Aristotle's influence in learned circles was such that even centuries after his death, he was known simply as the philosopher. Aristotle saw the cosmos divided into two realms, the terrestrial and the celestial. The terrestrial realm included the Earth, the Moon, and the space between them, called the Sublunar region. The terrestrial realm was marked by change in imperfection. The celestial realm was the region above the moon. Here there was complete order and perfection. Aristotle's model showed the planets in the celestial realm moving around the Earth in an orderly manner in perfect circles and with uniform motion, neither speeding up nor slowing down. As a philosophy, this model worked very well. However, it did not explain why planets appeared to slow down and speed up in their movements. Nevertheless, Aristotle's universe was the accepted model until the 17th century. Aristotle's model of the universe had trouble explaining some planetary phenomena. The most striking of these was retrograde. Action in retrograde motion. Each planet seems to slow down at times, then move in reverse or retrograde before resuming its course. Planets also grow brighter or dimmer as they move through the sky. Aristotle's model could account for neither phenomena very well. The most important solution to this problem was proposed by Claudius Toleman in the 3rd century AD. He argued that planets move on two sets of circles, a different and an epicycle. This explained retrograde motion while keeping the planets in their circular orbits around the Earth. Where this did not fit, Toleman proposed an eccentric an eccentric orbit, had a center different from the Earth, and accounted well for changes in a planet's brightness. Tolly's last device was the equant. In an equal to planet speed it up and slowed down.