sign in sign up
<<
Home , Chinese astronomy, Conjunction (astronomy), History of astronomy, Meton of Athens, Prutenic Tables, Reflecting telescope

Theme 1: in History

1.1 The Uses of Astronomy Astronomy is generally reckoned to be the oldest of the sciences. Most ancient civilisations practised something that we would recognise as astronomy; the first applications of mathe- matics to the understanding of the natural world involved astronomy; the mediaeval university syllabus included astronomy (the trivium of grammar, rhetoric and logic was followed by the more advanced quadrivium of arithmetic, astronomy, geometry and music). Astronomy also played a very large part in the foundation of “modern science” in the —the so- called “” is often considered to have begun with the publication of Coper- nicus’ De Revolutionibus in 1543. Why is this? Why did such an apparently abstract discipline— the study of the sky—play such an important role in ancient societies? There are two—perhaps three—principal reasons for the importance of astronomy in pre- modern eras. All require a certain level of observational precision and mathematical sophis- tication: this is why astronomy quickly became recognisable as a science, whereas other equally important disciplines—such as engineering and medicine—remained more in the nature of “crafts”.

1.1.1 Calendrics Knowing the time of is key to a successful agrarian society: you need to know when to plant crops, when to expect or river floods, how long the stored grain has to last, and so on. The time of year can be estimated quite accurately from natural phenomena such as the return of migratory animals, but one might reasonably suspect that a certain amount of prestige would be acquired by anyone who could predict such events in advance. A regular, calculable, observable phenomenon would be very useful for this purpose—and the night sky offers precisely that. In an era before street lighting, the phases of the and the shifting positions of the naked-eye (most of which are very bright compared to most ) would be very dramatic. The regularity of these motions is clear enough to be perceptible, but complicated enough to require careful study to be useful: for example, the year does not contain an exact number of full , so a lunar quickly gets out of step with the . Therefore there is an incentive to keep good records, and to develop mathematical models of the motion of those celestial bodies which are seen to move (this is the origin of the Greek term planetes, “wanderer”, which gives us our word “”).

1.1.2 The positions of celestial bodies can provide useful markers for navigation. The height of the at noon can easily be used to calculate , and the can provide the same service at night (though the use of the Pole to determine is only temporary, because of ). Navigation was not a major use of astronomy for most ancient societies—most sea-faring was done within sight of land. However, with the advent of transatlantic voyages in the 16th century navigation became a preoccupation of many European governments, and con- siderable effort was put into developing the necessary techniques and instrumentation (leading eventually to the development of the , which remained an important nautical instrument up to modern times). The most challenging problem for astronomical observations is the determination of ; the development of practical methods of longitude determination took up a great deal of astronomical manpower in the 17th and 18th centuries, but the problem was eventually solved by the development of reliable chronometers.

1.1.3 / To anyone living near the sea, it is obvious that the phases of the Moon are linked with the height of the . The changing length over the seasons is equally obviously linked to the position of sunrise and sunset. We accept these relations as causal—the -Sun-Moon geo- metry does affect the height of ; the angle of the Sun does determine the warmth of the , and its does determine the length of the day. It is not surprising that ancient peoples were prepared to accept other associations as causal, and to assume that the motion of the planets might affect human actions as well as natural phenomena. Thus astrology was, from earliest times to the 17th century, a major driver for astronomical observations ( is remembered as the preeminent naked-eye and the man who bequeathed Kepler the data he needed to construct his three laws; but Tycho also cast horoscopes). This can be clearly seen in the nature of the observations that were made: the European tradition viewed the hea- vens as basically unchanging and focused on the motions of the planets, whereas the Chinese thought transient celestial phenomena were astrologically important; therefore we have superb records of and supernovae from the Chinese records, but very little from the Europeans. Even when astrology became detached from mainstream religion, astronomical observations were still of importance for the timing of religious festivals. This ties in with the calendric func- tion of astronomy: lunar require intercalation of additional if they are to re- main in step with the seasons, and even purely solar calendars require the occasional extra day to account for the fact that the year is not precisely 365 days. Thus, from very early times there were incentives to observe the night sky and to attempt to predict the motions of celestial bo- dies. Since the making of precise, testable predictions is now regarded as one of the hallmarks of science, astronomy was “scientific” from its earliest days. However, note that most of the early incentives for studying astronomy relate primarily to the motion of the “planets” (in this sense including the Sun and Moon); the stars are simply useful signposts to assist in defining the locations of the planets. This is reflected in the : despite the name, which means “star-reckoning”, the stars do not really attract much attention until the 19th century. Note that none of these early motivations for the study of astronomy has really persisted to the present time. Our current standards for time measurement and navigation do not rely on as- tronomy, and of all the major only still ties its calendar directly to astronomical observations (of the “”, which in Islam means the first visible crescent, rather than the astronomical meaning of lunar , which produces no visible Moon at all). The modern reasons for studying astronomy can be divided into two (overlapping) categories:

