematics, astronomy, and hydraulics by his grandfather. He became an engineer, working for the emperor’s chief architect Liu Bingzhong. In the late 1250s, Kublai Khan took the throne and chose the region around the town of Dadu near the Yellow River to build the new capital of Khanbaliq, now known as Beijing. Guo was tasked with building a canal to bring spring water from the mountains to the new city. In the 1290s, Guo—by now the khan’s chief science and engineering adviser—connected Khanbaliq to the ancient Grand Canal system that linked to the Yangtze and other major rivers. In addition to continuing his astronomical work, Guo oversaw similar irrigation and canal projects across China, and his theoretical and technological innovations continued to influence Chinese society for centuries after his death. 30 WE HAVE RE-OBSERVED ALL OF THE STARS IN PTOLEMY’S CATALOG IMPROVED INSTRUMENTS IN CONTEXT KEY ASTRONOMER Ulugh Beg (1384–1449) BEFORE c.130 bce Hipparchus publishes a star catalog giving the positions of more than 850 stars. 150 ce Ptolemy publishes a star catalog in the Almagest, which builds on the work of Hipparchus and is seen as the definitive guide to astronomy for more than a millennium. 964 ce Abd al-Rahman al-Sufi adds the first references to galaxies in his star catalog. AFTER 1543 Nicolaus Copernicus places the sun as the center of the universe, not Earth. 1577 Tycho Brahe’s star catalog records a nova, showing that the “fixed stars” are not eternal and do change. F or more than 1,000 years, Ptolemy’s Almagest was the world’s standard authority on star positions. Translated into Arabic, Ptolemy’s work was also influential in the Islamic world up until the 15th century, when the Mongol ruler Ulugh Beg showed that a lot of the Almagest’s data were wrong. A grandson of the Mongol conqueror Timur, Ulugh Beg was just 16 years old when he became ruler of the family’s ancestral seat at Samarkand (in present-day Uzbekistan) in 1409. Determined Ulugh Beg The name Ulugh Beg means “Great Leader.” The sultan– astronomer’s birth name was Mirza Muhammad Taraghay bin Shahrukh. He was born on the move, as Timur’s army traveled through Persia. His grandfather’s death in 1405 brought the army to a halt in western China. The ensuing fight for control of his lands was eventually won by Ulugh Beg’s father, Shah Rukh. In 1409, Ulugh Beg was sent to Samarkand as his father’s to turn the city into a respected place of learning, Ulugh Beg invited scholars of many disciplines from far and wide to study at his new madrasa, an educational institution. Ulugh Beg’s own interest was in astronomy, and it may have been his discovery of serious errors in the star positions of the Almagest that inspired him to order the building of a gigantic observatory, the largest in the world at the time. Located on a hill to the north of the city, it took five years to construct and was regent, and by 1411, as he turned 18, his rule over the city was extended to include the surrounding province. Ulugh Beg’s flair for mathematics and astronomy was not matched by his leadership skills. When Shah Rukh died in 1447, Ulugh Beg assumed the imperial throne, but he did not command enough authority to keep it. In 1449, he was beheaded by his own son. Key work 1437 Zij-i Sultani FROM MYTH TO SCIENCE 31 See also: Shifting stars 22 ■ Consolidating knowledge 24–25 The Copernican model 32–39 ■ The Tychonic model 44–47 ■ Mapping the galaxies 27 The understanding of astronomy is based on the study of the work of past scholars. A precisely built sextant in a protected location gives more accurate measurements. completed in 1429. It was there, with his team of astronomers and mathematicians, that he set about compiling a new star catalog. Giant instruments Ptolemy’s catalog had largely been derived from the work of Hipparchus, and many of its star positions were not based on fresh observations. To measure accurately, Ulugh Beg built the observatory on an immense scale. Its most impressive instrument was the so-called Fakhri sextant. In fact, more like a quadrant (a quarter-circle rather The religions disperse, kingdoms fall apart, but works of science remain for all ages. Ulugh Beg With better instruments, the work of past astronomers is often found to contain errors. than a sixth), it is estimated to have had a radius of more than 130 ft (40 m) and would have been three stories high. The instrument was kept underground to protect it from earthquakes and rested in a curved trench along the north– south meridian. As the sun and the moon passed overhead, their light focused into the dark trench, and their positions could be ■ measured to within a few hundredths of a degree, as could the positions of the stars. In 1437, Zij-i Sultani (“The Sultan’s Catalog of Stars”) was published. Of the 1,022 stars included in the Almagest, Ulugh Beg corrected the positions of 922. Zij-i Sultani also contained new measurements for the solar year, planetary motion, and the axial tilt of Earth. These data became very important, enabling the prediction of eclipses, the time of sunrise and sunset, and the altitude of celestial bodies, which were needed to navigate. Ulugh Beg’s work remained the definitive star catalog until Tycho Brahe’s, nearly 200 years later. ■ All that remains of the Fakhri sextant is a 6½-ft (2-m) wide trench gouged in a hillside. The observatory was destroyed after Ulugh Beg’s death in 1449 and not discovered until 1908. FINALLY WE SHALL PLACE THE SUN HIMSELF AT THE CENTER OF THE UNIVERSE THE COPERNICAN MODEL 34 THE COPERNICAN MODEL IN CONTEXT KEY ASTRONOMER Nicolaus Copernicus (1473–1543) BEFORE c.350 bce Aristotle places Earth at the center of the universe. c.270 bce Aristarchus proposes a sun-centered (heliocentric) universe, with the stars a vast distance away. c.150 ce Ptolemy publishes the Almagest. AFTER 1576 English astronomer Thomas Digges suggests modifying the Copernican system, removing its outer edge and replacing it with a star-filled unbound space. 1605 Johannes Kepler discovers that orbits are elliptical. 