Kinematical Models in Astronomy

Early on, the Greeks explained the motion of the stars by assuming that they are all attached to a sphere that rotates uniformly about a fixed axis, while the immovable earth is at the center of the sphere. However, explaining the motion of the Sun, the Moon, and the five planets posed a difficult problem because their motions are not uniform. By combining uniform circular motions in a sophisticated way, the Greek astronomers succeeded in defining kinematic models that were consistent with the observations.