The Astronomy Book

COSMIC EXPANSION THE UNMOVING THE SEARCH FOR IS ACCELERATING STARS GO UNIFORMLY GRAVITY EXPLAINS WESTWARD THE MOTIONS OF THE PLANETS THE EXTRATERRESTRIAL UNIVERSE IS INTELLIGENCE IS A SEARCH FOR OURSELVES EXPANDING IN ALL DIRECTIONS RIPPLES THROUGH SPACETIME THE THE ASTRONOMYI FOUND WAY TO BOOKCOMET, FOR THE THAT IT IS A STARS IS IT HAS BIG IDEAS SIMPLY EXPLAINED OPEN CHANGED ITS PLACE THE MOST TRUE PATH FINALLY WE OF THE PLANET IS SHALL PLACE THE SUN HIMSELF AN ELLIPSE AT THE CENTER OF THE UNIVERSE A SLOW AN EXACT STARS ARE PROCESS OF SOLUTION TO FACTORIES FOR ANNIHILATION RELATIVITY THE CHEMICAL OF MATTER PREDICTS ELEMENTS BLACK HOLES









DK LONDON PRE-PRODUCTION PRODUCER First American Edition, 2017 Jacqueline Street-Elkayam Published in the United States by SENIOR EDITOR DK Publishing, 345 Hudson Street, Victoria Heyworth-Dunne SENIOR PRODUCER Mandy Inness New York, New York 10014 US EDITOR Margaret Parrish DK DELHI Copyright © 2017 Dorling Kindersley Limited SENIOR ART EDITORS JACKET DESIGNER DK, a Division of Penguin Gillian Andrews, Nicola Rodway Suhita Dharamjit Random House LLC MANAGING EDITOR EDITORIAL COORDINATOR 17 18 19 20 21 10 9 8 7 6 5 4 3 2 1 Gareth Jones Priyanka Sharma 001—283974—Sep/2017 SENIOR MANAGING ART EDITOR SENIOR DTP DESIGNER Lee Griffiths Harish Aggarwal All rights reserved. Without limiting the rights under the ART DIRECTOR MANAGING JACKETS EDITOR copyright reserved above, no part of this Karen Self Saloni Singh publication may be reproduced, stored in or introduced into a retrieval system, or ASSOCIATE PUBLISHING produced for DK by transmitted, in any form, or by any means DIRECTOR (electronic, mechanical, photocopying, Liz Wheeler TALL TREE LTD. recording, or otherwise), without the prior written permission of the copyright owner. PUBLISHING DIRECTOR EDITORS Jonathan Metcalf Rob Colson, David John Published in Great Britain by Dorling Kindersley Limited. SENIOR JACKET DESIGNER DESIGN Mark Cavanagh Ben Ruocco A catalog record for this book is available from the Library of Congress. JACKET EDITOR ILLUSTRATIONS Claire Gell James Graham ISBN: 978-1-4654-6418-7 JACKETS DESIGN original styling by DK books are available at special discounts DEVELOPMENT MANAGER when purchased in bulk for sales STUDIO 8 Sophia MTT promotions, premiums, fund-raising, or educational use. For details, contact: DK Publishing Special Markets, 345 Hudson Street, New York, New York 10014 [email protected] Printed in China A WORLD OF IDEAS: SEE ALL THERE IS TO KNOW www.dk.com

CONTRIBUTORS ROBERT DINWIDDIE JACQUELINE MITTON, CONSULTANT EDITOR Robert Dinwiddie is a science writer specializing in educational illustrated books on astronomy, cosmology, Jacqueline Mitton is the author of more than 20 books earth science, and the history of science. He has written on astronomy, including books for children. She has been or contributed to more than 50 books, including the DK a contributor, editor, and consultant for many other books. titles Universe, Space, The Stars, Science, Ocean, Earth, Becoming an astronomer was Jacqueline’s childhood and Violent Earth. He lives in southwest London and ambition. She studied physics at Oxford University and enjoys travel, sailing, and stargazing. then earned her Ph.D. at Cambridge, where she still lives.  PENNY JOHNSON DAVID W. HUGHES Penny Johnson started out as an aeronautical engineer, David W. Hughes is Emeritus Professor of Astronomy working on military aircraft for 10 years, before becoming at the University of Sheffield, UK. He is an international a science teacher, and then a publisher producing science authority on comets, asteroids, and the history of astronomy. courses for schools. Penny has been a full-time educational He has spent more than 40 years explaining the joys of writer for the last 15 years. astronomy and physics to his students, and has published well over 200 research papers, as well as books on the moon, TOM JACKSON the solar system, the universe, and the Star of Bethlehem. He was a co-investigator on the European Space Agency’s Tom Jackson is a science writer based in Bristol, UK. GIOTTO space mission to Halley's Comet and also on ESA’s He has written about 150 books and contributed to many Smart 1 mission to the moon. David has served on a host others, covering all kinds of subjects from fish to religion. of space and astronomy committees, and has been a vice Tom writes for adults and children, mostly about science president of both the Royal Astronomical Society and the and technology, with a focus on the histories of the British Astronomical Association. sciences. He has worked on several astronomy books, including collaborations with Brian May and Patrick Moore.

6 CONTENTS 10 INTRODUCTION 26 The unmoving stars go uniformly westward FROM MYTH Earth’s rotation TO SCIENCE 27 A little cloud in 600 BCE–1550 CE the night sky Mapping the galaxies 20 It is clear that Earth does not move 28 A new calendar for China The geocentric model The solar year 21 Earth revolves around the sun on the circumference 30 We have re-observed of a circle all of the stars in Early heliocentric model Ptolemy’s catalog 22 The equinoxes Improved instruments move over time Shifting stars 32 Finally we shall place the 64 A perfectly circular spot 23 The moon’s brightness sun himself at the center centered on the sun is produced by the of the universe The transit of Venus radiance of the sun The Copernican model 65 New moons around Saturn Theories about the moon Observing Saturn’s rings 24 All matters useful to the THE TELESCOPE 66 Gravity explains the theory of heavenly things REVOLUTION motions of the planets Consolidating knowledge Gravitational theory 1550–1750 74 I dare venture to foretell that the comet will return 44 I noticed a new again in the year 1758 and unusual star Halley’s comet The Tychonic model 48 Mira Ceti is a variable star A new kind of star 50 The most true path of 78 These discoveries are the the planet is an ellipse most brilliant and useful Elliptical orbits of the century 56 Our own eyes show us Stellar aberration four stars traveling 79 A catalog of the around Jupiter southern sky Galileo’s telescope Mapping southern stars

7 URANUS 100 A survey of the whole TO NEPTUNE surface of the heavens The southern hemisphere 1750–1850 102 An apparent movement of the stars Stellar parallax 84 I found that it is a comet, for it has changed its place 103 Sunspots appear in cycles Observing Uranus The surface of the sun 86 The brightness of the 104 A spiral form of 118 Photographing the stars star was altered arrangement was detected Astrophotography Variable stars Examining nebulae 120 A precise measurement 87 Our Milky Way is the 106 The planet whose position of the stars dwelling, the nebulae you have pointed out The star catalog are the cities actually exists Messier objects The discovery of Neptune 88 On the construction of the heavens THE RISE OF 122 Classifying the stars The Milky Way ASTROPHYSICS according to their 90 Rocks fall from space spectra reveals their Asteroids and meteorites 1850–1915 age and size The characteristics of stars 92 The mechanism 112 Sodium is to be found 128 There are two kinds of the heavens in the solar atmosphere of red star Gravitational disturbances The sun’s spectrum Analyzing absorption lines 94 I surmise that it could 113 Stars can be grouped 129 Sunspots are magnetic be something better by their spectra The properties of sunspots than a comet Analyzing starlight 130 The key to a distance The discovery of Ceres 114 Enormous masses scale of the universe of luminous gas Measuring the universe Properties of nebulae 138 Stars are giants 116 The sun’s yellow or dwarfs prominence differs from Refining star classification any terrestrial flame 140 Penetrating radiation The sun’s emissions is coming from space 117 Mars is traversed by Cosmic rays a dense network 141 A white hot star of channels that is too faint Mapping Mars’s surface Discovering white dwarfs

8 178 White dwarfs have 196 It took less than an hour a maximum mass to make the atomic nuclei The life cycles of stars The primeval atom ATOMS, STARS, 179 The radio universe 198 Stars are factories for AND GALAXIES Radio astronomy the chemical elements 180 An explosive transition Nucleosynthesis 1915–1950 to a neutron star 200 Sites of star formation Supernovae Dense molecular clouds 146 Time and space and gravitation have no 182 The source of energy in NEW WINDOWS separate existence stars is nuclear fusion ON THE UNIVERSE from matter Energy generation The theory of relativity 184 A reservoir of comets 1950–1975  exists beyond the planets The Kuiper belt 185 Some galaxies have active 206 A vast cloud surrounds regions at their centers the solar system Nuclei and radiation The Oort cloud 154 An exact solution 186 The match of lunar and 207 Comets are dirty snowballs to relativity predicts Earth material is too perfect The composition of comets black holes The origin of the moon Curves in spacetime 208 The way to the stars is open 156 The spiral nebulae 188 Important new discoveries The launch of Sputnik are stellar systems will be made with flying Spiral galaxies telescopes 210 The search for interstellar 162 Stars are dominated by Space telescopes communications hydrogen and helium Radio telescopes Stellar composition 164 Our galaxy is rotating 212 Meteorites can vaporize The shape of the Milky Way on impact 166 A slow process of Investigating craters annihilation of matter Nuclear fusion within stars 213 The sun rings like a bell The sun’s vibrations 168 A day without yesterday The birth of the universe 214 The data can best be explained as X-rays from sources outside the solar system Cosmic radiation 172 The universe is expanding 218 Brighter than a galaxy, in all directions but it looks like a star Beyond the Milky Way Quasars and black holes