1610 Galileo Galilei discovers the phases of Venus, and Jupiter’s moons, strengthening the heliocentric viewpoint. Nicolaus Copernicus T o most people in mid-15th century Europe, questions about Earth’s place in the cosmos had been answered in the 2nd century by the GrecoEgyptian mathematician Ptolemy, who had modified ideas first put forward by Aristotle. These ideas placed Earth at the center of the cosmos, and they carried an official stamp of approval from the Church. Yet the first convincing challenge to this orthodoxy was to come from a figure within the Church, the Polish canon Nicolaus Copernicus. A stationary Earth Of all discoveries and opinions, none may have exerted a greater effect on the human spirit than the doctrine of Copernicus. Johann von Goethe According to the version of the universe described by Aristotle and Ptolemy, Earth was a stationary point at the center of the universe, with everything else circling around it, and stars were fixed in a large, invisible, distant sphere, which rotated rapidly around Earth. The sun, moon, and planets also revolved at different speeds around Earth. This idea of the universe seemed like common sense. After all, one only had to stand outside and look up at the sky, and it appeared obvious that Earth stayed in one place, while everything else rose in the east, swung across the sky, and set in the west. Furthermore, the Bible seemed to state that the sun moves, whereas Earth does not, so anyone who contradicted this view risked being accused of heresy. Nicolaus Copernicus was born in Torun, Poland, in 1473. From 1491 to 1495, he studied mathematics, astronomy, and philosophy at the University of Kraków, then from 1496, canon (religious) law and astronomy at the University of Bologna, Italy. In 1497, he was appointed canon of the cathedral of Frombork, Poland, a post he retained for life. From 1501 to 1505, he studied law, Greek, and medicine at the University of Padua, Italy. Subsequently, he returned to Frombork, where he spent much of the rest of his life. By 1508, he had begun developing his sun-centered model of the universe. He did not complete this work until 1530, although he did publish a summary of his ideas in 1514. Realizing that he risked being ridiculed or persecuted, Copernicus delayed publishing the full version of his theory until the last weeks of his life. Nagging doubts The Earth-centered, or geocentric, model of the universe had never convinced everyone—in fact, doubts about it had surfaced from time to time for more than 1,800 years. The most serious Key works 1514 Commentariolus 1543 De revolutionibus orbium coelestium (On the Revolutions of the Celestial Spheres) FROM MYTH TO SCIENCE 35 See also: The geocentric model 20 ■ Early heliocentric model 21 ■ Consolidating knowledge 24–25 ■ The Tychonic model 44–47 ■ Elliptical orbits 50–55 ■ Galileo’s telescope 56–63 ■ Stellar aberration 78 ■ Al-Battani (Directory) 334 In so many and such important ways, then, do the planets bear witness to the Earth’s mobility. Nicolaus Copernicus Planet Center of deferent cle The most glaring anomaly was Mars, which had been carefully observed in ancient times by both the Babylonians and the Chinese. It appeared to speed up and slow down from time to time. If its movements were compared to those of the rapidly rotating outer sphere of fixed stars, Mars usually moved in a particular direction, but occasionally it reversed direction— a strange behavior described as “retrograde motion.” In addition, its brightness varied greatly over the course of a year. Similar, but less dramatic, irregularities were also observed in the other planets. To Center of epicycle Earth cy Ptolemy’s fixes Ptolemy tried to fix some of the anomalies in Aristotle’s geocentric model by proposing that each planet moved in a small circle called an epicycle. Each epicycle was embedded in a sphere called a deferent. Each planet’s deferent rotated around a point slightly displaced from Earth’s position in space. This point, in turn, continuously rotated around another point called an equant. Each planet had its own equant. i Ep Equant ef er en t concern related to predicting the movements and appearances of the planets. According to the Aristotelian version of geocentrism, the planets—like all other celestial bodies—were embedded in invisible concentric spheres that revolved around Earth, each rotating at its own steady speed. But if this were true, each planet should move across the sky at a constant pace and with an unvarying brightness—and this wasn’t what was observed. D address these problems, Ptolemy modified the original Aristotelian geocentric model. In his revised model, the planets were attached not to the concentric spheres themselves, but to circles attached to the concentric spheres. He called these circles “epicycles.” These were suborbits around which the planets circled while the central pivot points of these suborbits were carried around the sun. These modifications, Ptolemy thought, sufficed to explain the anomalies observed and matched observational data. However, his model became hugely complicated, as further epicycles needed to be added to keep prediction in line with observation. Alternative views From about the 4th century bce, a number of astronomers had suggested theories refuting the geocentric model. One of these ideas was that Earth spins on its own axis, which would account for a large proportion of the daily movements of celestial objects. The concept of a rotating Earth had initially been put forward by a Greek, Heraclides Ponticus, in about 350 bce and later by various ❯❯ 36 THE COPERNICAN MODEL Ptolemy’s Earthcentered model of the universe relies upon complex adjustments to explain observed data. Copernicus’s sun-centered model explains the same data with far fewer adjustments. Copernicus believes his model is more elegant, and thus more likely to be correct. Place the sun himself at the center of the universe. Arabic and Indian astronomers. Supporters of geocentrism rejected his idea as absurd, believing a spinning Earth would create huge winds, such that objects on Earth’s surface would simply fly