Skip to main content Accessibility help
  • Print publication year: 2019
  • Online publication date: March 2019

2 - Two spheres: modeling the heavens and the Earth


The stars move from east to west across the sky each night. The ancient Greeks realized that the apparent movement of the stars would make sense if the stars were stuck on the inner surface of a giant celestial sphere that rotated around the Earth once every sidereal day. The Sun also moves from east to west across the sky, but not quite in the same way as the stars. The Sun’s motion can be tracked using shadows, and it appears to move eastward relative to the sphere of the stars along a path that is tilted relative to the celestial equator. The Sun completes its motion around the celestial sphere in one year, traveling through the constellations of the zodiac along a path called the ecliptic. As an observer moves around on Earth the apparent motions of the stars and Sun change in a way that shows the Earth to be spherical. The stars also display a very slow motion known as the precession of the equinoxes with a period of about 26,000 years.