9 222 An ocean of whispers 298 Cosmic expansion left over from our is accelerating eruptive creations Dark energy Searching for the Big Bang 304 Peering back over 13.5 billion years 228 The search for Studying distant stars extraterrestrial 306 Our mission is to intelligence is a search land on a comet for ourselves Understanding comets Life on other planets 312 The violent birth of the solar system 236 It has to be some 268 Most of the universe The Nice model new kind of star is missing 314 A close-up view of an Quasars and pulsars Dark matter oddball of the solar system 240 Galaxies change over time 272 Negative pressures Studying Pluto Understanding stellar produce repulsive gravity 318 A laboratory on Mars evolution Cosmic inflation Exploring Mars 242 We choose to go 274 Galaxies appear to 326 The biggest eye on the sky to the moon be on the surfaces of Looking farther into space The Space Race bubblelike structures 328 Ripples through spacetime 250 The planets formed from Redshift surveys Gravitational waves a disk of gas and dust The nebular hypothesis 332 DIRECTORY 340 GLOSSARY 252 Solar neutrinos can 276 Stars form from 344 INDEX only be seen with the inside out 352 ACKNOWLEDGMENTS a very large detector Inside giant molecular clouds The Homestake experiment 280 Wrinkles in time 254 A star that we couldn’t see Observing the CMB Discovering black holes 286 The Kuiper belt is real 255 Black holes emit radiation Exploring beyond Neptune Hawking radiation 288 Most stars are orbited by planets THE TRIUMPH Exoplanets OF TECHNOLOGY 296 The most ambitious map of the universe ever 1975–PRESENT  A digital view of the skies 260 A grand tour of the 297 Our galaxy harbors giant planets a massive central Exploring the solar system black hole The heart of the Milky Way

INTRODU

CTION

12 INTRODUCTION T hroughout history, the aim through telescopes and detectors predictability of repetitive cycles of astronomy has been to of various kinds, one of our biggest had vital practical applications make sense of the universe. discoveries is what we do not in marking the passage of time. In the ancient world, astronomers understand at all: more than 95 puzzled over how and why the percent of the substance of the Archaeology provides abundant planets moved against the backdrop universe is in the form of “dark evidence that, even in prehistoric of the starry sky, the meaning of matter” and “dark energy.” times, astronomical phenomena the mysterious apparition of comets, were a cultural resource for societies and the seeming remoteness of The origins of astronomy around the world. Where there is the sun and stars. Today, the In many of the world’s most no written record, we can only emphasis has changed to new populated areas today, many of us speculate as to the knowledge questions concerning how the are barely aware of the night sky. and beliefs early societies held. universe began, what it is made We cannot see it because the blaze The oldest astronomical records of, and how it has changed. The of artificial lighting overwhelms to survive in written form come way in which its constituents, such the faint and delicate light of the from Mesopotamia, the region as galaxies, stars, and planets, fit stars. Light pollution on this scale that was between and around the into the larger picture and whether has exploded since the mid-20th valleys of the Tigris and Euphrates there is life beyond Earth are some century. In past times, the starry rivers, in present-day Iraq and of the questions humans still patterns of the sky, the phases of neighboring countries. Clay tablets endeavor to answer. the moon, and the meanderings inscribed with astronomical of the planets were a familiar information date back to about Understanding astronomy part of daily experience and a The baffling cosmic questions of perpetual source of wonder. Philosophy is written in the day have always inspired big this grand book, the universe, ideas to answer them. They have Few people fail to be moved the stimulated curious and creative first time they experience a clear which stands continually minds for millennia, resulting in sky on a truly dark night, in which open to our gaze. pioneering advances in philosophy, the magnificent sweep of the Milky Galileo Galilei mathematics, technology, and Way arches across the sky. Our observation techniques. Just ancestors were driven by a mixture when one fresh breakthrough of curiosity and awe in their search seems to explain gravitational for order and meaning in the great waves, another discovery throws vault of the sky above their heads. up a new conundrum. For all we The mystery and grandeur of the have learned about the universe’s heavens were explained by the familiar constituents, as seen spiritual and divine. At the same time, however, the orderliness and

INTRODUCTION 13 1600 bce. Some of the constellations century onward, however, moon’s phases. The combined (groupings of stars) we know today astronomy as a scientific activity motion in space of Earth, the sun, have come from Mesopotamian diverged from traditional astrology. and the moon also determines mythology going back even earlier, Today, astronomers reject astrology, the timing and magnitudes of the to before 2000 bce. because it is unfounded in scientific oceanic tides, which are of crucial evidence, but they have good reason importance to coastal communities Astronomy and astrology to be grateful to the astrologers of and seafarers. The Babylonians of Mesopotamia the past for leaving an invaluable were greatly concerned with historical record. Astronomy played an equally divination. To them, planets were important role in navigation, the manifestations of the gods. The Time and tide stars acting as a framework of mysterious comings and goings of The systematic astronomical reference points visible from the planets and unusual happenings observations once used for astrology anywhere at sea (cloud permitting). in the sky were omens from the started to become increasingly In 1675, British King Charles II gods. The Babylonians interpreted important as a means of both commissioned an observatory, the them by relating them to past timekeeping and navigation. Royal Observatory at Greenwich, experience. To their way of Countries had highly practical near London. The instruction to thinking, detailed records over long reasons—civil, as well as military — its director, the first Astronomer periods were essential to establish to establish national observatories, Royal, John Flamsteed, was to connections between the celestial as the world industrialized and apply himself diligently to making and the terrestrial, and the practice international trade grew. For many the observations needed “for the of interpreting horoscopes began in centuries, only astronomers had perfecting of the art of navigation.” ❯❯ the 6th century bce. Charts showed the skills and equipment to preside where the sun, moon, and planets over the world’s timekeeping. You have to have the appeared against the backdrop of This remained the case until the imagination to recognize a the zodiac at some critical time, development of atomic clocks in discovery when you make one. such as a person’s birth. the mid-20th century. Clyde Tombaugh For some 2,000 years, there was Human society regulates itself little distinction between astrology, around three natural astronomical which used the relative positions of clocks: Earth’s rotation, detectable celestial bodies to track the course by the apparent daily march of the of human lives and history, and the stars around the celestial sphere to astronomy on which it relied. The give us the day; the time our planet needs of astrology, rather than pure takes to make a circuit around the curiosity, justified observation of sun, otherwise known as a year; the heavens. From the mid-17th and the monthly cycle of the

14 INTRODUCTION Astronomy was largely discarded as surface inhabitants, the protection Arthur Eddington took advantage the foundation of navigation in the afforded by the atmosphere and of a total solar eclipse to observe 1970s, and replaced by artificial Earth’s magnetic field may make how the paths of starlight deviated satellites, which created a global us feel secure, but in reality we from a straight line when the light positioning system. are at the mercy of a harsh space passed through the gravitational environment, blasted by energetic field of the sun, just as relativity The purpose of astronomy particles and radiation, and at risk predicted. Then, in 1979, the first The practical reasons for pursuing of colliding with rocks. The more example of a gravitational lens astronomy and space science may we know about that environment, was identified, when the image have changed, but they still exist. the better equipped we are to deal of a quasar was seen to be double For example, astronomy is needed with the potential threats it presents. due to the presence of a galaxy to assess the risks our planet faces along the line of sight, again as from space. Nothing illustrated A universal laboratory relativity had predicted. The most Earth’s apparent fragility more There is another very important recent triumphant justification of powerfully than the iconic images, reason for doing astronomy. The Einstein’s theory came in 2015 with such as “Earthrise” and “Blue universe is a vast laboratory in the first detection of gravitational Marble,” taken from space by Apollo which to explore the fundamental waves, which are ripples in the astronauts in the 1960s. These nature of matter, and of time and fabric of spacetime, generated by images reminded us that Earth is space. The unimaginably grand the merging of two black holes. a small planet adrift in space. As scales of time, size, and distance, and the extremes of density, When to observe What a wonderful and pressure, and temperature go far One of the main methods scientists amazing scheme have we beyond the conditions we can use to test ideas and search for new readily simulate on Earth. It would phenomena is to design experiments here of the magnificent be impossible to test the predicted and carry them out in controlled vastness of the universe. properties of a black hole or watch laboratory conditions. For the most Christiaan Huygens what happens when a star explodes part, however, with the exception in an Earth-bound experiment. of the solar system—which is close enough for experiments to be Astronomical observations carried out by robots—astronomers have spectacularly confirmed the have to settle for a role as passive predictions of Albert Einstein’s collectors of the radiation and general theory of relativity. As elementary particles that happen Einstein himself pointed out, his to arrive on Earth. The key skill theory explained apparent anomalies astronomers have mastered is that in Mercury’s orbit, where Newton’s of making informed choices about theory of gravity failed. In 1919,