off. Another idea, first proposed by Aristarchus of Samos in about 250 bce, was that Earth might move around the sun. Not only did this go against deeply ingrained Aristotelian ideas, but supporters of geocentrism had also for centuries cited what seemed a scientifically valid reason for ruling it out—the “lack of stellar parallax.” They argued that if Earth moved around the sun, it would be possible to observe some variation in the relative positions of stars. No such variation could ever be detected so, they said, Earth could not move. In the face of such an established philosophical tradition with little observational evidence to contradict it, and the theological arguments in favor of it, the geocentric view of the universe went unchallenged for centuries. However, in about 1545, rumors began circulating in Europe of a highly convincing challenge that had appeared in the form of a book entitled De revolutionibus orbium coelestium (On the Revolutions of the Celestial Spheres), by a Polish scholar, Nicolaus Copernicus. Copernican revolution The work was extremely comprehensive, and proposed a new, detailed, mathematical, and geometrical model of how the universe works, based on years of astronomical observations. Copernicus’s theory was based on a number of basic propositions. First, Earth rotates on its axis daily, and this rotation accounts for most of the daily movements of the stars, sun, and planets across the sky. In his 1660 star atlas, German mapmaker Andreas Cellarius illustrated the cosmic systems of Ptolemy, Tycho Brahe, and Copernicus (shown here). All three still had their champions. FROM MYTH TO SCIENCE 37 Copernicus thought it was just too unlikely that thousands of stars were spinning rapidly around Earth every 24 hours. Instead, he considered them to be fixed and immovable in their distant, outer sphere, and that their apparent movement was actually an illusion caused by Earth’s spin. To refute the idea that a spinning Earth would create huge winds, and that objects on its surface would fly off, Copernicus pointed out that Earth’s oceans and atmosphere were part of the planet and were naturally part of this spinning motion. In his own words: “We would only say that not merely the Earth and the watery element joined with it have this motion, but also no small part of the air and whatever is linked in the same way to the Earth.” Second, Copernicus proposed that it is the sun that is at the center of the universe, not Earth, which is simply one of the planets, all of which circle the sun at differing speeds. In the Ptolemaic model (top), Earth is at the center and other celestial bodies go around Earth. In the Copernican system (bottom), Earth together with the moon have swapped position with the sun; the sphere of the fixed stars is much farther out. Sun Mercury Moon Mars Venus Earth Saturn Embedded “fixed” stars Jupiter Outer sphere with embedded “fixed” stars Elegant solution These two central tenets of Copernicus’s theory were of utmost importance because they explained the movements and variation in brightness of the planets without recourse to Ptolemy’s complicated adjustments. If Earth and another planet, such as Mars, both circle the sun and do so at different speeds, taking a different amount of time to complete each revolution, they will sometimes be close to each other on the same side as the sun and sometimes far from each other, on opposite sides to the sun. This, at a stroke, explained the observed variations in brightness of Mars and the other planets. The heliocentric system also elegantly explained apparent retrograde motion. In place of Ptolemy’s ❯❯ Earth Moon Mars Mercury Saturn Sun Venus Jupiter 38 THE COPERNICAN MODEL complicated epicycles, Copernicus explained that such motion could be attributed to changes in perspective caused by Earth and the other planets moving at different speeds. Distant stars Another of Copernicus’s tenets was that the stars are much farther away from Earth and the sun than had previously been believed. He said: “The distance between Earth and the sun is an insignificant fraction of the distance from Earth and sun to the stars.” Earlier astronomers knew that the stars were distant, but few suspected just how far away they were, and those who did, such as Aristarchus, had not managed to convince anyone. Even Copernicus probably Those things which I am saying now may be obscure, yet they will be made clearer in their proper place. Nicolaus Copernicus never realized quite how far away the stars are—it is now known that the very closest are about 260,000 times more distant than the sun. But his assertion was extremely important because of its implications for stellar parallax. For centuries, supporters of geocentrism had argued that the absence of parallax could only be due to Earth not moving. Now, there was an alternative explanation: the parallax was not absent, but because of the great distance to the stars, it was simply too tiny to be measured with the instruments of the time. Copernicus additionally proposed that Earth is at the center of the lunar sphere. Copernicus maintained that the moon circled Earth, as it did in the geocentric model. In his heliocentric model, the moon moved with Earth as it circled the sun. In this system, the moon was the only celestial object that did not primarily move around the sun. In the Ptolemaic model (left), the occasional retrograde (backward-moving) motion of Mars was regarded as due to loops that the planet makes in space. In the Copernican model (right), retrograde motion was caused simply by changes in perspective because Earth and Mars orbit the sun at different speeds. Earth would from time to time “overtake Mars on the inside” as shown here, causing Mars to reverse its apparent direction of movement for several weeks. Mars View as seen from Earth Motion of Mars Mars Earth Epicycle Sun Earth Earth’s orbit Mars’s deferent Mars’s orbit FROM MYTH TO SCIENCE 39 Though Copernicus’s work was widely circulated, it took a century or more before its basic ideas were accepted by most other astronomers, let alone the public