Ptolemy

The culminating work of Greek astronomy is the Almagest of Claudius Ptolemaeus (2nd century CE). Ptolemy built on the work of his predecessors—notably Hipparchus—but his work was so successful that it made older works of planetary astronomy superfluous, and they ceased to be read and copied. An innovation that appears for the first time in the Almagest is the equant point. As in the planetary theories of Hipparchus’s day, a planet travels uniformly around its epicycle while the centre of the epicycle moves around Earth on an off-centre circle. But in Ptolemy’s theory the motion of the epicycle’s centre is nonuniform—it speeds up and slows down—which was a radical departure from Aristotelian physics. However, the nonuniformity is expressed in the language of uniformity: the epicycle’s centre moves in such a way that it appears to go through equal angles in equal times as viewed from another point distinct from Earth, the equant point. Though this may seem like an unnecessary complication, it was just what an explanation of planetary motion required (for, in the modern view, planets really do move nonuniformly). In Ptolemy, for the first time, Greek geometrical planetary theory finally achieved real numerical accuracy. Ptolemy’s theory actually predicted the behaviour of the planets, and it dominated the practice of astronomy for 1,400 years.

The Almagest contains an account of the observations and a description of the mathematical procedures that Ptolemy used to deduce the parameters of his theories. It also provides tables that allow the user to work out the position of a planet from theory for any desired date. The advantage of the tables is that Ptolemy has done all the trigonometry. One need only follow Ptolemy’s precepts, take numbers out of the various tables, and combine them to get an answer for a planet’s position. The Almagest includes trigonometric tables and a catalog of about 1,000 stars, which was probably based substantially on an earlier catalog by Hipparchus but with additions and modifications by Ptolemy. It also contains Ptolemy’s improvement on Hipparchus’s lunar theory. As an aid to convenient calculation, Ptolemy also composed the Procheiroi kanones (Handy Tables), in which the astronomical tables of the Almagest were expanded and accompanied by directions for using them but were stripped of the theoretical discussion.

Historians have long debated how much credit to give Ptolemy and how much to assign to his predecessors. For an ancient scientist, he was unusually generous in crediting his predecessors, particularly Hipparchus, for discoveries. But he did not always mention the origin of his ideas. In any case, Ptolemy’s publications fundamentally changed the way astronomy was done in the Greek world. In the period between Hipparchus and Ptolemy, Greek astronomers had struggled without great success to make geometrical planetary theory work. From the evidence of Greek-inspired astronomical works that later turned up in India, it has been conjectured that Greek astronomers before Ptolemy may have experimented with nonuniform motion—something akin to the equant—but nothing remains of a finished project before Ptolemy.

The garbage dumps of Oxyrhynchus in Greek Egypt have yielded large quantities of papyri, including planetary tables used for computing horoscopes. Most of this material is from the 1st through the 4th century CE. The papyri show the astrologers of Greek Egypt happily using Greek versions of Babylonian arithmetical theories for computing planet positions. This material is found side by side with papyri based on Ptolemy’s Handy Tables. Thus, in Greek astronomy there was a high road based on philosophy of nature and rooted in geometrical methods, and there was a low road based on the convenient arithmetical methods adapted from the Babylonians, even if these could not be considered to rest on adequate physical or philosophical foundations. These two methods still existed side by side up to the time of Ptolemy, and even a little after, but the newly successful (and convenient) geometrical methods gradually won out.

Ptolemy also wrote a speculative cosmological work, the Hypotheseis ton planomenon (Planetary Hypotheses), in which he took the eccentric-and-epicycle astronomy of the Almagest as physically true. However, to give a satisfactory image of the cosmos, he needed the nested-spheres cosmology of Eudoxus. The eccentrics and epicycles were regarded as the equator circles of three-dimensional orbs. Assuming that there was no wasted or empty space in the cosmos (consistent with both Aristotelian and Stoic physics), Ptolemy supposed that the mechanism for Mercury must lie immediately above the mechanism for the Moon. The mechanism for Venus came next, and so on, out to the mechanism for Saturn, and finally the sphere of the fixed stars. (Historians are not all agreed on whether the ancients regarded these mechanisms as real, physical objects that moved the planets or as merely theoretical constructions, but Ptolemy probably considered them as real things.) The known distance of the Moon provided the scale. When the numbers were worked out, the distance of the fixed stars was about 20,000 Earth radii. This is an enormous cosmos (though much smaller than modern estimates). Accepting such a conclusion, based on planetary astronomy and a few auxiliary physical premises, required a certain courage of imagination.