Cambridge University Press 978-0-521-89994-9 - Encyclopedia of the History of Astronomy and Astrophysics David Leverington Excerpt More information PART 1 GENERAL ASTRONOMY © in this web service Cambridge University Press www.cambridge.org Cambridge University Press 978-0-521-89994-9 - Encyclopedia of the History of Astronomy and Astrophysics David Leverington Excerpt More information © in this web service Cambridge University Press www.cambridge.org Cambridge University Press 978-0-521-89994-9 - Encyclopedia of the History of Astronomy and Astrophysics David Leverington Excerpt More information 1 r Ancient (pre-telescopic) astronomy ANCIENT ASTRONOMY OVERVIEW the last month. When Upper and Lower Egypt were unified in about 3000 BC the Upper Egyptian scheme was used. Pre-telescopic astronomy is considered in this survey article Astrology was the main driver behind early Babylonian in four broad geographical areas based on cultural contacts, astronomy of the Hammurabi dynasty of about 1895 to 1595 regional developments and astronomical traditions. This arti- BC, particularly that connected with the visibility of the Moon cle is then followed by individual, more detailed articles as and planets (see Babylonian astronomy). This required the indicated in the text. priests to carefully observe the motions of these bodies across the sky, so they could be analysed and predictions made of their Africa, Europe, Middle East and India future positions. Over the centuries, the Babylonians developed Not only have the earliest human remains been found in Africa, many techniques for these predictions, in the process produc- but so has the earliest physical evidence of astronomical activity. ing highly accurate estimates of the synodic and sidereal periods This appears to be a lunar calendar found on an Ishango bone of the Sun, Moon and planets. Those of Jupiter, for example, which has been dated at about 23,000 to 18,000 BC (see Sub- in the second century BC, were accurate to within 0.01%. Saharan Africa). The earliest megalithic structure that appears The Babylonians saw the sky in two dimensions, whereas to be of astronomical significance is that at Nabta Playa, also the Greeks, who were philosophers, saw it in three (see Greek in Africa, which has been dated to around 5000 to 4000 BC astronomy). They wanted to understand the structure of the (see Megalithic astronomy). The earliest parts of Stonehenge in universe and explain why the celestial objects moved in the England, on the other hand, are at least 1,000 years younger, way they did. Thales of Miletus was apparently the first to as is the passage tomb at Newgrange in Ireland. do this in the sixth century BC, and he is thought by some There are thousands of megaliths all over the world, some ancient sources to have discovered the cause of eclipses. A of which have astronomical significance, built by civilisations little later, the Pythagoreans hypothesised that the universe of widely different cultures. These civilisations had no contact consisted of a spherical, non-spinning Earth surrounded by a with one another, as far as we know. But that is not true of series of concentric, crystalline spheres that carried the Sun, ancient Egypt, which was later linked to Babylon and Greece. Moon, planets and stars. In these three civilisations we have written records to assist In the fifth century BC, Hicetas of Syracuse suggested that our understanding, which are missing for the early megalithic the Earth spun on its axis, but this idea was generally rejected. builders. About two hundred years later Aristarchus of Samos suggested The Egyptians had studied the sky since before 3000 BC for that the Sun, not the Earth, was at the centre of the universe, both religious and practical reasons (see Egyptian astronomy). but this idea was also generally ignored until Copernicus res- Their most important gods were the Sun god, Ra, and Nut, urrected it about 1,700 years later. the goddess of the sky, who was depicted stretched across the Many of the later Greeks, including Hipparchus and sky as the Milky Way. The heliacal rising of Sirius (Sothis) was Ptolemy, lived in Egypt, where Eratosthenes measured the also important to the Egyptians, as they used it to predict the diameter of the Earth to a few percent of its correct value in annual Nile floods. the third century BC. In the next century, Hipparchus esti- The earliest known Egyptian calendar, which was used mated the rate of precession of the equinoxes and produced for both religious and agricultural purposes, consisted of the most accurate star catalogue to date. Three hundred years 12 months of 29 or 30 days divided into three 4-month seasons later, Ptolemy wrote his Almagest, which included his star cat- of ‘inundation’, or flooding of the Nile, ‘growth’ and ‘harvest’. alogue, and Planetary