The word planet is from the Greek planete , meaning “wanderer” . All of the known planets moved against the background stars; the historically-known planets included Mercury, Venus, Mars, Jupiter, and Saturn (and Earth, of course). Mercury and Venus stayed close to the Sun. Mars, Jupiter, and Saturn could be tracked as they moved across the sky. But occasionally the planets were observed to move backwards against the stars. This phenomenon is called retrograde .At the time, the thinking was that the Earth was the center of the Solar System and even the Universe. So the Sun, Moon, and known planets revolved around Earth – called the Geocentric Solar System ; Geo means Earth. How could this retrograde motion occur if Earth was the center of the Solar System and even the Universe? Thus, the planets presented problems.
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