1.1.4 Astronomy as a laboratory for extreme From the birth of modern science to the present day, astronomy has provided insights into physics which, because of the extreme conditions required, were not (at the time) deducible from conventional experimental physics. Some examples of this include:  evidence for the “universality” of Newton’s laws of motion and of gravity, as set out in the Principia (the same laws can describe falling bodies on Earth and the motion of the planets as codified by Kepler); • the finite speed of , as deduced by Römer from the timing of the of ’s moons; • evidence in favour of General Relativity, initially from the advance of the perihelion of Mer- cury (a “postdiction”, in that this was a known problem before 1915, but persuasive because GR could account for the discrepancy with good quantitative precision) and the observation of gravitational bending of starlight by Eddington and colleagues in 1919; in more modern epochs by studies of binary pulsars (which won Hulse and Taylor a Nobel Prize in 1993); • evidence for non-zero neutrino (from the solar neutrino deficit; the best current upper limit on what those masses actually are is also astronomical, deduced from the WMAP and studies of the cosmic background). This motivation has decreased somewhat in modern times, because modern experimental techniques can often probe extreme conditions in the laboratory. The most notable exception is the increasingly tight linkage between theoretical particle physics and theoretical , caused by the fact that energy scales in the very early exceed any that could be pro- duced in terrestrial accelerators.

1.1.5 Addressing Big Questions The primary motivation for modern astronomy is undoubtedly human curiosity. Astronomy addresses the “big questions” (Where do we come from? How did the world begin?) which have always attracted the attention of humanity, albeit usually in the guise of religion. This is the motivation usually expressed by students wishing to study astronomy at university, and pro- bably also explains the success of popular expositions of astronomy, and particularly cos- mology—e.g. Hawking’s A Brief History of Time and Bill Bryson’s A Short History of Nearly Everything. Similar “big questions” (What is the world made of? Why do things behave in the way that they do?) are also the primary rationale behind the study of particle physics—but particle physics is more abstruse and more difficult to explain than astronomy, and hence does not usually manage to resonate with nonscientists as astronomy does, though the new discipline of “particle cosmology” is challenging this, with books such as Brian Greene’s The Elegant Universe achieving popular success. In terms of popular support, it surely helps that astronomy produces arresting visual imagery—consider the number of HST images that make their way on to the front pages of newspapers, despite their lack of obvious relevance to the lives of the papers’ readers—but this is clearly not a dominant motivation in the persistence of the subject as a scientific discipline! Thus, the history of astronomy spans not only a vast amount of time, but also a vast change in the attitudes of practitioners and the general public. Initially, astronomy was very much an ap- plied science, in the service of religion and the calendar; later, it became an inspiration and a test-bed for the application of mathematical techniques to natural phenomena, and subse- quently for the development of empirically supported “laws of nature” which gave birth to mo- dern science; finally, it has become established as a “pure”, curiosity-driven, science aiming to shed light on fundamental questions about humanity and the universe. A study of the history of astronomy therefore touches on many aspects of history, , society and philosophy, as well as documenting our progress in understanding the cosmos. Some (by no means all) of these issues will be considered in this course; for more details on any given aspect, consult the extensive specialist literature. 1.2 A Timeline for the History of Astronomy In this course, we shall take a thematic (albeit broadly chronological) approach to the history of astronomy, as summarised in the course outline. This has the disadvantage that it may some- times obscure the overall picture of what is happening at any given time. Therefore, we start the course with a brief chronological account of the history of astronomy, together with relevant events in the wider and in the socio-political context. This timeline focuses primarily on Western and the Near East, because this is the tradition that ultimately led to modern astronomy; it should be noted that from ~1000 BC much excellent observational work was carried out in and neighbours (Japan, Korea, Vietnam, Cambodia). This is of particular interest because, owing to a different astrological system, the Chinese recorded trans- ient phenomena ignored by Western observers; the Chinese records are thus our main source for information on phenomena such as supernovae and comets. However, the Chinese under- standing of astronomy as a system did not really influence the development of modern scientific astronomy, because of the lack of real contact between Europe and China in the relevant period. (This contrasts with the situation with Islamic astronomy, which inherited much of the Greek and Mesopotamian tradition through the Islamic conquest of the eastern Mediterranean and then transmitted it—augmented by additional work and greatly improved mathematical techniques and notation—to Europe, particularly via Spain.) Date Astronomy Other Sciences Social/Political

