The motion of the Earth around the Sun has a major effect on the apparent motions of the other planets, especially those at a greater distance from the Sun. According to Kepler's Laws the greater the distance of the planet from the Sun, the slower its speed in its orbit. This results in the Earth overtaking the superior planets at regular intervals (the synodic period). When the Earth passes one of the superior planets the line of sight to the planet moves backwards with respect to the fixed stars in the background and the planet appears to move backwards. This motion was considered to be a real motion rather than an observed phenomena by ancient astronomers and was a complicating factor in geocentric models of the universe.