FIGURE 3.5 Retrograde motion. In modern heliocentric theory, the relative motions of the planets around the Sun causes the outer planets, those further away from the Sun than Earth, to appear to move backwards relative to the fixed stars on a regular basis. Note also that the inner planets are never seen at a large angular separation from the Sun. These two features were well known even to prehistoric observers, and any explanation of the structure of the solar system had to explain these motions. SOURCE: Abell, 1969.