Hypotheses, in which he proposed a geo- An extra month was added every two or three years in Lower centric model of the universe based on epicycles and equants. Egypt to ensure that the festival of the birth of Ra occurred in Its largest deficiency was probably that it predicted that the the last month of the year. The same was done in Upper Egypt Moon’s apparent diameter, as seen from Earth, would vary by to ensure that the festival of the heliacal rising of Sothis was in a factor of two, which it clearly did not do. 3 © in this web service Cambridge University Press www.cambridge.org Cambridge University Press 978-0-521-89994-9 - Encyclopedia of the History of Astronomy and Astrophysics David Leverington Excerpt More information 4 Encyclopedia of the history of astronomy and astrophysics It is unclear how much Babylonian work was known to the Aristotle’s philosophy was taught in European universities Greeks. But the victory of Alexander the Great over the Persian in the Middle Ages. That, and the constraints placed on astron- empire in 331 BC certainly improved communications between omy by the teachings of the Christian Church, had a profound Greece and Babylon for a while, until the Seleucid empire col- effect on the development of the subject. But some thinkers lapsed in the following century. In the case of India, however, attacked various of Aristotle’s teachings. For example, in the there had been some contact between India and Babylon prior fourteenth century, Thomas Bradwardine attacked the Aris- to Darius I’s conquest of north-west India in 515 BC, but this totelian idea that the universe was finite in size, and Jean Buri- conquest naturally produced a greater influx of Babylonian dan dismissed Aristotle’s idea that the planets were in motion ideas into India (see Indian astronomy). So in the fifth century only because they were each subjected to a continuous force. BC, for example, Indian astronomy was a mixture of indige- Instead, Buridan suggested that the planets had been set in nous astronomy, which involved the sky being divided up into motion at their creation and were still moving as they were 27 lunar mansions, and methods for predicting the movement subject to no resistance. of celestial bodies which clearly came from the Babylonians. In 1543, Copernicus published De Revolutionibus Orbium In 326 BC, Alexander the Great brought Greek influence Caelestium in which he described his heliocentric theory of the to India when he conquered the north-west of the country. universe. His idea of a spinning Earth in a heliocentric uni- Over the next few centuries, Indians translated various Greek verse was not new, having been proposed by Aristarchus in the and Babylonian works into Sanskrit. Then, in about 425 AD, third century BC. But the time was now ripe in a Renaissance Indian astronomers devised a new model of the solar system that was eager for new ideas. Copernicus’ theory was based that eliminated Ptolemy’s equant. A little later Aryabhata wrote on circular motion and, like Ptolemy’s theory, depended on a book, later called the Aryabhatiya, in which he recognised that epicycles, although he had deleted the equant. But Copernicus the apparent rotation of the heavens was caused by the Earth had broken with the Aristotelian concept of a non-spinning rotating on its axis. He produced an accurate estimate of the Earth at the center of the universe. Then in 1577 Tycho Brahe size of the Earth, and determined the length of the sidereal year disproved another of Aristotle’s ideas. Aristotle had believed to within just 4 minutes. that comets were in the Earth’s atmosphere, but Tycho was The prophet Muhammad was born in about 570. Over the unable to measure any clear parallax for the comet of 1577. So next 150 years the Islamic empire expanded to include Spain it could not have been in the atmosphere, and must have been in the west, and north-west India in the east (see Islamic astron- appreciably further away than the Moon. omy). The caliph Al Mansur¯ decided in 762 to move his capital to Baghdad. He then started a major attempt to obtain copies China and Japan of as many astronomical texts as possible and have them trans- lated into Arabic. This resulted in many Indian and Greek A rock carving on a cliff at Jiangjumya, which depicts the texts being translated. As a result, early Islamic astronomy was Milky Way, shows that Chinese astronomy dates back to at a mixture of indigenous work, together with Indian, Persian least 2000 BC (see Chinese astronomy). Oracle bone fragments and Greek astronomy. from An Yang show that by 1400 BC the Chinese had adopted a In the tenth century, Abd al-Rahman al-Suf¯ ¯ı produced the lunisolar calendar, which consisted of 12 months of alternately first significant revision of Ptolemy’s star catalogue, followed a 29 and 30 days.