at large. One difficulty was that, although it resolved many of the problems of the Ptolemaic system, his model also contained faults that had to be amended by later astronomers. Many of these faults were due to the fact that, for philosophical reasons, Copernicus clung to the belief that all the movements of celestial bodies occurred with the objects embedded in invisible spheres I am deterred by the fate of our teacher Copernicus who, although he had won immortal fame with a few, was ridiculed and condemned by countless people (for very great is the number of the stupid). Galileo Galilei and that these movements must be perfect circles. This therefore forced Copernicus to retain some of Ptolemy’s epicycles in his model. The work of Johannes Kepler later replaced the idea of circular orbits with that of elliptical orbits, eliminating most of the remaining faults in Copernicus’s model. It wasn’t until the 1580s and the work of Danish astronomer Tycho Brahe that the idea of celestial spheres was abandoned in favor of free orbits. Banned by the Church De revolutionibus initially met with little or no resistance from the Roman Catholic Church, although some Protestants denounced it as heretical. In 1616, however, the Catholic Church condemned Copernicus’s book and it remained proscribed reading for more than 200 years. The Church’s decision coincided with a dispute it was having at the time with the astronomer Galileo Galilei. Galileo was an avid champion of the Copernican theory and had made discoveries in 1610 that strongly supported the heliocentric view. The dispute with Galileo caused the Church authorities to examine De revolutionibus with intense scrutiny, and the fact that Mars’s apparent retrograde motion occurs about every 26 months and lasts for 72 days. Its orbit is on a slightly different plane from Earth’s, contributing to the apparent loop. some of its propositions went against Biblical texts probably led to the ban. Viewed somewhat ambivalently at first by astronomers, and prohibited by the Catholic Church, Copernicus’s heliocentric model therefore took considerable time to catch on. Several centuries passed before some of its basic propositions were demonstrated to be true beyond dispute: that Earth moves in relation to the stars was eventually proved conclusively by English astronomer James Bradley in 1729. Proof that Earth spins came with the first demonstration of Foucault’s pendulum in 1851. Copernicus’s theory was a serious blow to old ideas about how the world and wider universe work—many of them dating from the time of Aristotle. As such, it is often cited as ushering in the “Scientific Revolution”—a series of sweeping advances in many areas of science that occurred between the 16th and 18th centuries. ■ THE TEL REVOLU 1550–1750 ESCOPE TION 42 INTRODUCTION Tycho Brahe builds a large observatory on the island of Hveen, from where he makes observations for 20 years. Dutch eyeglass-maker Hans Lippershey applies for a patent for a telescope with three-times magnification. Johannes Kepler describes the elliptical orbits of planets with his three laws of planetary motion. 1608 1619 1576 1600 1610 1639 Italian friar Giordano Bruno is burned at the stake as a heretic after expressing a view that the sun and Earth are not central or special in the universe. Using a telescope with 33-times magnification, Galileo Galilei discovers four moons orbiting Jupiter. English astronomer Jeremiah Horrocks observes the transit of Venus across the face of the sun. T he Dane Tycho Brahe was the last great astronomer of the pre-telescope era. Realizing the importance of trying to record more accurate positions, Tycho built some high-precision instruments for measuring angles. He accumulated an abundance of observations, far superior to those available to Copernicus. Magnifying the image The realm of heavenly bodies still seemed remote and inaccessible to astronomers at the time of Tycho’s death in 1601. However, the invention of the telescope around 1608 suddenly brought the distant universe into much closer proximity. Telescopes have two important advantages over eyes on their own: they have greater light-gathering power, and they can resolve finer detail. The bigger the main lens or mirror, the better the telescope on both counts. Starting in 1610, when Galileo made his first telescopic observations of the planets, the moon’s rugged surface, and the star clouds of the Milky Way, the telescope became the primary tool of astronomy, opening up unimagined vistas. formulated his three laws of planetary motion describing the geometry of how planets move. Kepler had solved the problem of how planets move, but there remained the problem of why they move as they do. The ancient Greeks had imagined Planetary dynamics After Tycho Brahe died, the records of his observations passed to his assistant Johannes Kepler, who was convinced by Copernicus’s arguments that the planets orbit the sun. Armed with Tycho’s data, Kepler applied his mathematical ability and intuition to discover that planetary orbits are elliptical, not circular. By 1619, he had If I have seen further it is by standing on the shoulders of giants. Isaac Newton THE TELESCOPE REVOLUTION 43 Dutch astronomer Christiaan Huygens correctly describes the shape of Saturn’s rings for the first time. Dane Ole Rømer measures the speed of light by observing eclipses of Jupiter’s moon Io. English astronomer Edmond Halley predicts the return of the comet that now bears his name. 1676 1705 1659 1675 1687 1725 Giovanni Domenico Cassini spots a gap in Saturn’s rings and concludes correctly that they are not solid. Isaac Newton publishes Principia, in which he lays out his universal law of gravitation. James Bradley proves that Earth is moving by demonstrating an effect called stellar aberration. that the planets were carried on invisible spheres, but Tycho had demonstrated that comets travel unhindered through interplanetary space, seeming to contradict this idea. Kepler thought that some influence from the sun impelled the planets, but he had no scientific means to describe