The motion of the planets

Greek thinking about the motion of the planets began by about 400 BCE. Eudoxus of Cnidus constructed the first Greek theory of planetary motion of which any details are known. In a book, On Speeds (which is lost but was briefly discussed by Aristotle and Simplicius), Eudoxus regarded each celestial body as carried on a set of concentric spheres, which nest one inside another. For each planet, three different motions must be accounted for, and Eudoxus proposed to do this with four spheres. The daily revolution to the west is accounted for by the outermost sphere (1). Next inside is sphere 2, whose axle fits into sphere 1 at an offset of about 24°; sphere 2 turns to the east in the planet’s zodiacal period (12 years for Jupiter, 30 years for Saturn). The third motion is retrograde motion. For this, Eudoxus used a combination of two spheres (3 and 4). The planet itself rides on the equator circle of sphere 4. The axle of 4 fits inside sphere 3 with a slight angular offset. Spheres 3 and 4 turn in opposite directions but at the same speed. The motion of the planet resulting from the gyrations of spheres 3 and 4 is a figure eight, which lies in the spherical surface. Eudoxus likely understood the mathematical characteristics of this curve, as he gave it the name hippopede (horse fetter). The two-sphere assembly of 3 and 4 is inserted into the inner surface of sphere 2. Thus, all three motions are accounted for, at least qualitatively: the daily motion to the west by sphere 1, the slow motion eastward around the zodiac by sphere 2, and the occasional retrograde motion by the two-sphere assembly of 3 and 4. Eudoxus’s theory is sometimes called the theory of homocentric spheres, as all the spheres have the same centre, Earth.

At this stage, Greek astronomers were more interested in providing plausible physical accounts of the universe and in proving geometrical theorems than in providing numerically accurate descriptions of planetary motion. Eudoxus’s successor Callippus made some improvements to the model. Nevertheless, the homocentric spheres were criticized for their failure to account for the fact that some planets (notably Mars and Venus) are much brighter at some times of their cycles than at others. Eudoxus’s system was soon abandoned as a theory for the motion of the planets, but it exerted a profound influence in cosmology, for the cosmos continued to be regarded as a set of concentric spheres until the Renaissance.

Late in the 3rd century BCE, alternative theoretical models were developed, based on eccentric circles and epicycles. (An eccentric circle is a circle that is slightly off-centre from Earth, and an epicycle is a circle that is carried and rides around on another circle.) This innovation is usually attributed to Apollonius of Perga (c. 220 BCE), but it is not conclusively known who first proposed these models. In considering the Sun’s motion, Eudoxus’s theory of homocentric spheres ignored the fact that the Sun appears to speed up and slow down in the course of the year as it moves around the zodiac. (This is clear from spring’s being several days longer than fall.) An eccentric (i.e., off-centre) circle can explain this fact. The Sun is still considered to travel at constant speed around a perfect circle, but the centre of the circle is slightly displaced from Earth. When the Sun is closest to Earth, it appears to travel a little more rapidly in the zodiac. When it is farthest away, it appears to travel a little more slowly. As far as is known, Hipparchus was the first to deduce the amount and direction of the off-centredness, basing his calculations on the measured length of the seasons. According to Hipparchus, the off-centredness of the Sun’s circle is about 4 percent of its radius. The eccentric-circle theory was capable of excellent accuracy in accounting for the observed motion of the Sun and remained standard until the 17th century.

The standard theory of the planets involved an eccentric circle, which carried an epicycle. Imagine looking down on the plane of the solar system from above its north pole. The planet moves counterclockwise on its epicycle. Meanwhile, the centre of the epicycle moves counterclockwise around the eccentric circle, which is centred near (but not quite exactly at) Earth. As viewed from Earth, the planet will appear to move backward (that is, go into retrograde motion) when it is at the inner part of the epicycle (closest to Earth), for this is when the westward motion of the planet on the epicycle is more than enough to overcome the eastward motion of the epicycle’s centre forward around the eccentric.

Hipparchus played a major role in introducing Babylonian numerical parameters into Greek astronomy. Indeed, an important shift in Greek attitudes toward astronomy occurred about this time. The Babylonian example served as a sort of wake-up call to the Greeks. Previous Greek planetary thinking had been more about getting the right big picture, based on philosophical principles and geometrical models (whether using Eudoxus’s concentric spheres or Apollonius’s epicycles and eccentrics). The Babylonians had no geometrical models but instead focused on devising arithmetical theories that had real predictive power. Hipparchus achieved numerically successful geometrical theories for the Sun and the Moon, but he did not succeed with the planets. He contented himself with showing that the planetary theories then in circulation did not agree with the phenomena. Nevertheless, Hipparchus’s insistence that a geometrical theory, if it is true, ought to work in detail marked a major step in Greek astronomy.

Another of Hipparchus’s contributions was the discovery of precession, the slow eastward motion of the stars around the zodiac caused by wobbling, over a period of 25,772 years, in the orientation of Earth’s axis of rotation. Hipparchus’s writings on this subject have not survived, but his ideas can be reconstructed from summaries given by Ptolemy. Hipparchus used observations of several fixed stars, taken with respect to the eclipsed Moon, which had been made by some of his predecessors. On comparing these with eclipse observations he had made himself, he deduced that the fixed stars move eastward not less than 1° in 100 years. The Babylonians, in their theories, revised their locations of the equinoxes and solstices. For example, in one version of the Babylonian theory, the spring equinox is said to occur at the 10th degree of Aries; in another version, at the 8th degree. Some historians have maintained that this reflects a Babylonian awareness of precession, on which Hipparchus might have drawn. Other historians have argued that the evidence is not clear and that these differing norms for the equinox may represent nothing more than alternative conventions.