INTRODUCTION 15 what, how, and when to observe. which is the branch of astronomy science.” This is the combination For instance, it was through the concerned with the movement of technology and practical gathering and analysis of telescopic of bodies, especially in the solar applications that blossomed with data that the rotation of galaxies system. The term “planetary the establishment of the “space could be measured. This, in turn, science” encompasses every age” in the mid-20th century. quite unexpectedly led to the aspect of the study of planets, discovery that invisible “dark matter” including Earth. Solar physics Collaboration of science must exist. In this way, astronomy’s is another important discipline. Every space telescope and mission contribution to fundamental to explore the worlds of the solar physics has been immense. Technology and innovation system makes use of space With the spawning of so many science, so sometimes it is hard Astronomy’s scope branches of enquiry connected to separate it from astronomy. Up to the 19th century, astronomers with everything in space, including This is just one example of how could only chart the positions and Earth as a planet, the meaning of developments in other fields, movements of heavenly bodies. the word “astronomy” has evolved especially technology and This led the French philosopher once again to become the collective mathematics, have been crucial Auguste Comte to state in 1842 name encompassing the whole of in propelling astronomy forward. that it would never be possible the study of the universe. However, Astronomers were quick to take to determine the compositions one closely related subject does advantage of the invention of of planets or stars. Then, some not come under astronomy: “space telescopes, photography, novel two decades later, new techniques ways of detecting radiation, for the spectrum analysis of light If astronomy teaches and digital computing and data began to open up the possibility anything, it teaches that handling, to mention but a few of investigating the physical man is but a detail in the technological advances. Astronomy nature of stars and planets. A new evolution of the universe. is the epitome of “big science”—a word was invented to distinguish large-scale scientific collaboration. this new field from traditional Percival Lowell astronomy: astrophysics. Understanding our place in the universe goes to the heart of Astrophysics became just one our understanding of ourselves: of many specialisms in the study the formation of Earth as a life- of the universe in the 20th century. supporting planet; the creation of Astrochemistry and astrobiology the chemical building blocks from are more recent branches. They join which the solar system formed; and cosmology—the study of the origin the origin of the universe as a and evolution of the universe as a whole. Astronomy is the means by whole—and celestial mechanics, which we tackle these big ideas. ■

FTROOSMCIME 600 BCE–1550 CE

NYCTHE

18 INTRODUCTION Anaximander of In his On the Heavens, Aristotle In Alexandria, Eratosthenes Miletus produces one outlines an Earth-centered measures the circumference of the earliest attempts model of the universe. Many of his ideas will dominate of Earth and estimates the at a scientific thinking for 2,000 years. distance to the sun. explanation of the universe. C.550 BCE 350 BCE C.200 BCE C.530 BCE C.220 BCE C.150 CE Pythagoras establishes a Aristarchus of Ptolemy writes the school in Croton, where he Samos proposes a Almagest, which sets sun-centered model out an Earth-centered promotes the idea of a of the universe, but his model of the universe cosmos in which bodies idea does not gain move in perfect circles. wide acceptance. that becomes widely accepted. T he traditions on which put forward two centuries later inconsistencies, his philosophy modern astronomy is by Aristotle (384–322 bce) were to was adopted as the most acceptable built began in ancient underpin the whole of astronomy overall framework of ideas for science Greece and its colonies. In nearby until the 16th century. and was later incorporated into Mesopotamia, although the Christian theology. Babylonians had become highly Aristotle’s beliefs proficient at celestial forecasting Aristotle was a pupil of Plato, Geometrical order using complicated arithmetic, and both were influenced by the Mathematically, much of Greek their astronomy was rooted in thinking of Pythagoras and his astronomy was based on geometry, mythology, and their preoccupation followers, who believed that the particularly motion in circles, was with divining the future. To natural world was a “cosmos” as which were considered to be the them, the heavens were the realm opposed to “chaos.” This meant most perfect shapes. Elaborate of the gods, outside the scope of that it is ordered in a rational way geometrical schemes were created rational investigation by humans. rather than incomprehensible. for predicting the positions of the planets, in which circular By contrast, the Greeks tried Aristotle stated that the motions were combined. In 150 ce, to explain what they observed heavenly realms are unchanging the Graeco–Egyptian astronomer happening in the sky. Thales and perfect, unlike the world of Ptolemy, working in Alexandria, put of Miletus (c.624–c.546 bce) is human experience, but he promoted together the ultimate compendium regarded as the first in a line of ideas that were consistent with of Greek astronomy. However, by philosophers who thought that “common sense.” Among other 500 ce, the Greek approach to immutable principles in nature things, this meant Earth was astronomy had lost momentum. could be revealed by logical stationary and at the center of the In effect, after Ptolemy, there were reasoning. The theoretical ideas universe. Although it contained

In the Aryabhatiya, Italian scholar Gerard of FROM MYTH TO SCIENCE 19 Indian astronomer Cremona makes Arabic texts, Aryabhata suggests including Ptolemy’s Almagest, Mongol ruler Ulugh Beg that the stars move corrects many of the across the sky because accessible in Europe by postions of stars Earth is rotating. translating them into Latin. found in the Almagest. 499 CE C.1180 1437 1543 1025 1279 Arab scholar Ibn Chinese astronomer Nicolaus Copernicus’s al-Haytham produces Guo Shoujing produces an book De revolutionibus a work that criticizes accurate measurement of the orbium coelestium is the Ptolomaic model published, outlining a length of the solar year. sun-centered cosmos. of the universe for its complexity. no significant new ideas in astronomy progress. Chinese, Arab, and The invention of the printing in this tradition for nearly 1,400 Japanese astronomers recorded press in the mid-15th century years. Independently, great cultures the 1054 supernova in the widened access to books. Nicolaus in China, India, and the Islamic constellation Taurus, which Copernicus, who was born in 1473, world developed their own traditions made the famous Crab nebula. collected books throughout his life, through the centuries when Although it was much brighter including the works of Ptolemy. To astronomy in Europe made little than Venus, there is no record of its Copernicus, Ptolemy’s geometrical appearance being noted in Europe. constructions failed to do what It is the duty of an astronomer the original Greek philosophers to compose the history of the The spread of learning saw as their objective: describe Ultimately, Greek science returned nature by finding simple underlying celestial motions through to Europe via a roundabout route. principles. Copernicus intuitively careful and expert study. From 740 ce, Baghdad became a understood that a sun-centered Nicolaus Copernicus great center of learning for the method could produce a much Islamic world. Ptolemy’s great simpler system, but in the end compendium was translated into his reluctance to abandon circular Arabic, and became known as the motion meant that real success Almagest, from its Arabic title. eluded him. Nevertheless, his In the 12th century, many texts in message that physical reality Arabic were translated into Latin, so should underpin astronomical the legacy of the Greek philosophers, thinking arrived at a pivotal as well as the writings of the Islamic moment to set the scene for the scholars, reached Western Europe. telescopic revolution. ■

