History of astronomy

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The history of astronomy describes the ideas of earlier civilisations. It also discusses the beginnings of modern astronomy.

Early history[change | edit source]

The Babylonian astronomers viewed the Universe as a flat disk floating in the ocean. This forms the start for early Greek maps like those of Anaximander and Hecataeus of Miletus.

Ancient Greece[change | edit source]

Greek philosophers, seeing the movement of heavenly bodies, developed models of the Universe based more on empirical evidence. The first model was proposed by Eudoxus of Cnidos. According to this model, space and time are infinite and eternal, the Earth is spherical and stationary, and all other matter is confined to rotating concentric spheres.

This model was brought into agreement with astronomical observations by Ptolemy of Alexandria, in the second century AD. His Syntaxis mathematica, known as the Almagest, was translated from Greek to Arabic, and from Arabic to Latin. It was first printed in 1515, with another edition in 1528.[1]

However, not all Greek scientists accepted the geocentric (Earth-centered) model of the Universe. The Pythagorean philosopher Philolaus thought the center of the Universe was a "central fire" around which the Earth, Sun, Moon and Planets revolved in uniform circular motion.[2] The Greek astronomer Aristarchus of Samos was the first person to propose a heliocentric (Sun-centered) model of the universe. Though the original text has been lost, a reference in Archimedes' book The Sand Reckoner describes Aristarchus' heliocentric theory. Archimedes wrote: (translated into English)

You King Gelon are aware the 'Universe' is the name given by most astronomers to the sphere the center of which is the center of the Earth, while its radius is equal to the straight line between the center of the Sun and the center of the Earth. This is the common account as you have heard from astronomers. But Aristarchus has brought out a book consisting of certain hypotheses, wherein it appears, as a consequence of the assumptions made, that the universe is many times greater than the 'Universe' just mentioned. His hypotheses are that the fixed stars and the Sun remain unmoved, that the Earth revolves about the Sun on the circumference of a circle, the Sun lying in the middle of the orbit, and that the sphere of fixed stars, situated about the same center as the Sun, is so great that the circle in which he supposes the Earth to revolve bears such a proportion to the distance of the fixed stars as the center of the sphere bears to its surface.

Aristarchus thus believed the stars to be very far away, and saw this as the reason why there was no visible parallax, that is, an observed movement of the stars relative to each other as the Earth moved around the Sun. The stars are in fact much farther away than was generally assumed in ancient times, which is why stellar parallax is only detectable with telescopes. The rejection of the heliocentric view was apparently quite strong, as the following passage from Plutarch suggests (On the apparent face in the orb of the Moon):

Cleanthes [a contemporary of Aristarchus and head of the Stoics] thought it was the duty of the Greeks to indict Aristarchus of Samos on the charge of impiety for putting in motion the Hearth of the universe [i.e. the Earth], . . . supposing the heaven to remain at rest and the earth to revolve in an oblique circle, while it rotates, at the same time, about its own axis.

The only other astronomer from antiquity known by name who supported Aristarchus' heliocentric model was Seleucus of Seleucia. He was a Babylonian astronomer a century after Aristarchus.[3][4][5] According to Plutarch, Seleucus was the first to prove the heliocentric system through reasoning, but it is not known what arguments he used. Seleucus' arguments for a heliocentric theory were probably related to the phenomenon of tides.[6] According to Strabo (1.1.9), Seleucus was the first to state that the tides are due to the attraction of the Moon, and that the height of the tides depends on the Moon's position relative to the Sun.[7]

Copernicus[change | edit source]

Model of the Copernican universe by Thomas Digges in 1576, with the amendment that the stars are no longer confined to a sphere, but spread uniformly throughout the space surrounding the planets.

The geocentric model was accepted in the Western world for about two thousand years, until Copernicus revived Aristarchus' theory that the sun was placed at the center of the Solar System.

In the center rests the sun. For who would place this lamp of a very beautiful temple in another or better place than this wherefrom it can illuminate everything at the same time?
Copernicus Book 1, Chapter 10 of De Revolutionibus Orbium Coelestrum, 1543

As noted by Copernicus himself, the suggestion that the Earth rotates (goes round on its axis) was very old, dating at least to Philolaus (c. 450 BC). About a century before Copernicus, Nicholas of Cusa also proposed that the Earth rotates on its axis in his book, On Learned Ignorance (1440).[8]p754

The first empirical evidence for the Earth's rotation on its axis, using the phenomenon of comets, was given by Tusi (1201–1274).[9][10]

Johannes Kepler published the Rudolphine Tables containing a star catalog and planetary tables using Tycho Brahe's measurements.

