Historically, there have been nine planets in the solar system: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto. However, in 2006, the International Astronomical Union decided to reclassify Pluto as a dwarf planet, reducing the official number of planets to eight.
Ancient civilizations knew about five of the planets because Mercury, Venus, Mars, Jupiter and Saturn are visible to the naked eye. Tracking the movements of these bodies, the ancient Greeks observed that they did not behave like the other "fixed" stars. Periodically, these bodies seemed to switch direction as they moved across the sky, and for a while, they would appear to move in a direction opposite to the other bodies in the system before resuming motion in the normal direction. This apparent retrograde motion is an illusion caused by viewing the movements of the planets from another moving planet, Earth. The remaining planets in the solar system are too distant and dim to be seen from Earth under normal conditions, and were identified with the use of a telescope. The last of the original nine, Pluto, was only discovered in 1930. The sun is at the center of the solar system, and as each planet rotates on its axis, it revolves around the sun.