it. Universal gravitation It fell to Isaac Newton to describe the force responsible for the movement of the planets, with a theory that remained unchallenged until Einstein. Newton concluded that celestial bodies pull on each other and he showed mathematically that Kepler’s laws follow as a natural consequence if the pulling force between two bodies decreases in proportion to the square of the distance between them. Writing about this force, Newton used the word gravitas, Latin for weight, from which we get the word gravity. Improving telescopes Newton not only created a new theoretical framework for astronomers with his mathematical way of describing how objects move, but he also applied his genius to practical matters. Early telescope makers found it impossible to obtain images free from colored distortion with their simple lenses, although it helped to make the telescope enormously long. Giovanni Domenico Cassini, for example, used long “aerial” telescopes without a tube to observe Saturn in the 1670s. In 1668, Newton designed and made the first working version of a reflecting telescope, which did not suffer from the color problem. Reflecting telescopes of Newton’s design were widely used in the 18th century, after English inventor John Hadley developed methods for making large curved mirrors of precisely the right shape from shiny speculum metal. James Bradley, Oxford professor and later Astronomer Royal, was one astronomer who was impressed and acquired a reflector. There were also developments in lens-making. In the early-18th century, English inventor Chester Moore Hall designed a two-part lens that greatly reduced color distortion. The optician John Dollond used this invention to build much-improved refracting telescopes. With high-quality telescopes now widely available, practical astronomy was transformed. ■ 44 I NOTICED A NEW AND UNUSUAL STAR THE TYCHONIC MODEL IN CONTEXT KEY ASTRONOMER Tycho Brahe (1546–1601) BEFORE 1503 The most accurate star positions to date are recorded by Bernhard Walther at Nuremberg. 1543 Copernicus introduces the idea of a sun-centered cosmos, improving the prediction of planetary positions. These, however, are still inaccurate. AFTER 1610 Galileo’s use of the telescope starts a revolution that eventually supersedes naked-eye astronomy. 1620 Johannes Kepler completes his laws of planetary motion. 1670s Major observatories are established in all the capitals of Europe. I n the 16th century, the exact orbits of the planets were a mystery. Danish nobleman Tycho Brahe realized that accurate observations would need to be taken over an extended period of time if this problem were to be solved. The need for better data was underlined by the fact that a conjunction of Jupiter and Saturn in 1562, when Tycho was 17, occurred days away from the time predicted by the best available astronomical tables. Tycho undertook to take measurements along the entirety of the planets’ visible paths. The astronomy of Tycho’s time still followed the teachings that Aristotle had laid down nearly THE TELESCOPE REVOLUTION 45 See also: The geocentric model 20 ■ Consolidating knowledge 24–25 Elliptical orbits 50–55 ■ Hevelius (Directory) 335 ■ The Copernican model 32–39 ■ The appearance of a new star challenges Aristotle’s insistence that the stars never change. Careful measurement shows that the new star is not an atmospheric phenomenon. Further careful measurements of the Great Comet show that it is much farther away than the moon. Careful measurements are the key to accurate models of the solar system. 1,900 years earlier. Aristotle had stated that the stars in the heavenly firmament were fixed, permanent, and unchanging. In 1572, when Tycho was 26, a bright new star was seen in the sky. It was in the constellation of Cassiopeia and stayed visible for 18 months before fading from view. Influenced by the prevailing Aristotelian dogma, most observers assumed that this was an object high in the atmosphere, but below the moon. Tycho’s careful measurements of the new object convinced him that it did not move in relation to nearby stars, so he concluded that it was not an atmospheric phenomenon but a real star. The star was later discovered to be a supernova, and the remnant of this stellar explosion is still visible in the sky as Cassiopeia B. The observation of a new star was an extremely rare event. Only eight naked-eye observations of supernovae have ever been recorded. This sighting showed that the star catalogs in use at the time did not tell the whole story. Greater precision was needed, and Tycho led the way. Precision instruments To accomplish his task, Tycho set about constructing a collection of reliable instruments (quadrants and sextants (p.31), and armillary spheres) that could measure the position of a planet in the sky to an accuracy of about 0.5 arcminute (± 1 ⁄120º). He personally measured planetary positions over a period of around 20 years, and for this purpose Tycho used his immense wealth to design and build fine instruments, such as this armillary sphere, which was used to model the night sky as seen from Earth. in 1576 he oversaw the building of a large complex on the small island of Hven in the Øresund Strait, between what is now Denmark and Sweden. This was one of the first research institutes of its kind. Tycho carefully measured the positions of the stars and recorded them on brass plates on a spherical wooden globe about 5 ft 3 in (1.6 m) in diameter at his observatory on Hven. By 1595, his globe had around 1,000 stars recorded on it. It could spin around a polar axis, and a horizontal ring was used so that stars positioned above the horizon at any given time could be distinguished from those below the horizon. Tycho carried the globe with him on his travels, but it was destroyed in a fire in Copenhagen in 1728. ❯❯ 46 THE TYCHONIC MODEL Further evidence of a changing cosmos came from Tycho’s observation of the Great Comet in 1577. Aristotle had claimed that comets were atmospheric phenomena, and this was still generally believed to be the case in the 16th century. Tycho compared measurements