Astronomy in the Classical Age
According to the astronomers of ancient Greece, the Earth was the center of the Universe. The Moon, Sun, stars, and five known planets - Mercury, Venus, Mars, Jupiter, and Saturn - revolved around the Earth in great big circles. This view of the Universe is called a "geocentric cosmology."
To the ancient Greeks in a time when telescopes had not yet been invented, the planets were pinpoints of light that looked like the other stars. However, unlike the stars, they changed position from night to night, usually eastward compared to the background stars. The word "planet" means "wandering star."
The Greeks also saw that the planets didn't always move in the same direction. Sometimes they would slow down, stop, and even move westward for a short time (weeks to months) before resuming their usual eastward direction. This backward movement is called "retrograde motion."
To explain the motion of the planets, Greek astronomer Hipparchus proposed a system of epicycles, which are smaller circles that travel along an overall big circle called a deferent. A planet that had stopped moving forward was simply moving in the reverse direction of its epicycle. Ptolemy, one of the last great Greek astronomers, painstakingly worked out every last detail about these epicycles - how big they were, how fast the planets moved on them, how fast the epicycles moved on their deferents, how big the deferents were, and so on. He published his work in the 13-volume Almagest in about 150 CE, and it became the established cosmology for the next thousand years.