20  IDTTHOIEASTSCENLAOERTATRMHOVE THE GEOCENTRIC MODEL IN CONTEXT O ne of the most influential stayed the same, and spun daily of all Western philosophers, around Earth. The moon, sun, and KEY ASTRONOMER Aristotle, from Macedonia planets, too, appeared to move in Aristotle (384–322 bce) in northern Greece, believed that unchanging orbits around Earth. the universe was governed by Their motion, he believed, was BEFORE physical laws. He attempted to circular and their speed constant. 465 bce Greek philosopher explain these through deduction, Empedocles thinks that there philosophy, and logic. His observations of the shadow are four elements: earth, water, cast by Earth on the moon’s air, and fire. Aristotle contends Aristotle observed that the surface during a lunar eclipse that the stars and planets are positions of the stars appeared to convinced him that Earth was made of a fifth element, aether. be fixed in relation to each other, a sphere. His conclusion was and that their brightness never that a spherical Earth remained 387 bce Plato’s student changed. The constellations always stationary in space, never spinning Eudoxus suggests that the or changing its position, while the planets are set in transparent Earth casts a circular shadow cosmos spun eternally around it. rotating spheres. on the moon during a lunar eclipse. Earth was an unmoving object This convinced Aristotle that at the center of the universe. AFTER Earth was a sphere. 355 bce Greek thinker Aristotle believed that Earth’s Heraclides claims that the sky Earth’s shadow atmosphere, too, was stationary. is stationary and Earth spins. At the top of the atmosphere, moon friction occurred between the 12th century Italian Catholic sun’s atmospheric gases and the rotating priest Thomas Aquinas begins rays sky above. Episodic emanations teaching Aristotle’s theories. of gases from volcanoes rose to Earth the top of the atmosphere. Ignited 1577 Tycho Brahe shows that by friction, these gases produced the Great Comet is farther comets, and, if ignited quickly, from Earth than the moon. they produced shooting stars. His reasoning remained widely 1687 Isaac Newton explains accepted until the 16th century. ■ force in his Philosophiae Naturalis Principia Mathematica. 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 EOTAAHRFREOATUCCHNIIRDRRCCETUVLHMOEELFSVEUERSNENOCNE   EARLY HELIOCENTRIC MODEL IN CONTEXT A n astronomer and Aristarchus was the mathematician from the real originator of the KEY ASTRONOMER Greek island of Samos, Copernican hypothesis. Aristarchus (310–230 bce) Aristarchus is the first person Sir Thomas Heath known to have proposed that the BEFORE sun, not Earth, is at the center Mathematician and classical scholar 430 bce Philolalus of Craton of the universe, and that Earth proposes that there is a revolves around the sun. the case until the 15th century, when huge fire at the center of the the heliocentric viewpoint was universe, around which the Aristarchus’s thoughts on this revived by Nicolaus Copernicus. sun, moon, Earth, five planets, matter are mentioned in a book and stars revolve. by another Greek mathematician, Aristarchus also believed that Archimedes, who states in The the stars were much farther away 350 bce Aristotle states that Sand Reckoner that Aristarchus than had previously been imagined. Earth is at the center of the had formulated a hypothesis that He made estimates of the distances universe and everything else “the fixed stars and sun remain to the sun and moon, and their moves around it. unmoved” and “Earth revolves sizes relative to Earth. His estimates about the sun.” regarding the moon were reasonably AFTER accurate, but he underestimated 150 ce Ptolemy publishes Unfashionable idea the distance to the sun, mainly his Almagest, describing an Aristarchus persuaded at least because of an inaccuracy in one Earth-centered (geocentric) one later astronomer—Seleucus of of his measurements. ■ model of the universe. Seleucia, who lived in the second century bce—of the truth of his 1453 Nicolaus Copernicus heliocentric (sun-centered) view proposes a heliocentric of the universe, but otherwise it (sun-centered) universe. seems his ideas did not gain wide acceptance. By the time of Ptolemy, 1838 German astronomer in about 150 ce, the prevailing view Friedrich Bessel is the was still a geocentric (Earth- first to obtain an accurate centered) one, and this remained measurement of the distance to a star, using a See also: The geocentric model 20 ■ Consolidating knowledge 24–25 ■ method known as parallax. The Copernican model 32–39 ■ Stellar parallax 102

22   MTHOEVEEOOUVIENROXTEIMSE SHIFTING STARS IN CONTEXT I n about 130 bce, the Greek exactly defined points and curves astronomer and mathematician on the surface of this sphere as KEY ASTRONOMER Hipparchus of Nicaea noticed references for describing the Hipparchus (190–120 bce) that a star named Spica had moved positions of stars and other celestial 2o east of a point on the celestial objects. The sphere has north and BEFORE sphere, called the fall equinox south poles, and a celestial equator, 280 bce Greek astronomer point, compared to its position which is a circle lying above Earth’s Timocharis records that the recorded 150 years earlier. Further equator. The ecliptic is another star Spica is 8° west of the fall research showed him that the important circle on the sphere, equinox. positions of all stars had shifted. which traces the apparent path This shift became known as of the sun against the background AFTER “precession of the equinoxes.” of stars over the course of the year. 4th century ce Chinese The ecliptic intersects the celestial astronomer Yu Xi notices The celestial sphere is an equator at two points: the spring and measures precession. imaginary sphere surrounding and fall equinox points. These mark Earth, in which stars are found at the positions on the celestial sphere 1543 Nicolaus Copernicus specific points. Astronomers use that the sun reaches on the explains precession as a equinoxes in March and September. motion of Earth’s axis. Industrious, and a The precession of the equinoxes great lover of the truth. refers to the gradual drift of these 1687 Isaac Newton two points relative to star positions. demonstrates precession to Ptolemy be a consequence of gravity. Hipparchus put this precession describing Hipparchus down to a “wobble” in the movement 1718 Edmond Halley discovers of the celestial sphere, which he that, except for the relative believed to be real and to rotate motion between stars and around Earth. It is now known reference points on the that the wobble is actually in celestial sphere, stars have a the orientation of Earth’s spin gradual motion relative to each axis, caused by the gravitational other. This is because they are influence of the sun and the moon. ■ moving in different directions and at different speeds. See also: Gravitational theory 66–73 ■ Halley’s comet 74–77

FROM MYTH TO SCIENCE 23 RBPTRHRAODEIGDIMAHUNOTCCNOEEENDS’OSBSFYITSTHHEESUN  THEORIES ABOUT THE MOON IN CONTEXT T he Chief Astrologer at the The sun is like fire court of Chinese emperor and the moon like water. KEY ASTRONOMER An-ti, Zhang Heng was The fire gives out light Zhang Heng (78–139 ce) a skilled mathematician and a and the water reflects it. careful observer. He cataloged BEFORE 2,500 “brightly shining” stars and Zhang Heng 140 bce Hipparchus discovers estimated that there were a further how to predict eclipses. 11,520 “very small” ones. sun is fully lit, and the side that is away from it is dark.” He also 1st century bce Jing Fang Also a distinguished poet, described a lunar eclipse, during advances the “radiating Zhang expressed his astronomical which the sun’s light cannot reach influence” theory, stating that ideas through simile and metaphor. the moon because Earth is in the the light of the moon is the In his treatise Ling Xian, or The way. He recognized that the planets reflected light of the sun. Spiritual Constitution of the were similarly subject to eclipses. Universe, he placed Earth at the AFTER center of the cosmos, stating that Zhang’s work was developed 150 ce Ptolemy produces “the sky is like a hen’s egg, and is further in the 11th century by tables for calculating the as round as a crossbow pellet, and another Chinese astronomer, positions of celestial bodies. Earth is the yolk of the egg, lying Shen Kuo. Shen demonstrated alone at the center.” that the waxing and waning of 11th century Shen Kuo’s the moon proved that the moon Dream Pool Essays explains Shape but no light and sun were spherical. ■ that heavenly bodies are round Zhang concluded that the moon like balls rather than flat. had no light of its own, but rather reflected the sun “like water.” In 1543 Nicolaus Copernicus’s this, he embraced the theories of On the Revolutions of the his compatriot Jing Fang who, a Celestial Spheres describes century earlier, had declared that a heliocentric system. “the moon and the planets are Yin; they have shape but no light.” Zhang 1609 Johannes Kepler saw that “the side that faces the explains the movements of the planets as free-floating See also: The Copernican model 32–39 ■ Elliptical orbits 50–55 bodies, describing ellipses.

24 HTAOLELATVMHEEANTLTTYHEETROHSRINYUGSOSEFFUL CONSOLIDATING KNOWLEDGE IN CONTEXT I n his greatest known work, the The constellations devised by Almagest, the Graeco-Egyptian Ptolemy are used in this 17th-century KEY ASTRONOMER astronomer Ptolemy produced star map. The number of stars per Ptolemy (85–165 ce) a summary of all the astronomical constellation ranges from two (Canis knowledge of his time. Rather than Minor) to 42 (Aquarius). BEFORE producing radical new ideas of his 12th century bce The own, Ptolemy mostly consolidated Ptolemy’s model of the solar system Babylonians organize the and built upon previous knowledge, had a stationary Earth at its center, stars into constellations. particularly the works of the Greek with the heavens spinning daily astronomer Hipparchus, whose around it. His model required 350 bce Aristotle asserts star catalog formed the basis complicated additions to make it that the stars are fixed in of most of the calculations in the match the data and allow it to be place and Earth is stationary. Almagest. Ptolemy also detailed used to calculate the positions of the the mathematics required to planets; nonetheless, it was largely 135 bce Hipparchus produces calculate the future positions of unchallenged until Copernicus a catalog of over 850 star the planets. His system would be placed the sun at the center of positions and brightnesses. used by generations of astrologers. the cosmos in the 16th century. 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.

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 stone, and its shadow gave a Claudius Ptolemy of 1,022 star positions and listed precise indication of the height 48 constellations in the part of of the sun at noon. Ptolemy took Ptolemy was a polymath and the celestial sphere known to the daily measurements to obtain produced works on a wide Greeks—everything that could be accurate estimates of the time of range of topics, including seen from a northern latitude of the solstices and equinoxes, which astronomy, astrology, about 32o. Ptolemy’s constellations confirmed previous measurements geography, music, optics, are still used today. Many of their showing that the seasons were and mathematics. names can be traced even further different lengths. He believed that back to the ancient Babylonians, the orbit of the sun around Earth Very little is known about including Gemini (twins), Cancer was circular, but his calculations him, but he probably spent (crab), Leo (lion), Scorpio (scorpion), led him to the conclusion that all his life in Alexandria, the and Taurus (bull). The Babylonian Earth could not be at the exact Egyptian seaport with a constellations are named on a center of that orbit. reputation for scholarship cuneiform tablet called the Mul and a great library, where he Apin, which dates back to the Ptolemy the astrologist was taught by the renowned 7th century bce, however, they are Like most thinkers of his day, mathematician Theon of thought to have been compiled Ptolemy believed that the Smyrna. Many of his prolific about 300 years earlier. movements of the heavenly bodies writings have survived. They profoundly affected events on were translated into Arabic Early quadrant Earth. His book on astrology, and Latin, disseminating his To improve his measurements, Tetrabiblos, rivaled the Almagest ideas across the medieval Ptolemy built a plinth. One of the in popularity over the following world. Geography listed the earliest examples of a quadrant, 1,000 years. Ptolemy had not only locations of most of the places his plinth was a huge rectangular provided a means to calculate in the known world, and block of stone, one of whose vertical planetary positions, but he had was carried by Christopher sides accurately aligned in the also produced a comprehensive Columbus on his voyages of north–south plane. A horizontal interpretation of the ways those discovery in the 15th century. bar protruded from the top of the movements affected humans. ■ The Almagest remained in continual use in academia Sun Sun’s height until about 1643, a century after Ptolemy’s model of the Horizontal bar Stone plinth universe had been challenged by Copernicus. 0o Key works Sun’s shadow c.150 ce Geography c.150 ce Almagest Ptolemy describes the 90o c.150 ce Tetrabiblos design of his stone plinth in the Almagest. It was a quadrant, an instrument that measures angles between 0° and 90°.