of the comet’s position that he had taken on Hven with those that had been taken at the same time by Bohemian astronomer Thaddaeus Hagecius in Prague. In both instances, the comet was observed in roughly the same place, but the moon was not, suggesting that the comet was much farther away. Tycho’s observations of the way the comet moved across the sky over the months also convinced him that it was traveling through the solar system. This overturned another theory that had been believed for the previous 1,500 years. The great Graeco-Egyptian astronomer Ptolemy had been convinced that the planets were embedded in real, solid, ethereal, transparent crystalline spheres, and that the spinning of these spheres moved the planets across the sky. However, Tycho observed that the comet seemed to move unhindered, and he concluded that the spheres could not exist. He therefore proposed that the planets moved unsupported through space, a daring concept at the time. No parallax Tycho was also very interested in Copernicus’s proposition that the sun, rather than Earth, was at the center of the cosmos. If Copernicus was right, the nearby stars should appear to swing from side to side as Earth traveled annually on its orbit around the sun—a phenomenon known as parallax. Tycho searched hard, but could not find any stellar parallax. There were two possible conclusions. The first was that the stars were too far away, meaning that the change in their position was too small for Tycho to measure with the instruments of the day. (It is now known that the parallax of even the closest star is about 100 times smaller than the typical accuracy of Tycho’s observations.) The second possibility was that Tycho Brahe’s observatory complex on the island of Hven attracted scholars and students from all over Europe between its founding in 1576 and its closure in 1597. THE TELESCOPE REVOLUTION 47 Copernicus was wrong and that Earth did not move. This was Tycho’s conclusion. The Tychonic model In reaching this conclusion, Tycho trusted his own direct experience. He did not feel Earth moving. In fact, nothing that he observed convinced him that the planet was moving. Earth appeared to be stationary and the external universe was the only thing that appeared to be in motion. This led Tycho to discard the Copernican cosmos and introduce his own. In his model of the cosmos, all the planets except Earth orbited the sun, but the sun and the moon orbited a stationary Earth. For many decades after his death in 1601, Tycho’s model was popular among astronomers who were dissatisfied with Ptolemy’s Earthcentric system but who did not wish to anger the Catholic Church by adopting the proscribed Copernican model. However, Tycho’s own insistence on observational accuracy provided the data that would lead to his idea being discredited shortly after his death. His accurate observations helped Johannes Kepler Tycho Brahe The Tychonic model kept Earth at the center of the cosmos as in the Ptolemaic model, but the five known planets were now orbiting the sun. Although he was impressed by the Copernican model, Tycho believed that Earth did not move. Mars Jupiter Venus Mercury Sun Saturn Earth Moon Outer ring of stars to demonstrate that the planets’ orbits are ellipses and to create a model that would displace both the Tychonic and Copernican models. Tycho’s improved measurements would also allow English astronomer Edmond Halley to discover the proper motion of stars (the change in position due to the stars’ motion through space) in 1718. Halley realized that the bright stars Sirius, Arcturus, and Aldebaran had, by Tycho’s time, moved over half a degree away from the positions recorded by Hipparchus 1,850 years earlier. Not only were the stars not fixed in the sky, but the changing positions of the closer stars could also be measured. Stellar parallax was not detected until 1838. ■ Born a nobleman in 1546 in Scania (then Denmark, but now Sweden), Tyge Ottesen Brahe (Tycho is the Latinized version of his first name) became an astronomer after sighting a predicted solar eclipse in 1560. In 1575, King Frederick II gave Tycho the island of Hven in the Øresund Strait, where he built an observatory. Tycho later fell out with Frederick’s successor, Christian IV, over the potential transfer of the island to his children and closed the observatory. In 1599, he was appointed Imperial Mathematician to Emperor Rudolph II in Prague. There, Tycho appointed Johannes Kepler as his assistant. Tycho was famed for his distinctive metal nose, the legacy of a duel he fought as a student. He died in 1601, allegedly of a burst bladder, having refused out of politeness to take a toilet break during a long royal banquet. Key work 1588 Astronomiæ Instauratæ Progymnasmata (Introduction to the New Astronomy) 48 MIRA CETI IS A VARIABLE STAR A NEW KIND OF STAR IN CONTEXT KEY ASTRONOMER David Fabricius (1564–1617) BEFORE 350 bce Greek philosopher Aristotle asserts that the stars are fixed and unchanging. AFTER 1667 Italian astronomer Geminiano Montanari notes that the star Algol varies in brightness. 1784 John Goodricke discovers Delta Cephei, a star that varies in brightness over five days; English astronomer Edward Pigott discovers the variable Eta Aquilae. The star Mira Ceti is observed to change in brightness periodically. Mira Ceti is a variable star. Some stars are variable. 19th century Different kinds of variable star are discovered, including long-period variables, cataclysmic variables, novae, and supernovae stars. 