~30000 Records of lunar phases in cave Nothing known. BC paintings and bone carvings 3500 – Astronomically aligned Little known of the society 2500 monuments, e.g. , that built megalithic BC Newgrange. monuments. Early calendars, e.g. Egyptian, Writing developed Sumerian (Sumerian , then Probable astronomical record- Egyptian hieroglyphic) keeping, in many . Egyptian 1st Dynasty, ~3150 Sumerian dynastic period, ~2900 Pyramids of Giza, 26th century. 2500 – Definite evidence of astronomical Place-value mathematical Akkad conquers . 1500 record-keeping, e.g. record of notation introduced in Egyptian Middle Kingdom, BC lunar in Ur (Sumeria), . ~2160-1780. 2094 BC Geometry texts in Egypt and Chinese dynastic period Lunisolar calendars. . begins, ~2100. Minoan civilisation in . 1500 – Start of eclipse records in China. Egyptian New Kingdom 1000 Records of planetary ephemerides (Tutankhamun, ~1330) BC and heliacal risings of stars in Mycenaean period in Babylonia. Olmec civilisation in Mesoamerica, from ~1400 (first of the Mesoamerican pre-Columbian civilisations) 1000 – Systematic records of Indian develops. Foundation of Rome, 600 BC astronomical phenomena in ~750. Babylonia. Prediction of lunar Neo-Babylonian Empire, . Establishment of 626 – 539. zodiacal constellations. Development of Greek ~700 BC, Hesiod’s Works and Days city-states. sets out defined by