26    STWHTEAESRUTSNWGMAOORVUDINNIGFORMLY EARTH’S ROTATION IN CONTEXT F rom the 4th century bce He was the father of the to the 16th century ce, the Indian cyclic astronomy … KEY ASTRONOMER prevailing view throughout Aryabhata (476–550 ce) the Western world was that Earth that determines more is stationary and located at the accurately the true positions BEFORE center of the universe. Suggestions and distances of the planets. 350 bce Heraclides Ponticus, that Earth might be rotating were a pupil of Plato, proposes that dismissed on the grounds that Helaine Selin Earth rotates once a day on this would cause objects on Earth’s its axis. The idea does not surface to fly off into space. In India, Historian of astronomy become widespread because however, an astronomer named it contradicts Aristotle, who is Aryabhata was convinced that considered more authoritative. the movement of stars across the night sky was due not to the stars 4th century bce Aristotle revolving in a distant sphere around states that Earth is stationary Earth, but to Earth itself rotating. in space. An illusory movement Essentially a compendium of the AFTER According to Aryabhata, the stars fundamentals of astronomy and 950 ce Iranian astronomer were stationary and their apparent relevant mathematics, it greatly al-Sijzi supports the idea movement toward the west was an influenced Arabic astronomy. that Earth rotates. illusion. His notion of a spinning Earth was not widely accepted Among other achievements, 1543 Nicolaus Copernicus until the mid-17th century—a Aryabhata calculated the length states that Earth rotates century after Nicolaus Copernicus of the sidereal day (the time it as part of his heliocentric had endorsed the idea. takes Earth to rotate once in (sun-centered) model of relation to the stars) to a high the universe. Aryabhata’s achievements were degree of accuracy, and devised considerable. His book Aryabhatiya original and accurate ways of 1851 The first demonstration was the most important work of compiling astronomical tables. ■ of Léon Foucault’s pendulum astronomy in the 6th century. in Paris provides the final scientific proof that Earth See also: The geocentric model 20 ■ The Copernican model 32–39 ■ is rotating. The Tychonic model 44–47 ■ Elliptical orbits 50–55

FROM MYTH TO SCIENCE 27 IANLTIHTETLNEIGCHLTOUSDKY  MAPPING THE GALAXIES IN CONTEXT A bd al-Rahman al-Sufi, once also consulted Arab merchants better known in the West who traveled to the south and KEY ASTRONOMER as Azophi, was a Persian east, and who saw more of the sky. Abd al-Rahman al-Sufi astronomer who made the first His work centered on translating (903–986 ce) record of what are now understood Ptolemy’s Almagest into Arabic. to be galaxies. To al-Sufi, these In the process, al-Sufi tried to BEFORE fuzzy, nebulous objects looked like merge the Hellenistic constellations 400 bce Democritus suggests clouds in the night’s sky. (which dominate star maps today) that the Milky Way is made with their Arab counterparts, most of a dense mass of stars. Al-Sufi made most of his of which were totally different. observations in Isfahan and Shiraz, 150 ce Ptolemy records several in what is now central Iran, but he The fruit of this labor was nebulae (or cloudy objects) Kitab suwar al-kawakib, or the Book in the Almagest. The Large Magellanic Cloud, of Fixed Stars, published in 964 ce. seen here above the ESO’s Paranal The work contained an illustration AFTER observatory in Chile, can be easily of “a little cloud,” which is now 1610 Galileo sees stars observed with the naked eye from know to be the Andromeda Galaxy. in the Milky Way using the southern hemisphere. This object was probably known a telescope, confirming to earlier Persian astronomers, but Democritus’s theory. al-Sufi’s mention is the earliest record. Similarly, The Book of 1845 Lord Rosse makes the Fixed Stars includes the White first clear observation of a Ox, another cloudy object. This is spiral nebula, now known now named the Large Magellanic as the Whirlpool Galaxy. Cloud and is a small galaxy that orbits the Milky Way. Al-Sufi would 1917 Vesto Slipher discovers not have been able to observe that spiral nebulae are rotating this object himself, but would independently of the Milky Way. have received reports of it from astronomers in Yemen and sailors 1929 Edwin Hubble shows who crossed the Arabian Sea. ■ that many spiral nebulae are far beyond the Milky Way See also: Consolidating knowledge 24–25 ■ Examining nebulae 104–05 ■ and are galaxies themselves. Spiral galaxies 156–61 ■ Beyond the Milky Way 172–77

28 FAONREWCHCINAALENDAR THE SOLAR YEAR IN CONTEXT T he traditional Chinese China’s calculations were ahead calendar is a complex blend of the West’s: 50 years later, this KEY ASTRONOMER of lunar and solar cycles, same period was used by Julius Guo Shoujing (1231–1314) with 12 or 13 lunar months matched Caesar to create the Roman up to the solar-derived seasons. Empire’s Julian system. BEFORE It had first been formalized in the 100 bce Emperor Wu of 1st century bce during the Han By the time the Mongol leader the Han Dynasty establishes Dynasty, and used a solar year of Kublai Khan conquered most of the Chinese calendar based 365.25 days (365 days and 6 hours). China in 1276, a variant of the on a solar year. original calendar, the Daming calendar, was in use, but was 46 bce Julius Caesar reforms centuries old and in need of the Roman calendar using a correction. The khan decided to year-length of 365 days and impose his authority with a new, 6 hours, and adds a leap day more accurate calendar, which every four years. became known as the Shoushi (“well-ordered”) calendar. The task AFTER of creating it was entrusted to 1437 The Timurid astronomer Guo Shoujing, the khan’s brilliant Ulugh Beg measures the Chinese chief astronomer. solar year as 365 days, 5 hours, 49 minutes, and A trained engineer, Guo Shoujing Measuring the year 15 seconds using a 164-ft invented a water-powered version Guo’s job was to measure the (50-m) gnomon (the central of an armillary sphere, which is an length of the solar year, and to column of a sundial). instrument used to model the this end he set up an observatory positions of celestial bodies. in Khanbaliq (the “City of the 1582 Pope Gregory adopts Khan”), a new imperial capital the Gregorian calendar as a that would one day become known reform of the ancient Julian as Beijing. The observatory may calendar by using a 365.25-day have been the largest anywhere year, the same year as Guo’s in the world at the time. Shoushi calendar. 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 ■ Zu Chongzhi (Directory) 334 The two men traveled widely, The calendar has 365 days and setting up another 26 observatories 6 hours in the year, but does not match the across China. In 1279, the pair announced that there were motion of the sun through the year. 29.530593 days to a month, and that the true solar year was To measure the There is a need to 365.2524 days long (365 days, length of the year, better create a new calendar 5 hours, 49 minutes, and 12 seconds). This is just 26 seconds instruments must that matches the longer than the current accepted be created. solar year. measurement. Again, China was ahead of the West. The same figure The solar year is found to be 365 days, 5 hours, was not independently measured 49 minutes, and 12 seconds. There is a and adopted for the universal new calendar for China. Gregorian calendar in Europe until 300 years later. measurements. This allowed Guo serving official calendar in Chinese to measure the angle of the sun history. China officially adopted the Enduring calendar with far greater accuracy. Gregorian calendar in 1912, but the A great technological innovator, traditional calendar, today known Guo invented several new The Shoushi calendar was as the rural or former calendar, still observational devices and made widely regarded as the most plays a role in Chinese culture, enhancements to the Persian accurate calendar in the world determining the most propitious equipment that had begun to at the time. As a testament to its dates to hold weddings, family arrive in China under Kublai success, it continued to be used celebrations, and public holidays. ■ Khan’s rule. Most importantly, he for 363 years, making it the longest- 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 Guo Shoujing was born into a poor Guo was tasked with building family in the north of China, in the a canal to bring spring water years when the Mongols were from the mountains to the new consolidating their control over city. In the 1290s, Guo—by now the region. A child prodigy who the khan’s chief science and had built a highly advanced water engineering adviser—connected clock by the age of 14, Guo was Khanbaliq to the ancient Grand taught mathematics, astronomy, Canal system that linked to and hydraulics by his grandfather. the Yangtze and other major He became an engineer, working rivers. In addition to continuing for the emperor’s chief architect his astronomical work, Guo Liu Bingzhong. In the late 1250s, oversaw similar irrigation and Kublai Khan took the throne canal projects across China, and and chose the region around the his theoretical and technological town of Dadu near the Yellow innovations continued to River to build the new capital of influence Chinese society Khanbaliq, now known as Beijing. for centuries after his death.