Copernicus' heliocentric model allowed the stars to be placed uniformly through the (infinite) space surrounding the planets, as first proposed by Thomas Digges in his Perfit Description of the Caelestiall Orbes according to the most aunciente doctrine of the Pythagoreans, latelye revived by Copernicus and by Geometricall Demonstrations approved (1576).[8] Giordano Bruno accepted the idea that space was infinite and filled with solar systems similar to our own; for the publication of this view (and other beliefs), he was burnt at the stake in Rome on 17 February 1600.[8]

This cosmology was accepted provisionally by Isaac Newton, Christiaan Huygens and later scientists,[8] but it had several paradoxes which were resolved only with general relativity. The first paradox was that it assumed that space and time were infinite, and that the stars in the universe had been burning forever; however, since stars are constantly radiating energy, a finite star seems inconsistent with the radiation of infinite energy. Secondly, Edmund Halley (1720) noted that an infinite space filled uniformly with stars would lead to the prediction that the nighttime sky would be as bright as the sun itself; this became known as Olbers' paradox in the 19th century.[11][8][12] Third, Newton himself showed that an infinite space uniformly filled with matter would cause infinite forces and instabilities crushing matter inwards inwards under gravity.[8] This instability was later clarified in 1902.[13]

A significant astronomical advance of the 18th century was the realization by Thomas Wright, Immanuel Kant and others that stars are not distributed uniformly throughout space; rather, they are grouped into galaxies.[8]p756

Modern era[change | edit source]

The modern era of physical cosmology began in 1917, when Albert Einstein first applied his general theory of relativity to model the structure and dynamics of the universe.[14]

References[change | edit source]

  1. Stillwell, Margaret B. 1970. The awakening interest in science during the first century of printing. New York: Bibliographical Society of America, 30-32.
  2. Boyer C. A history of mathematics. Wiley, p54.
  3. Neugebauer, Otto E. 1945. The history of ancient astronomy: problems and methods. Journal of Near Eastern Studies 4 (1), p. 1–38.
  4. Sarton, George 1955. Chaldaean astronomy of the last three centuries B.C. Journal of the American Oriental Society 75 (3), pp. 166–173 [169]: "The heliocentrical astronomy invented by Aristarchos of Samos and still defended a century later by Seleucos the Babylonian".
  5. Wightman, William P.D. (1951, 1953), The growth of scientific ideas, Yale University Press p.38, where Wightman calls him Seleukos the Chaldean.
  6. Lucio Russo, Flussi e riflussi, Feltrinelli, Milano, 2003. ISBN 88-07-10349-4.
  7. Bartel Leendert van der Waerden 1987."The heliocentric system in Greek, Persian and Hindu astronomy. Annals of the New York Academy of Sciences 500 (1): 525–545 [527]
  8. 8.0 8.1 8.2 8.3 8.4 8.5 8.6 Misner C.W; Thorne K. & Wheeler J.A. 1973. Gravitation. San Francisco: W.H. Freeman. ISBN 978-0-7167-0344-0
  9. Ragep, F. Jamil (2001a). "Tusi and Copernicus: the Earth's motion in context". Science in Context (Cambridge University Press) 14 (1–2): 145–63.
  10. Ragep, F. Jamil (2001b). "Freeing astronomy from philosophy: an aspect of Islamic influence on science". Osiris, 2nd Series 16 (Science in theistic contexts: cognitive dimensions): 49–64 & 66–71.
  11. Reprinted as Appendix I in Dickson FP (1969). The bowl of night: the physical universe and scientific thought. Cambridge, MA: M.I.T. Press. ISBN 978-0262540032.
  12. Jean-Philippe de Cheseaux (1744). Traité de la Comète. Lausanne. pp. 223ff.. Reprinted as Appendix II in Dickson, above ref.
  13. Jeans J.H. 1902. Philosophical Transactions Royal Society of London, Series A, 199, 1.
  14. Einstein, A (1917). "Kosmologische Betrachtungen zur allgemeinen Relativitätstheorie". Preussische Akademie der Wissenschaften, Sitzungsberichte 1917 (part 1): 142–152.