heliacal risings. 600 – 585 BC, predicts Pythagoras, ~580–500, various Classical Greece. 350 BC . topics in mathematics and Athenians and allies defeat 5th century: both Meton of astronomy. Persia at Marathon and and Babylonia know the 19-year Democritus, ~460–370, Salamis, 480s —it’s not clear if the proposes that matter consists of discoveries were independent or, atoms. if not, who got it from whom. ’s Academy founded, 387. Eudoxus (408-355) developed the Eudoxus develops more first model of the planetary abstract mathematical and system using combinations of geometrical principles, later spheres to account for retrograde built on by Euclid. motion. 340- (384-322): empirical Aristotle works extensively in Reign of Alexander the 323 BC evidence for spherical shape of various sciences, developing Great. Earth; distinction between highly influential body of work. Alexandrian empire unites “terrestrial” and “celestial” Greece and Persia: fruitful phenomena; Earth as natural contact between Greek centre of cosmos. model-builders and Heraclides (~390-310) suggests Babylonian calculators. that the Earth rotates on its axis. 323- Hellenistic astronomy: Aristarchos Euclid (325-265): development Alexander’s empire 100 BC (310-230): distances of Sun and of formal geometry, especially fragments into a number Moon; heliocentric model of his book The Elements (of of Greek-ruled successor planetary system. mathematics—no connection states. Under the (276-195): size of the Earth. with Aristotle’s theories of , Apollonius (262-190): eccentric chemistry). develops as centre of orbits, epicycles. Hipparchos Archimedes (287-212): scientific learning. (~190-120): , mathematics, mechanics (e.g. Rise of the Roman accurate length of year, precession Archimedes’ Principle), Republic. Wars with of , theory of lunar and machines (e.g. Archimedes’ Carthage, 264-146. planetary orbits. screw). First , publishes (terracotta warriors), 258- embodying Hipparchos’ model of work on conic sections, ~210 210. the planetary system, ~100. 100 BC- developed, 46 BC Lucretius’ De rerum natura Establishment of Imperial AD 14 (replacing with summarises current Rome (Augustus, 63 BC – pure solar calendar). understanding of science AD 14). text Zhoubi Jesus of Nazareth, ~4 BC – suanjing, covering positional ~AD 30. astronomy and surveying. First “long count” date stelae in Mesoamerica. AD 14- (~100–170) produces Hero of Alexandria (~10-70) Roman empire at peak. AD 200 the most developed version of the builds machines powered by Hadrian’s Wall built, AD Greek geocentric planetary model, steam and by wind. 122–128. which continues to be used until (78-139) invents the 16th century. the seismograph. Chinese record SN 185, the Galen (129-200 or 216) writes earliest probable highly influential treatises on record. medicine, anatomy and pharmacology. AD A̅ryabhaṭa (476-550) writes the A̅ryabhaṭiya also includes an Classic Maya period, 200- A̅ryabhaṭiya (499), a work on excellent approximation to π ~250-950. 600 astronomy and mathematics (62832/20000 = 3.1416) and Decline of Roman empire, which includes a tables of trigonometric func- especially in west. of the , based on tions, introducing modern sine Foundation of earlier Greek work but function in place of Greek chord. Constantinople and independent of and in some Although it does not use place- adoption of Christianity respects better then Ptolemy’s value notation, the nature of under Constantine (incorporates rotation of the some of the calculations (reigned 306-337). Earth). Also includes good suggests that A̅ryabhaṭa knew algorithms for eclipse prediction. this system. Western Roman empire ends, 476. Maya civilisation developed accurate understanding of motions of , Jupiter and , and probably eclipse pre- dictions, but no model building. AD Islamic scholars translate and Indians develop first real Mohammed, 570–632. 600- thus preserve much Greek science, understanding of zero as a Meteoric rise of Islam; 1000 refine and develop Ptolemy’s number (instead of just a conquest of much of North model, invent modern spherical placeholder for a missing digit Africa and Asia Minor. trigonometry, and improve in place-value notation). Conquest of most of Spain, instruments, e.g. . Indian numerals come into use 711-719. Astronomical tables or zīj in Islamic world. Modern European states produced by Islamic scholars: not Islamic mathematics: al- beginning to emerge. only tables of planetary positions Khwārizmi writes treatise on Æthelstan (894-939), (from Indian or Ptolemaic models) algebra, 830 (his name gives us grandson of Alfred the but also mathematical tables. the word algorithm; the title of Great, is first king of his book gives us algebra). England (recognisably the modern country), unifying the various Anglo-Saxon and Danish kingdoms. AD Astronomical ephemerides spread University of Bologna founded, Norman conquest, 1066. 1000- to Europe, particularly Toledo 1088 (first university in modern First Crusade, 1099. 1200 Tables by al-Zarqālī (1027-1087). sense; Oxford University, Moorish rule in Spain These were translated into ~1096, is the second). recedes: kingdoms of in the 12th century and were Greek and Islamic science texts Castile, Aragon and highly influential. begin to be translated into Portugal established. in Far East observe Latin, 12th century, largely as SN 1006 (the brightest naked-eye consequence of reconquest of supernova ever recorded) and SN Spain. 1054 (which formed the Crab Movable type invented in China, ). 1040. AD Astronomy taught in universities More universities established, Greater stability in Europe 1200- as part of standard curriculum, including Cambridge in England and rise of merchant class. 1500 and textbooks written/translated. and St Andrews, Glasgow and More education and of planetary Aberdeen in (yes, more greater levels of literacy positions, 1252. in Scotland than in England, among laypeople. from 1495 till 1832!). More developments in Black Death (1346-53) instrumentation: cross-staff, Beginnings of a concept of devastates Europe, killing . experimental science (Robert ~half the population. Grosseteste, ). (1436-76): Artillery (cannon) being systematic programme of press developed in used in warfare, 15th cent. Europe, (Gutenberg, 1439, then observations, of very good very rapid spread: ~1000 Fall of Constantinople accuracy, and ambitious textbook presses in operation in Europe (1453): end of last printing programme (sadly by 1500). This is critically vestiges of Roman Empire. terminated by his early death). important for the dissemination Contact with New World of new ideas. (1492) emphasises need Islamic mathematical methods for navigational spread to Europe. instruments—ships now undertaking long voyages out of sight of land. AD Copernicus (1473-1543) writes Paracelsus (1493-1541) revives Protestant Reformation 1500- De Revolutionibus (1453): study of chemistry, toxicology (Martin Luther, 1483- 1600 reintroduction of heliocentric and medicine in Europe. 1546; his “95 Theses”, model. Basis of , Mathematical sophistication 1517; Ulrich Zwingli, 1551. increases: Tartaglia (1499- 1484-1531; Jean Calvin, Giordano Bruno (1548-1600) 1557) translates Euclid and 1509-1564). espouses Copernican theory and devises a method for solving Henry VIII establishes concludes that there must be cubic equations. Church of England, 1534. many life-bearing worlds. Burned Significant progress in anatomy Increasing levels of at stake for heresy, 1600. and zoology. education and intellectual Tycho Brahe (1546-1601) shows Galileo discovers constancy of independence. Higher that the “new star” of 1572 (a period of pendulum, 1583. education no longer supernova) and the of 1577 confined to clerics. Invention of in are not sublunary; constructs star Netherlands, ~1590. Destruction of Meso- catalogue; makes extensive american civilisations by William Gilbert, De Magnete observations of Mars in Spanish conquistadors. (1600), describes Earth’s unsuccessful attempt to measure Foundation of Spanish magnetic field and distinguishes parallax. Empire. electricity from . developed, 1588 Spanish Armada

