Eudoxus

• Outside the earth are concentric transparent spheres.
• They carry the moon, sun and planets.
• They rotate at different speeds about different axes.
• The picture illustrates the spheres for Mars.
• The outer sphere rotates once per Earth day: its motion makes Mars rise and set each day.
• The next sphere rotates once each Mars year. It carries Mars around the ecliptic once per Mars year.
• The next two spheres have almost opposite motions and (approximately) give Mars its retrograde motion (see picture below).
• Mars is on the innermost sphere, approximately between the viewer of this drawing and Earth.