Overview

Today we know that the planets are nine large spherical objects that rotate around the Sun and reflect light from the Sun and that Earth is a planet. This knowledge has come from observations and theories made and refined over the centuries and is quite different from the concept of planets held by the ancients. Indeed, some of the planets we know today were not known in ancient times because they are not visible without the use of a telescope.

The so-called fixed stars hold constant positions relative to each other and all move together in a regular manner. During each night they appear to travel together eastward across the sky. Each morning at sunrise their positions have shifted slightly eastward in the sky relative to the day before so that a star's sunrise position seems to...