‘Wanderers’ and the problem of retrograde motion
The most basic problem with the core conception just outlined is the apparent 'retrograde motion' of the five planets.
Planets as ‘wanderers’. Against the backdrop of the innumerable fixed stars, closer study of the night sky reveals seven anomalies: heavenly bodies which are not ‘fixed’ in this way. When viewed on successive nights, their apparent position changes against the backdrop of the fixed stars. Because they are not fixed, the Greeks called them πλανήτης or planētēs, meaning ‘wanderers’.
All of these planets ‘wander’ across a narrow band of the sky known as the ‘ecliptic’. It was to track the positions of these ‘wanderers’ that the zodiac was divided into twelve segments, each identified by its constellation of fixed stars. The animation below recreates the motion of the planets in a portion of the sky as observed from earth between 27 June 2008 and 22 December 2012.
The simple motion of the Sun and Moon. The two brightest and largest of these planets, when viewed from earth, move in a very simple manner. Every night, the Moon rises about 50 minutes earlier than the night before, and it therefore races through the zodiac every 29 days. The Sun moves through the zodiac at a slower pace, taking 365 days to circle the heavens. Moreover, like the fixed stars, these two great planets always move in the same direction at virtually uniform speed (variations in their apparent speed can only be detected with careful study).
The retrograde motion of the other five planets. The other five planets move in a much more complicated and baffling manner. Their average motion (viewed over the space of a year) is always in the same direction as the Sun and Moon: from east to west along the ecliptic (that is, from upper right to lower left in this animation). But at slightly irregular intervals, these five planets go into what is called ‘retrograde motion’: their apparent motion from right to left slows, stops, and then reverses for a while before slowing, stopping again, and resuming their apparent motion from east to west. Moreover (as indicated in the image above), it is while these five planets are in this retrograde motion that they appear brightest from earth.
The retrograde motion of these five planets was the most puzzling problem confronting astronomers for millennia. Studying the animation below is an excellent means of understanding the phenomenon they were attempting to model and explain.
The pattern of retrograde motion for each planet is unique. The swiftest (Mercury) and the slowest (Saturn) display retrograde motion repeatedly during this interval captured in this animation; the other planets only once (or not at all in the case of Jupiter, which is not visible in this portion of the night sky for years at a time). Here is a guide to help spot all ten instances of retrograde motion depicted:
- 0:08-0:11 Mercury
- 0:18-0:33 Saturn
- 0:44-0:46 Mercury
- 0:57-1:11 Saturn
- 01:18-02:20 Mercury
- 01:24-01:27 Venus
- 01:34-01:48 Saturn
- 02:10 Mars
- 02:12-02:25 Saturn
- 02:40-02:42 Mercury
The single greatest challenge confronting astronomers from antiquity to the seventeenth century was to model this strange behaviour mathematically and to explain it physically.
Commentary. Howard Hotson (January 2024)