1912 Henrietta Swan Leavitt discovers a relationship between the periods and the brightness of variable stars such as Delta Cephei. B efore the work of German astronomer David Fabricius, it was thought that there were only two types of star. The first were those of constant brightness, such as the 2,500 or so that can be seen with the naked eye above the horizon on a clear dark night. The second type were the “new stars,” such as those seen by Tycho Brahe in 1572 and Johannes Kepler in 1604. The constant stars were synonymous with the fixed, permanent stars in the ancient Greek cosmos—those that mapped out the patterns in the constellations and never changed. The new stars, by contrast, would appear unexpectedly, apparently from nowhere, then fade away, never to be seen again. A third kind of star Aristotle was wrong when he asserted that the stars are fixed and eternal. While observing the star Mira Ceti (also called Omicron Ceti), in the constellation of Cetus the whale, Fabricius realized that there was a third type of star in the sky—one that regularly varied in brightness. He made his discovery in August 1596 as he was plotting the movement of Jupiter across the sky in relation to a nearby star. THE TELESCOPE REVOLUTION 49 See also: The geocentric model 20 ■ The Tychonic model 44–47 Variable stars 86 ■ Measuring the universe 130–37 ■ Elliptical orbits 50–55 ■ An artist’s impression shows material flowing from Mira A (right) onto the hot disk around its companion white dwarf Mira B (left). The hot gas in the system emits X-rays. the sun rotated, providing further proof of the variable nature of heavenly bodies. However, the book they published on the subject in 1611 was mostly overlooked, and the credit for describing the movement of sunspots went to Galileo, who published his results in 1613. Double-star system To Fabricius’s amazement, a few days later, the brightness of this star had increased by a factor of about three. After a few weeks, it disappeared from view altogether, only to reappear some years later. In 1609, Fabricius confirmed that Mira Ceti was a periodic variable star, showing that, contrary to the prevailing Greek philosophy that the cosmos was unchanging, stars were not constant. Working with his son Johannes, Fabricius also used a camera obscura to look at the sun. They studied sunspots, observing that the spots moved across the sun’s disk from east to west at a constant speed. They then disappeared, only to reappear on the other side, having been out of sight for the same time that it had taken them to move across the sun’s disk. This was the first concrete evidence that David Fabricius In short, this new star signifies peace … as well as change in the [Holy Roman] Empire for the better. David Fabricius in a letter to Johannes Kepler David Fabricius was born in 1564 in Esens, Germany, and studied at the University of Helmsted. He later became a Lutheran pastor for a group of churches in Frisia. Together with his son Johannes (1587–1615), he was fascinated by astronomy and an avid user of early telescopes, which his son had brought back with him from a trip to the Netherlands. Fabricius corresponded extensively with Johannes Kepler, with It is now known that Mira Ceti is a double-star system 420 light-years away. Mira A is an unstable red giant star, about 6 billion years old and in a late phase of its evolution. It pulses in and out, changing not only its size but also its temperature. During the cooler part of its cycle, it emits much of its energy as infrared radiation rather than light, so its brightness diminishes dramatically. Mira B is a white dwarf star surrounded by a disk of hot gas that is flowing from Mira A. ■ whom Fabricius pioneered the use of a camera obscura to observe the sun. Little is known of Fabricius’s life beyond his letters and publications. He died in 1617 after he was struck on the head with a shovel by a local goose thief, whom he had denounced from the pulpit. Key work 1611 Narration on Spots Observed on the Sun and their Apparent Rotation with the Sun (with his son Johannes) THE MOST TRUE PATH OF THE PLANET IS AN ELLIPSE ELLIPTICAL ORBITS 52 ELLIPTICAL ORBITS IN CONTEXT KEY ASTRONOMER Johannes Kepler (1571–1630) BEFORE 530–400 bce The works of Plato and Pythagoras convince Kepler that the cosmos can be explained using mathematics. 1543 Copernicus’s suncentered cosmos helps astronomers to visualize a physical solar system but still gives no indication as to the true shape of a planetary orbit. 1600 Tycho Brahe convinces Kepler of the reliability of his planetary observations. AFTER 1687 Isaac Newton realizes that an inverse square law of gravitational force explains why the planets obey Kepler’s laws. 1716 Edmond Halley uses observations of the transit of Venus to convert Kepler’s ratios of planetary distance from the sun into absolute values. Kepler was never satisfied by a moderate agreement between theory and observation. The theory had to fit exactly otherwise some new possibility had to be tried. Fred Hoyle B efore the 17th century, all astronomers were also astrologers. For many, including German astronomer Johannes Kepler, casting horoscopes was the main source of their income and influence. Knowing where the planets had been in the sky was important, but of greater significance for constructing astrological charts was the ability to predict where the planets would be over the next few decades. To make predictions, astrologers assumed that the planets moved on specific paths around a central object. Before Copernicus, in the 16th century, this central body was thought by most to be Earth. Copernicus showed how the mathematics of planetary prediction could be simplified by assuming that the central body was the sun. However, Copernicus assumed that orbits were circular, and to provide any reasonable predictive accuracy, his system still required the planets to Kepler’s most productive years came in Prague under the patronage of Holy Roman Emperor Rudolf II (r.1576–1612). Rudolf was particularly interested in astrology and alchemy. move around a small circle, the center of which moved around a larger circle. These circular velocities were always assumed to be constant. Kepler supported the Copernican system, but the planetary tables it produced could still easily be out by a day or two. The planets, the sun, and the moon always appeared in a certain band of the sky, known as the ecliptic, but actual paths of individual planets around the sun were still a mystery, as was the mechanism that made them move. Finding the paths To improve the predictive tables, Danish astronomer Tycho Brahe spent more than 20 years observing the planets. He