30 WALELHOAFVTEHREES-OTBASRESRIVNED PTOLEMY’S CATALOG   IMPROVED INSTRUMENTS IN CONTEXT F or more than 1,000 years, to turn the city into a respected Ptolemy’s Almagest place of learning, Ulugh Beg KEY ASTRONOMER was the world’s standard invited scholars of many Ulugh Beg (1384–1449) authority on star positions. disciplines from far and wide Translated into Arabic, Ptolemy’s to study at his new madrasa, BEFORE work was also influential in the an educational institution. c.130 bce Hipparchus Islamic world up until the 15th publishes a star catalog century, when the Mongol ruler Ulugh Beg’s own interest giving the positions of more Ulugh Beg showed that a lot of was in astronomy, and it may than 850 stars. the Almagest’s data were wrong. have been his discovery of serious errors in the star positions of the 150 ce Ptolemy publishes A grandson of the Mongol Almagest that inspired him to a star catalog in the conqueror Timur, Ulugh Beg was order the building of a gigantic Almagest, which builds on just 16 years old when he became observatory, the largest in the the work of Hipparchus and ruler of the family’s ancestral seat world at the time. Located on a is seen as the definitive guide at Samarkand (in present-day hill to the north of the city, it took to astronomy for more than Uzbekistan) in 1409. Determined five years to construct and was a millennium. Ulugh Beg regent, and by 1411, as he 964 ce Abd al-Rahman al-Sufi turned 18, his rule over the adds the first references to The name Ulugh Beg means city was extended to include galaxies in his star catalog. “Great Leader.” The sultan– the surrounding province. astronomer’s birth name was AFTER Mirza Muhammad Taraghay Ulugh Beg’s flair for 1543 Nicolaus Copernicus bin Shahrukh. He was born mathematics and astronomy places the sun as the center on the move, as Timur’s army was not matched by his of the universe, not Earth. traveled through Persia. leadership skills. When Shah Rukh died in 1447, Ulugh Beg 1577 Tycho Brahe’s star His grandfather’s death assumed the imperial throne, catalog records a nova, in 1405 brought the army to but he did not command enough showing that the “fixed a halt in western China. The authority to keep it. In 1449, he stars” are not eternal ensuing fight for control of his was beheaded by his own son. and do change. lands was eventually won by Ulugh Beg’s father, Shah Rukh. Key work In 1409, Ulugh Beg was sent to Samarkand as his father’s 1437 Zij-i Sultani

FROM MYTH TO SCIENCE 31 See also: Shifting stars 22 ■ Consolidating knowledge 24–25 ■ Mapping the galaxies 27 ■ The Copernican model 32–39 ■ The Tychonic model 44–47 The understanding of astronomy is based measured to within a few on the study of the work of past scholars. hundredths of a degree, as could the positions of the stars. A precisely built sextant With better instruments, in a protected location the work of past In 1437, Zij-i Sultani (“The astronomers is Sultan’s Catalog of Stars”) gives more accurate often found to was published. Of the 1,022 measurements. contain errors. stars included in the Almagest, Ulugh Beg corrected the positions completed in 1429. It was there, than a sixth), it is estimated to of 922. Zij-i Sultani also contained with his team of astronomers and have had a radius of more than new measurements for the solar mathematicians, that he set about 130 ft (40 m) and would have been year, planetary motion, and the compiling a new star catalog. three stories high. The instrument axial tilt of Earth. These data was kept underground to protect became very important, enabling Giant instruments it from earthquakes and rested in the prediction of eclipses, the time Ptolemy’s catalog had largely a curved trench along the north– of sunrise and sunset, and the been derived from the work of south meridian. As the sun and altitude of celestial bodies, which Hipparchus, and many of its star the moon passed overhead, their were needed to navigate. Ulugh positions were not based on light focused into the dark trench, Beg’s work remained the definitive fresh observations. and their positions could be star catalog until Tycho Brahe’s, nearly 200 years later. ■ To measure accurately, Ulugh Beg built the observatory on an All that remains of the Fakhri immense scale. Its most impressive sextant is a 6½-ft (2-m) wide trench instrument was the so-called gouged in a hillside. The observatory Fakhri sextant. In fact, more like was destroyed after Ulugh Beg’s death a quadrant (a quarter-circle rather in 1449 and not discovered until 1908. The religions disperse, kingdoms fall apart, but works of science remain for all ages. Ulugh Beg

THE SUNFINALLY WE SHALL PLACE HCIEMSNETLFEART OTHFE THE UNIVERSE THE COPERNICAN MODEL



34 THE COPERNICAN MODEL IN CONTEXT To most people in mid-15th Of all discoveries and century Europe, questions opinions, none may have KEY ASTRONOMER about Earth’s place in exerted a greater effect on Nicolaus Copernicus the cosmos had been answered the human spirit than the (1473–1543) in the 2nd century by the Greco- doctrine of Copernicus. Egyptian mathematician Ptolemy, Johann von Goethe BEFORE who had modified ideas first put c.350 bce Aristotle places Earth forward by Aristotle. These ideas Earth stayed in one place, while at the center of the universe. placed Earth at the center of the everything else rose in the east, cosmos, and they carried an official swung across the sky, and set in c.270 bce Aristarchus proposes stamp of approval from the Church. the west. Furthermore, the Bible a sun-centered (heliocentric) Yet the first convincing challenge seemed to state that the sun moves, universe, with the stars a vast to this orthodoxy was to come from whereas Earth does not, so anyone distance away. a figure within the Church, the who contradicted this view risked Polish canon Nicolaus Copernicus. being accused of heresy. c.150 ce Ptolemy publishes the Almagest. A stationary Earth Nagging doubts According to the version of the The Earth-centered, or geocentric, AFTER universe described by Aristotle and model of the universe had never 1576 English astronomer Ptolemy, Earth was a stationary convinced everyone—in fact, Thomas Digges suggests point at the center of the universe, doubts about it had surfaced modifying the Copernican with everything else circling from time to time for more than system, removing its outer around it, and stars were fixed 1,800 years. The most serious edge and replacing it with in a large, invisible, distant a star-filled unbound space. sphere, which rotated rapidly around Earth. The sun, moon, 1605 Johannes Kepler discovers and planets also revolved at that orbits are elliptical. different speeds around Earth. 1610 Galileo Galilei discovers This idea of the universe the phases of Venus, and seemed like common sense. Jupiter’s moons, strengthening After all, one only had to stand the heliocentric viewpoint. outside and look up at the sky, and it appeared obvious that Nicolaus Copernicus Nicolaus Copernicus was born in developing his sun-centered Torun, Poland, in 1473. From 1491 model of the universe. He did to 1495, he studied mathematics, not complete this work until astronomy, and philosophy at the 1530, although he did publish University of Kraków, then from a summary of his ideas in 1496, canon (religious) law and 1514. Realizing that he risked astronomy at the University of being ridiculed or persecuted, Bologna, Italy. In 1497, he was Copernicus delayed publishing appointed canon of the cathedral the full version of his theory of Frombork, Poland, a post he until the last weeks of his life. retained for life. From 1501 to 1505, he studied law, Greek, Key works and medicine at the University of Padua, Italy. Subsequently, 1514 Commentariolus he returned to Frombork, where 1543 De revolutionibus orbium he spent much of the rest of his coelestium (On the Revolutions life. By 1508, he had begun 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 concern related to predicting Ptolemy tried to fix some of the anomalies in Aristotle’s the movements and appearances geocentric model by proposing that each planet moved in a of the planets. According to the small circle called an epicycle. Each epicycle was embedded Aristotelian version of geocentrism, in a sphere called a deferent. Each planet’s deferent rotated the planets—like all other celestial around a point slightly displaced from Earth’s position in bodies—were embedded in space. This point, in turn, continuously rotated around another invisible concentric spheres point called an equant. Each planet had its own equant. that revolved around Earth, each rotating at its own steady speed. Center of But if this were true, each planet epicycle should move across the sky at a constant pace and with an Planet unvarying brightness—and this wasn’t what was observed. Center of deferent Ptolemy’s fixes The most glaring anomaly was Earth cycle Mars, which had been carefully Epi observed in ancient times by both the Babylonians and the Chinese. Equant It appeared to speed up and slow down from time to time. If its eferent movements were compared to D those of the rapidly rotating outer sphere of fixed stars, Mars usually address these problems, Ptolemy as further epicycles needed to moved in a particular direction, but modified the original Aristotelian be added to keep prediction in occasionally it reversed direction— geocentric model. In his revised line with observation. a strange behavior described as model, the planets were attached “retrograde motion.” In addition, its not to the concentric spheres Alternative views brightness varied greatly over the themselves, but to circles attached From about the 4th century bce, course of a year. Similar, but less to the concentric spheres. He a number of astronomers had dramatic, irregularities were also called these circles “epicycles.” suggested theories refuting the observed in the other planets. To These were suborbits around geocentric model. One of these which the planets circled while ideas was that Earth spins on its In so many and such important the central pivot points of these own axis, which would account ways, then, do the planets bear suborbits were carried around the for a large proportion of the daily witness to the Earth’s mobility. sun. These modifications, Ptolemy movements of celestial objects. thought, sufficed to explain the The concept of a rotating Earth Nicolaus Copernicus anomalies observed and matched had initially been put forward by observational data. However, his a Greek, Heraclides Ponticus, in model became hugely complicated, about 350 bce and later by various ❯❯