1582. defeated AD 1602 Tycho’s star catalogue Many foundations of modern 1603 Death of Elizabeth. 1600- published. science laid, e.g. gas laws (Boyle, Union of Crowns of 1700 1604 Kepler’s supernova Hooke), mechanics (Galileo, England and Scotland. Hooke, Newton), calculus 1608 Galileo begins telescopic 1605 Foundation of East (Newton, Leibniz). Rise of observations India Company. properly designed controlled Danish, Dutch, English and 1609 Kepler’s and predictive st nd Portuguese all establish (Kepler’s 1 & 2 laws) mathematical models. Concept presence in India for 1610 Galileo, Sidereus Nuncius of universal physical laws. Final trading purposes. 1610 Galileo discovers satellites of demise of . 1607 Foundation of Jupiter. English Colony of Virginia 1611 Kepler publishes improved telescope design Increasing importance of long-distance travel makes 1613 Galileo writes letter on 1614 John Napier invents navigation and accurate logarithms maps a critical issue. 1619 Harmonice Mundi, Kepler’s 1621 Snell discovers his law of 1639-1651 Wars of the 3rd law refraction. Galileo shows that (English 1632 Galileo, Dialogue of the Two under gravity is Civil War). Execution of Chief World Systems independent of of body Charles I. Protectorate of 1636 Oliver Cromwell, 1653-8. 1639 Huygens recognises ’s founded 1660 Restoration of rings 1644 Descartes, Principles of Charles II. 1655 Huygens discovers Titan Philosophy 1663 James Gregory publishes 1662 Royal Society founded design for reflecting telescope 1665 Phil. Trans. started

