The ancient Greeks turned astronomy into a science. Ptolemy, who was born around AD100, published his Almaghest in AD140. This was an encyclopedia of the patterns of the stars and planets, and he used it to support his argument that the Earth was at the centre of the Universe. His “system of the world” claimed that surrounding Earth were seven transparent spheres, each containing a moving object such as the Sun, the Moon, or a planet. An eighth sphere, which surrounded everything, held the stars. His theory, which we now know to be incorrect, was a very accurate way of predicting the motions of the planets, and was the dominant theory in astronomy for over 1 000 years.
Virtually nothing is known about Ptolemy’s life except what can be inferred from his writings. His first major astronomical work, the Almagest, was completed about 150 CE and contains reports of astronomical observations that Ptolemy had made over the preceding quarter of a century. The size and content of his subsequent literary production suggests that he lived until about 170 CE.
The book that is now generally known as the Almagest (from a hybrid of Arabic and Greek, “the greatest”) was called by Ptolemy he mathematic syntaxes (“The Mathematical Collection”) because he believed that its subject, the motions of the heavenly bodies, could be explained in mathematical terms. The opening chapters present empirical arguments for the basic cosmological framework within which Ptolemy worked. Earth, he argued, is a stationary sphere at the centre of a vastly larger celestial sphere that revolves at a perfectly uniform rate around Earth, carrying with it the stars, planets, Sun, and Moon—thereby causing their daily risings and settings. Through the course of a year the Sun slowly traces out a great circle, known as the ecliptic, against the rotation of the celestial sphere. (The Moon and planets similarly travel backward—hence, the planets were also known as “wandering stars”—against the “fixed stars” found in the ecliptic.) The fundamental assumption of the Almagest is that the apparently irregular movements of the heavenly bodies are in reality combinations of regular, uniform, circular motions.
How much of the Almagest is original is difficult to determine because almost all of the preceding technical astronomical literature is now lost. Ptolemy credited Hipparchus (mid-2nd century BCE) with essential elements of his solar theory, as well as parts of his lunar theory, while denying that Hipparchus constructed planetary models. Ptolemy made only a few vague and disparaging remarks regarding theoretical work over the intervening three centuries, yet the study of the planets undoubtedly made great strides during that interval. Moreover, Ptolemy’s veracity, especially as an observer, has been controversial since the time of the astronomer Tycho Brahe (1546–1601). Brahe pointed out that solar observations Ptolemy claimed to have made in 141 are definitely not genuine, and there are strong arguments for doubting that Ptolemy independently observed the more than 1,000 stars listed in his star catalog. What is not disputed, however, is the mastery of mathematical analysis that Ptolemy exhibited.
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