36 THE COPERNICAN MODEL Ptolemy’s Earth- Copernicus’s Copernicus believes centered model of the sun-centered model his model is more explains the same data universe relies upon elegant, and thus more complex adjustments to with far fewer likely to be correct. adjustments. explain observed data. Place the sun himself at the center of the universe. Arabic and Indian astronomers. Aristotelian ideas, but supporters of In the face of such an established Supporters of geocentrism rejected geocentrism had also for centuries philosophical tradition with his idea as absurd, believing a cited what seemed a scientifically little observational evidence to spinning Earth would create huge valid reason for ruling it out—the contradict it, and the theological winds, such that objects on Earth’s “lack of stellar parallax.” They arguments in favor of it, the surface would simply fly off. argued that if Earth moved around geocentric view of the universe Another idea, first proposed by the sun, it would be possible to went unchallenged for centuries. Aristarchus of Samos in about observe some variation in the However, in about 1545, rumors 250 bce, was that Earth might relative positions of stars. No such began circulating in Europe of a move around the sun. Not only did variation could ever be detected highly convincing challenge that this go against deeply ingrained so, they said, Earth could not move. 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 In the Ptolemaic model Sun too unlikely that thousands of (top), Earth is at the center Mars stars were spinning rapidly around and other celestial bodies Earth every 24 hours. Instead, he go around Earth. In the Mercury Moon Venus considered them to be fixed and Copernican system immovable in their distant, outer (bottom), Earth together sphere, and that their apparent with the moon have movement was actually an illusion swapped position caused by Earth’s spin. To refute with the sun; the the idea that a spinning Earth sphere of the fixed would create huge winds, and that stars is much objects on its surface would fly off, farther out. Copernicus pointed out that Earth’s oceans and atmosphere were part Saturn Earth of the planet and were naturally part of this spinning motion. In his Embedded own words: “We would only say “fixed” stars that not merely the Earth and the watery element joined with it have Jupiter this motion, but also no small part of the air and whatever is linked in Outer sphere the same way to the Earth.” with embedded Second, Copernicus proposed “fixed” stars 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. Elegant solution Earth Moon These two central tenets of Mercury Mars Copernicus’s theory were of utmost importance because they explained Saturn Sun the movements and variation in brightness of the planets without Venus recourse to Ptolemy’s complicated adjustments. If Earth and another Jupiter 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 ❯❯

38 THE COPERNICAN MODEL complicated epicycles, Copernicus Those things which I am of its implications for stellar explained that such motion could saying now may be obscure, parallax. For centuries, supporters be attributed to changes in yet they will be made clearer of geocentrism had argued that perspective caused by Earth the absence of parallax could and the other planets moving in their proper place. only be due to Earth not moving. at different speeds. Nicolaus Copernicus Now, there was an alternative explanation: the parallax was not Distant stars never realized quite how far absent, but because of the great Another of Copernicus’s tenets away the stars are—it is now distance to the stars, it was simply was that the stars are much farther known that the very closest are too tiny to be measured with the away from Earth and the sun than about 260,000 times more distant instruments of the time. had previously been believed. He than the sun. But his assertion said: “The distance between Earth was extremely important because Copernicus additionally and the sun is an insignificant proposed that Earth is at the fraction of the distance from center of the lunar sphere. Earth and sun to the stars.” Earlier Copernicus maintained that astronomers knew that the stars the moon circled Earth, as it did were distant, but few suspected in the geocentric model. In his just how far away they were, and heliocentric model, the moon those who did, such as Aristarchus, moved with Earth as it circled the had not managed to convince sun. In this system, the moon was anyone. Even Copernicus probably the only celestial object that did not primarily move around the sun. In the Ptolemaic model (left), the occasional retrograde View as (backward-moving) motion of Mars was regarded as due to loops that seen from the planet makes in space. In the Copernican model (right), retrograde motion was caused simply by changes in perspective because Earth 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 Mars reverse its apparent direction of movement for several weeks. Mars Motion of Mars Epicycle Earth Sun Earth Earth’s orbit Mars’s deferent Mars’s orbit

FROM MYTH TO SCIENCE 39 Though Copernicus’s work was and that these movements must Mars’s apparent retrograde motion widely circulated, it took a century be perfect circles. This therefore occurs about every 26 months and or more before its basic ideas forced Copernicus to retain some lasts for 72 days. Its orbit is on a were accepted by most other of Ptolemy’s epicycles in his model. slightly different plane from Earth’s, astronomers, let alone the public The work of Johannes Kepler contributing to the apparent loop. at large. One difficulty was that, later replaced the idea of circular although it resolved many of the orbits with that of elliptical orbits, some of its propositions went problems of the Ptolemaic system, eliminating most of the remaining against Biblical texts probably his model also contained faults faults in Copernicus’s model. It led to the ban. that had to be amended by later wasn’t until the 1580s and the astronomers. Many of these faults work of Danish astronomer Tycho Viewed somewhat ambivalently were due to the fact that, for Brahe that the idea of celestial at first by astronomers, and philosophical reasons, Copernicus spheres was abandoned in favor prohibited by the Catholic Church, clung to the belief that all the of free orbits. Copernicus’s heliocentric model movements of celestial bodies therefore took considerable time occurred with the objects Banned by the Church to catch on. Several centuries embedded in invisible spheres De revolutionibus initially met passed before some of its basic with little or no resistance from propositions were demonstrated I am deterred by the fate of the Roman Catholic Church, to be true beyond dispute: that our teacher Copernicus who, although some Protestants Earth moves in relation to the although he had won immortal denounced it as heretical. In 1616, stars was eventually proved fame with a few, was ridiculed however, the Catholic Church conclusively by English astronomer and condemned by countless condemned Copernicus’s book and James Bradley in 1729. Proof that people (for very great is the it remained proscribed reading for Earth spins came with the first more than 200 years. The Church’s demonstration of Foucault’s number of the stupid). decision coincided with a dispute it pendulum in 1851. Galileo Galilei was having at the time with the astronomer Galileo Galilei. Galileo Copernicus’s theory was was an avid champion of the a serious blow to old ideas Copernican theory and had made about how the world and wider discoveries in 1610 that strongly universe work—many of them supported the heliocentric view. dating from the time of Aristotle. The dispute with Galileo caused As such, it is often cited as the Church authorities to examine ushering in the “Scientific De revolutionibus with intense Revolution”—a series of sweeping scrutiny, and the fact that advances in many areas of science that occurred between the 16th and 18th centuries. ■

RTHEEVOTLEUL 1550–1750

ETSIOCNOPE

42 INTRODUCTION Dutch eyeglass-maker Johannes Kepler describes Hans Lippershey the elliptical orbits of Tycho Brahe builds a large observatory applies for a patent for planets with his three laws a telescope with of planetary motion. on the island of three-times Hveen, from where he magnification. makes observations for 20 years. 1576 1608 1619 1600 1610 1639 Italian friar Giordano Bruno is Using a telescope with English astronomer burned at the stake as a heretic 33-times magnification, Jeremiah Horrocks after expressing a view that the observes the transit sun and Earth are not central Galileo Galilei of Venus across the discovers four moons or special in the universe. face of the sun. orbiting Jupiter. T he Dane Tycho Brahe was power, and they can resolve finer formulated his three laws of the last great astronomer detail. The bigger the main lens planetary motion describing the of the pre-telescope era. or mirror, the better the telescope geometry of how planets move. Realizing the importance of trying on both counts. Starting in 1610, to record more accurate positions, when Galileo made his first Kepler had solved the problem Tycho built some high-precision telescopic observations of the of how planets move, but there instruments for measuring angles. planets, the moon’s rugged surface, remained the problem of why He accumulated an abundance of and the star clouds of the Milky they move as they do. The observations, far superior to those Way, the telescope became the ancient Greeks had imagined available to Copernicus. primary tool of astronomy, opening up unimagined vistas. If I have seen further it Magnifying the image is by standing on the The realm of heavenly bodies still Planetary dynamics shoulders of giants. seemed remote and inaccessible After Tycho Brahe died, the records to astronomers at the time of of his observations passed to his Isaac Newton Tycho’s death in 1601. However, assistant Johannes Kepler, who the invention of the telescope was convinced by Copernicus’s around 1608 suddenly brought arguments that the planets orbit the distant universe into much the sun. Armed with Tycho’s data, closer proximity. Kepler applied his mathematical ability and intuition to discover Telescopes have two important that planetary orbits are elliptical, advantages over eyes on their own: not circular. By 1619, he had they have greater light-gathering