1665 Cassini discovers Great Red 1665 Hooke publishes Spot Micrographia, first discussion of microscopic sctructures 1666 Paris Académie des Sciences founded 1667 Paris founded

1668 Newton builds reflecting telescope 1672 Newton uses prism to 1672 Cassegrain publishes design decompose white light for reflecting telescope 1675 Greenwich Observatory founded 1676 Römer deduces finite speed of light

1677-8 Halley catalogues

southern stars

1687 Newton publishes Principia 1693 Halley works out the 1695 Halley predicts return of 1 1 1 equation, + = . comet in 1758/9 푢 푣 푓

AD Rapid development of Emergence of modern Much social upheaval 1700- astronomical instrumentation. chemistry (Priestley, Lavoisier, (Jacobite rebellions in 1800 Numerous star catalogues. Early Dalton). Scotland, 1715, 1745; studies of “nebulae”, and first Advances in Newtonian American independence, attempts to understand the mechanics by Laplace, 1776; French Revolution, . Many detailed Newtonian Lagrange, Euler, etc. 1789). Beginnings of calculations of planetary and modern parliamentary First systematic studies of heat satellite orbits. democracies. United and electricity. 1729 Bradley discovers stellar Kingdom formed by Act of

aberration Union, 1707. 1738 Daniel Bernoulli invents Beginning of Industrial kinetic theory of gases Revolution: 1712

1742 Celsius invents Celsius Newcomen’s steam 1750 Wright: early speculations scale engine; 1776 Watt’s (much on structure of Galaxy 1751 Franklin publishes book improved) steam engine; on electricity 1761 onwards development of canal 1753 Linnaeus invents modern system; mechanisation of 1758 Dollond builds the first biological naming convention textile production; achromatic industrial iron production 1759 Halley’s Comet duly returns, and bulk production of and acquires its name 1761 Joseph Black introduces chemicals; mechanisation 1761, 1769 Transits of Venus concept of latent heat of (1701 Tull, 1767 Michell deduces, by 1761 Longitude problem solved seed drill; 1730 Foljambe, statistical arguments, existence of by development of reliable iron plough; 1784 Meikle, binary stars and star clusters chronometers (Harrison’s H4) threshing machine), which freed up labour for heavy 1781 Messier’s catalogue of “fuzzy industry and accelerated things that aren’t comets” 1783 Carnot describes urbanisation. 1781 discovers operation of heat engines 1776 Adam Smith 1788 Hutton, Theory of the publishes The Wealth of 1783 Goodricke studies variable Earth, introduces doctrine of Nations (origin of stars ( and δ Cephei) uniformitarianism in geology capitalism as economic 1796 Laplace proposes the and very great age of the Earth theory) for the (“no vestige of a beginning, no formation of the solar system prospect of an end”) AD Major advances in “Classical physics” perfected: Continuing advance of 1800- instrumentation: improved wave theory of light (Young, industrialisation and 1900 refractors, then silver-on-glass 1801; Fresnel, 1830); development of new reflectors; ; thermodynamics (Carnot, technologies. Machine . Meyer, Joule, Kelvin, Helmholtz, tools. Strong emphasis on Maxwell, etc.); classical science and engineering in electrodynamics (Oersted, support of this.

Faraday, Ampère, Maxwell, etc.) Heyday of British Empire.