next tried to ascertain a path of each planet THE TELESCOPE REVOLUTION 53 See also: The Copernican model 32–39 ■ The Tychonic model 44–47 ■ Galileo’s telescope 56–63 ■ Gravitational theory 66–73 ■ Halley’s comet 74–77 through space that would fit the observational data. This is where the mathematical abilities of Kepler, Brahe’s assistant, came into play. He considered specific models for the solar system and the paths of the individual planets in turn, including circular and ovoid (egg-shaped) orbits. After many calculations, Kepler determined whether or not the model led to predictions of planetary positions that fit into Tycho’s precise observations. If there was not exact agreement, he would discard the idea and start the process again. Abandoning circles In 1608, after 10 years of work, Kepler found the solution, which involved abandoning both circles and constant velocity. The planets made an ellipse—a kind of stretched-out circle for which the amount of stretching is measured by a quantity called an eccentricity (p.54). Ellipses have two foci. The distance of a point on an ellipse from one focus plus the distance from the other focus is always constant. Kepler found that the sun was at one of these two foci. These two facts made up his first law of planetary motion: the motion of the planets is an ellipse with the sun as one of the two foci. Kepler also noticed that the speed of a planet on its ellipse was always changing, and that this change followed a fixed law (his second): a line between the planet and the sun sweeps out equal areas in equal times (p.54). These two laws were published in his 1609 book Astronomia Nova. Kepler had chosen to investigate Mars, which had strong astrological significance, thought to influence human desire and action. Mars took variable retrograde loops— periods during which it would reverse its direction of movement— and large variations in brightness. It also had an orbital period of only 1.88 Earth years, meaning that Mars went around the sun about 11 times in Tycho’s data ❯❯ Neither circular nor ovoid orbits fit Tycho Brahe’s data on Mars. An ellipse fits the data, so the path of Mars is an ellipse. The success of the predictions shows that the orbits of all the planets are ellipses. The Three Laws of Planetary Motion allow for new, improved predictive tables. Johannes Kepler Born prematurely in 1571, Kepler spent his childhood in Leonberg, Swabia, in his grandfather’s inn. Smallpox affected his coordination and vision. A scholarship enabled him to attend the Lutheran University of Tübingen in 1589, where he was taught by Michael Maestlin, Germany’s top astronomer at the time. In 1600, Tycho Brahe invited Kepler to work with him at Castle Benátky near Prague. On Tycho’s death in 1601, Kepler succeeded him as Imperial Mathematician. In 1611, Kepler’s wife died, and he became a teacher in Linz. He remarried and had seven more children, five of whom died young. His work was then disrupted between 1615 and 1621 while he defended his mother from charges of witchcraft. The Catholic Counter-Reformation in 1625 caused him further problems, and prevented his return to Tübingen. Kepler died of a fever in 1630. Key works 1609 Astronomia Nova 1619 Harmonices Mundi 1627 Rudolphine Tables 54 ELLIPTICAL ORBITS When just one body goes around a larger body undisturbed, the paths it can follow are known as Kepler orbits. These are a group of curves called conic sections, which include ellipses, parabolas, and hyperbolas. The shape of the orbit is defined by a property called eccentricity. An eccentricity of 0 is a circle (A). Eccentricity between 0 and 1 is an ellipse (B). Eccentricity equal to 1 produces a parabola (C), and greater than 1 a hyperbola (D). C searched for a divine purpose within his scientific work. Since he saw six planets, he presumed that the number six must have a profound significance. He produced an ordered geometric model of the solar system in which the suncentered spheres that contained each planetary orbit were inscribed and circumscribed by a specific regular “platonic” solid (the five possible solids whose faces and internal angles are all equal). The sphere containing the orbit of Mercury was placed inside an octahedron. The sphere that just touched the points of this regular solid contained the orbit of Venus. This in its turn was placed inside an icosahedron. Then followed the orbit of Earth, a dodecahedron, Mars, a tetrahedron, Jupiter, a cube, and finally Saturn. The system was beautifully ordered, but inaccurate. B set. Kepler was lucky to have chosen Mars, since its orbit has a high eccentricity, or stretch: 0.093 (where 0 is a circle and 1 is a parabola). This is 14 times the eccentricity of Venus. It took him another 12 years to show that the other planets also had elliptical orbits. Studying Brahe’s observations, Kepler was also able to work out the planets’ orbital periods. Earth goes around the sun in one year, Mars in 1.88 Earth years, Jupiter in 11.86, and Saturn in 29.45. Kepler realized that the square of the orbital period was proportional to the cube of the planet’s average distance from the sun. This became his third law and he published it in 1619 in his book Harmonices Mundi, alongside lengthy tracts on astrology, planetary music, and platonic figures. The book had taken him 20 years to produce. A Large body D Today, astronomers might look at a list of planetary orbital sizes and eccentricities and regard them as the result of the planetary formation process coupled with a few billion years of change. To Kepler, however, the numbers needed explanation. A deeply religious man, Kepler F A Planet near aphelion E Focus 1 (the sun) Focus 2 (empty point in space) B Searching for meaning Kepler was fascinated by patterns he found in the orbits of the planets. He noted that, once you accepted the Copernican system for the cosmos, the size of the orbits of the six planets—Mercury, Venus, Earth, Mars, Jupiter, and Saturn—appeared in the ratios 8 : 15 : 20 : 30 : 115 : 195. Planet near perihelion C Elliptical orbit D According to Kepler’s second law, the l