A Modern Almagest

Ptolemy Today

The ancient science of the sky, retold with modern diagrams

The yearly motion

Over longer stretches of time (days, weeks, months) something else becomes apparent. While the stars keep the same positions relative to each other, the sun, moon, and planets slowly shift eastward against the background of stars. This motion is opposite to the daily rotation: it goes east rather than west. It is much slower, so it is hidden by the faster daily motion and only reveals itself if you observe patiently over time.

If this second motion simply moved the sun and planets eastward along the equator, one could imagine that the daily rotation was just a little slower for these bodies than for the stars. But that is not what happens. The sun, moon, and planets also drift north and south of the equator.

Figure 5 — The Sun, Moon, and Jupiter all share the daily east-to-west rotation with the fixed stars, but they also drift slowly eastward on their own. Watch the diagram for a moment and notice that the three wandering bodies appear both above and below the celestial equator at different times. Try pausing the daily rotation to isolate their slow, independent motion against the frozen stars. The Moon moves fastest; Jupiter barely moves at all.

The sun, moon, and planets do not simply lag behind the stars; they wander. They share a common direction of drift (eastward, opposite to the daily motion) and they all pass through roughly the same band of sky. But their speeds are different: the moon completes a full circuit in about a month, the sun in a year, and Jupiter in about twelve years. And all of them, as they travel, drift north and south of the equator in a regular pattern.

Wandering stars

The Ancient Greeks could refer to all celestial bodies as asteres, the root for the word astronaut. They called the bodies that move independently from the daily motion (the sun, the moon, and the planets) asteres planetai, which is where the English word planet comes from. Planets don't have their own light like stars, but they look like stars to the naked eye, if slightly brighter. For that reason, Greeks would often refer to "regular" stars as "fixed stars" (the stars "attached" to the celestial sphere, making one rotation a day) and the planets as "wandering stars", or simply "wanderers".

Several things about this wandering turn out to be true, though some will only be demonstrated in later chapters:

  1. The north-south drift is not random; it follows a smooth, repeating pattern.
  2. This pattern can be explained if the wandering bodies are all moving along a single circle that is tilted at an angle to the equator.
  3. This tilted circle is the same for all of them: the Sun traces it most precisely, while the Moon and planets travel close to it but can stray slightly to either side, each within its own narrow band.
  4. The Sun drifts equally far north of the equator as it does south: which means this tilted circle must be a great circle, dividing the celestial sphere into two equal halves, just as the equator does.

All of this points to the existence of a second fundamental circle on the celestial sphere: a great circle tilted to the equator, shared by all the wandering bodies, precisely traced by the Sun. This circle is called the ecliptic, and understanding it is the next step.