1800 Herschel discovers 1801 Piazzi discovers 1 radiation 1802 Olbers discovers 2 Pallas. 1801 Jacquard invents 1801 Young’s two-slit Herschel first uses the word punched-card loom “” for these small (arguably first planetary bodies (most people programmable machine) just call them planets) 1803 Herschel confirms binary stars by observing orbital motion 1807-12 Davy presents a series 1804 3 Juno discovered of lectures developing modern 1807 discovered chemistry

1807 Young coins the term “energy”

1808 Dalton presents modern

atomic theory 1814-15 Fraunhofer maps solar 1811 Fourier develops theory of spectrum Fourier series

1824 Carnot devises Carnot 1829 Stephenson’s Rocket 1826 Olbers propounds his cycle wins Rainhill Trails. Start paradox 1820s studies of emission of railway era.

spectra of salts via flame spectroscopy

1830 Lyell’s Principles of 1832 Durham University Geology establishes modern founded (first new geological science. university in UK for ~300 1831 Faraday uses a moving 1838 Bessel makes first reliable ) magnetic field to generate parallax measurement, of 61 electricity Cygni. (Struve and Henderson make less reliable measurements, 1839 Daguerre introduces the

of and α Cen respectively) process—first practical photography 1840s Domestic gas 1843 11-year solar cycle lighting introduced discovered 1845 onwards: many further main belt discovered—no longer regarded as full-status planets (term “asteroid” becomes

commonly used)

1846 Existence of deduced by Adams and Leverrier by analysis of perturbations of 1851 Frederick Scott Archer Uranus’ orbit; Neptune invents wet collodion process consequently discovered by Galle 1854 Lord Kelvin coins the 1857 Maxwell analyses dynamics word thermodynamics of Saturn’s rings 1859 Darwin, Origin of Species 1859 Kirchhoff-Bunsen laws of spectroscopy 1859 Maxwell derives the Maxwellian distribution 1861 Maxwell interprets light as an electromagnetic wave

1861 Maxwell (again!) and Sutton take first colour photo 1864 Huggins observes emission 1865 Maxwell publishes his line spectra from planetary electromagnetic theory nebulae and concludes that they

are gaseous

1868 Secchi develops first spectral classification system for 1871 Richard Leach Maddox stars invents dry plate process 1872 Draper photographs 1870s Maxwell, Boltzmann, spectrum of Vega Clausius and Gibbs develop the theory of thermodynamics 1880 Silver-on-glass technology introduced 1880s Hertz produces radio waves and proves that they are 1888 Lick refractor (36")—with electromagnetic waves as Yerkes 40" refractor (1897), last predicted by Maxwell of the great research refractors 1890s development of Diesel 1890 First version of Harvard engine classification system (Pickering and Fleming) 1895 Röntgen discovers X-rays 1897 Maury’s (short-lived) 1896 Becquerel discovers classification system, including radioactivity first spectral identification of giant 1897 JJ Thomson discovers the stars electron 1898 Keeler refurbishes Crossley reflector—first major reflector in a professional observatory (Lick) AD Advances in physics (quantum Dramatic shift from ~1900 view Two world wars: 1900- mechanics and general relativity) of physics as essentially a emergence of USA (and, 1950 lead to the development of two finished article. after 1945, USSR) as global new branches of astronomy: superpower. Decline of and cosmology. British Empire. 1900 Planck’s formula for blackbody radiation, Increasing levels of 1905 Hertzsprung introduces the introducing E = hν Government funding for universities and “pure idea of red giants 1905 Einstein’s “annus research”—previously 1906 Kapteyn’s model of the mirabilis”—special relativity, Government funding Galaxy the photoelectric effect, tended to be for applied Brownian motion 1908 Mt Wilson 60" reflector— work, e.g. (for astronomy) first of the purpose-built research 1909 Geiger and Marsden navigation and reflectors (working for Rutherford) timekeeping. conduct α particle scattering Widespread car experiments leading to 1911 Hertzsprung constructs ownership: beginning of discovery of atomic nucleus colour-magnitude diagrams for decline of railway system. 1911 Development of metal the and Hyades Electricity becomes filament lightbulb 1912 Leavitt discovers Cepheid ubiquitous for domestic period-luminosity relation 1913 Bohr’s semiclassical power and lighting theory of the atom 1913 Russell draws first modern-