THE TELESCOPE REVOLUTION 43 Dutch astronomer Dane Ole Rømer measures English astronomer Christiaan Huygens the speed of light by Edmond Halley observing eclipses of predicts the return of correctly describes Jupiter’s moon Io. the comet that now the shape of bears his name. Saturn’s rings for the first time. 1659 1676 1705 1675 1687 1725 Giovanni Domenico Isaac Newton publishes James Bradley Cassini spots a gap in Principia, in which he lays proves that Earth Saturn’s rings and out his universal law is moving by concludes correctly that of gravitation. demonstrating an effect called they are not solid. stellar aberration. that the planets were carried on about this force, Newton used the not suffer from the color problem. invisible spheres, but Tycho had word gravitas, Latin for weight, from Reflecting telescopes of Newton’s demonstrated that comets travel which we get the word gravity. design were widely used in the unhindered through interplanetary 18th century, after English inventor space, seeming to contradict this Improving telescopes John Hadley developed methods idea. Kepler thought that some Newton not only created a for making large curved mirrors influence from the sun impelled new theoretical framework for of precisely the right shape from the planets, but he had no scientific astronomers with his mathematical shiny speculum metal. James means to describe it. way of describing how objects Bradley, Oxford professor and move, but he also applied his later Astronomer Royal, was one Universal gravitation genius to practical matters. astronomer who was impressed It fell to Isaac Newton to describe Early telescope makers found it and acquired a reflector. the force responsible for the impossible to obtain images free movement of the planets, with a from colored distortion with their There were also developments theory that remained unchallenged simple lenses, although it helped in lens-making. In the early-18th until Einstein. Newton concluded to make the telescope enormously century, English inventor Chester that celestial bodies pull on each long. Giovanni Domenico Cassini, Moore Hall designed a two-part other and he showed mathematically for example, used long “aerial” lens that greatly reduced color that Kepler’s laws follow as a natural telescopes without a tube to distortion. The optician John consequence if the pulling force observe Saturn in the 1670s. Dollond used this invention to build between two bodies decreases much-improved refracting telescopes. in proportion to the square of the In 1668, Newton designed and With high-quality telescopes distance between them. Writing made the first working version of now widely available, practical a reflecting telescope, which did astronomy was transformed. ■

44 IN CONTEXT IUSANNTNAOUERSTWIUCAAELNDD KEY ASTRONOMER Tycho Brahe (1546–1601)  THE TYCHONIC MODEL 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 ■ The Copernican model 32–39 ■ Elliptical orbits 50–55 ■ Hevelius (Directory) 335 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 Tycho used his immense wealth key to accurate models of the solar system. to design and build fine instruments, such as this armillary sphere, which was used to model the night sky as seen from Earth. 1,900 years earlier. Aristotle visible in the sky as Cassiopeia B. in 1576 he oversaw the building of a had stated that the stars in the The observation of a new star large complex on the small island of heavenly firmament were fixed, was an extremely rare event. Hven in the Øresund Strait, between permanent, and unchanging. Only eight naked-eye observations what is now Denmark and Sweden. In 1572, when Tycho was 26, a of supernovae have ever been This was one of the first research bright new star was seen in the recorded. This sighting showed institutes of its kind. sky. It was in the constellation that the star catalogs in use of Cassiopeia and stayed visible at the time did not tell the whole Tycho carefully measured the for 18 months before fading from story. Greater precision was positions of the stars and recorded view. Influenced by the prevailing needed, and Tycho led the way. them on brass plates on a spherical Aristotelian dogma, most observers wooden globe about 5 ft 3 in (1.6 m) assumed that this was an object Precision instruments in diameter at his observatory high in the atmosphere, but To accomplish his task, Tycho set on Hven. By 1595, his globe had below the moon. Tycho’s careful about constructing a collection of around 1,000 stars recorded on it. measurements of the new object reliable instruments (quadrants It could spin around a polar axis, convinced him that it did not and sextants (p.31), and armillary and a horizontal ring was used move in relation to nearby stars, spheres) that could measure the so that stars positioned above so he concluded that it was not an position of a planet in the sky to the horizon at any given time atmospheric phenomenon but a real an accuracy of about 0.5 arcminute could be distinguished from those star. The star was later discovered (± 1⁄120º). He personally measured below the horizon. Tycho carried to be a supernova, and the remnant planetary positions over a period of the globe with him on his travels, of this stellar explosion is still around 20 years, and for this purpose but it was destroyed in a fire in Copenhagen in 1728. ❯❯

46 THE TYCHONIC MODEL Further evidence of a changing position that he had taken on Hven Tycho’s observations of the way the cosmos came from Tycho’s with those that had been taken comet moved across the sky over observation of the Great Comet at the same time by Bohemian the months also convinced him in 1577. Aristotle had claimed astronomer Thaddaeus Hagecius that it was traveling through the that comets were atmospheric in Prague. In both instances, the solar system. This overturned phenomena, and this was still comet was observed in roughly another theory that had been generally believed to be the case in the same place, but the moon believed for the previous 1,500 the 16th century. Tycho compared was not, suggesting that the years. The great Graeco-Egyptian measurements of the comet’s comet was much farther away. 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 The Tychonic model Mars Earth did not move. This was kept Earth at the center Tycho’s conclusion. of the cosmos as in the Jupiter Ptolemaic model, but The Tychonic model the five known planets Venus Mercury Saturn In reaching this conclusion, Tycho were now orbiting the Sun trusted his own direct experience. sun. Although he was He did not feel Earth moving. In impressed by the Earth fact, nothing that he observed Copernican model, convinced him that the planet Tycho believed was moving. Earth appeared to that Earth did be stationary and the external not move. universe was the only thing that appeared to be in motion. This led Moon Tycho to discard the Copernican cosmos and introduce his own. In Outer ring his model of the cosmos, all the of stars planets except Earth orbited the sun, but the sun and the moon to demonstrate that the planets’ realized that the bright stars Sirius, orbited a stationary Earth. orbits are ellipses and to create a Arcturus, and Aldebaran had, by model that would displace both the Tycho’s time, moved over half a For many decades after his death Tychonic and Copernican models. degree away from the positions in 1601, Tycho’s model was popular recorded by Hipparchus 1,850 years among astronomers who were Tycho’s improved measurements earlier. Not only were the stars not dissatisfied with Ptolemy’s Earth- would also allow English astronomer fixed in the sky, but the changing centric system but who did not wish Edmond Halley to discover the positions of the closer stars could to anger the Catholic Church by proper motion of stars (the change also be measured. Stellar parallax adopting the proscribed Copernican in position due to the stars’ motion was not detected until 1838. ■ model. However, Tycho’s own through space) in 1718. Halley 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 Born a nobleman in 1546 in Rudolph II in Prague. There, Scania (then Denmark, but now Tycho appointed Johannes Sweden), Tyge Ottesen Brahe Kepler as his assistant. (Tycho is the Latinized version of his first name) became an Tycho was famed for his astronomer after sighting a distinctive metal nose, the predicted solar eclipse in 1560. legacy of a duel he fought as a student. He died in 1601, In 1575, King Frederick II allegedly of a burst bladder, gave Tycho the island of Hven having refused out of politeness in the Øresund Strait, where he to take a toilet break during built an observatory. Tycho later a long royal banquet. fell out with Frederick’s successor, Christian IV, over the potential Key work transfer of the island to his children and closed the observatory. In 1588 Astronomiæ Instauratæ 1599, he was appointed Imperial Progymnasmata (Introduction Mathematician to Emperor to the New Astronomy)

48 MVAIRRAIACBELTEISISTAAR A NEW KIND OF STAR IN CONTEXT The star Mira Ceti is B efore the work of German observed to change in astronomer David Fabricius, KEY ASTRONOMER brightness periodically. it was thought that there David Fabricius (1564–1617) were only two types of star. Mira Ceti is a The first were those of constant BEFORE variable star. brightness, such as the 2,500 or 350 bce Greek philosopher so that can be seen with the naked Aristotle asserts that the stars Some stars are eye above the horizon on a clear are fixed and unchanging. variable. dark night. The second type were the “new stars,” such as those AFTER Aristotle was seen by Tycho Brahe in 1572 1667 Italian astronomer wrong when he asserted and Johannes Kepler in 1604. Geminiano Montanari notes that the star Algol varies that the stars are fixed The constant stars were in brightness. and eternal. synonymous with the fixed, permanent stars in the ancient 1784 John Goodricke discovers Greek cosmos—those that Delta Cephei, a star that varies mapped out the patterns in the in brightness over five days; constellations and never changed. English astronomer Edward The new stars, by contrast, would Pigott discovers the variable appear unexpectedly, apparently Eta Aquilae. from nowhere, then fade away, never to be seen again. 19th century Different kinds of variable star are discovered, A third kind of star including long-period variables, While observing the star Mira Ceti cataclysmic variables, novae, (also called Omicron Ceti), in the and supernovae stars. constellation of Cetus the whale, Fabricius realized that there was 1912 Henrietta Swan Leavitt a third type of star in the sky—one discovers a relationship that regularly varied in brightness. between the periods and the He made his discovery in August brightness of variable stars 1596 as he was plotting the such as Delta Cephei. movement of Jupiter across the sky in relation to a nearby star.