style HR diagram 1914-18 World War I 1913 Hertzsprung uses period-

luminosity relation to infer distance of Magellanic Clouds

1914 Shapley interprets Cepheids as pulsating variables 1915 Slipher publishes first 1915 Einstein’s general theory 1915 UK Department of Doppler shifts of “spiral of relativity Scientific and Industrial nebulae”—11/15 are 1916 Karl Schwarzschild solves Research set up to fund 1917 100" Hooker telescope on Einstein’s field equations for a university research Mt Wilson non-rotating black hole (replaced by system of Research Councils in 1918 Shapley’s model of Galaxy 1965) 1918-24 Cannon and Pickering: final version of and Harvard spectral classification

1919 Eddington expedition measures gravitational bending of starlight 1920s: development of

1920 over status of (1923 de spiral nebulae Broglie, λ = h/p; 1925 1920 Eddington proposes stars Schrödinger equation, matrix fuelled by hydrogen fusion mechanics; 1927 Heisenberg’s 1920 Saha equation for ionisation uncertainty principle, Dirac states of atoms in stellar equation, origins of quantum atmospheres field theory) 1922-24 Friedmann’s expanding universe solutions of GR 1923 Hubble finds Cepheids in nearby spirals, establishes existence of external

1924 Payne determines solar composition 1924 Eddington, mass-luminosity relation 1926 Eddington, Internal 1926 Central Electricity Constitution of the Stars Board set up in UK to

1927 Lemaître rediscovers construct the National

Friedmann’s solutions to GR Grid 1928 Gamow explains α decay 1929, 1931 Hubble (& Humason), by quantum tunnelling -distance relation

1929 Atkinson and Houtermans apply quantum tunnelling to problem of fusion in stars 1930 Pauli postulates neutrino

1931 Chandrasekhar, theory of 1931 Urey discovers deuterium white dwarfs 1932 Anderson discovers

1932 Jansky detects radio positron; Chadwick discovers emission from neutron 1933 Hitler appointed Chancellor of 1934 Baade and Zwicky distin- 1934 Oliphant postulates after Nazi party wins guish novae and supernovae, and existence of 3He postulate that latter arise when election massive star collapses to form neutron star 1938-39 Bethe and colleagues work out theory of hydrogen fusion (pp chain and CNO cycle) 1944 Baade resolves bulge of M31 1939-45 World War II and introduces concept of stellar Radar development in WWII paves way for radio populations astronomy 1944 van der Hulst predicts existence of 21 cm line of neutral hydrogen (first observed 1951) 1945 Jodrell Bank established; also around this time, facilities in Cambridge and Sydney 1946 V2 rockets used for astronomy 1948 (200" reflector, 48" Schmidt) 1948 Birth of modern cosmology: αβγ paper on hot dense early universe, Bondi, Gold and Hoyle on steady state model

From 1950 to the present day, the line between “astronomy” and “history of astronomy” be- comes increasingly blurred, and the expansion of the subject and its increasingly rapid develop- ment make constructing a timeline very difficult (multiple lines of enquiry are progressing in parallel). The difference between astronomy ~1900 and ~1950 is almost as striking as be- tween ~1600 and ~1700: in 1900, only the dynamics of the solar system had a strong theore- tical basis, there were no distances for anything beyond the nearest stars, the physical basis of stellar energy generation had yet to be understood, and the question of the origin and evolution of the universe as a whole was basically ignored; whereas in 1950 the theoretical framework is more or less in place, with only particle physics still to be understood, and the research pro- gramme is recognisably modern (though it will be greatly expanded over the next couple of decades by the advent of space-based instrumentation, foreshadowed by the use of V2 rockets in 1946). This drives home the idea that astronomy is at once the oldest and one of the youngest of the sciences—with the exception of orbital mechanics and spherical trigonometry, very little of the undergraduate astronomy curriculum dates from before the 20th century (in contrast to physics, where the 19th-century edifice of classical physics is still very relevant des- pite the complete restructuring of its basic theoretical framework in the early 20th century).