What is an inferior planet?

Since some of the planets of our Solar System are so much bigger than others, we might think that the inferior planets are the small ones. But Mars, smaller than earth, and tiny Pluto rate as superior planets, while Mercury rates as an inferior planet. It seems that size does not make a planet inferior or superior.

According to the astronomers, the inferior planets are closer to the sun than is the earth. Mercury and Venus are the inferior planets. The superiors are those further from the sun than we are    bars, Jupiter, Saturn, Neptune, Uranus and little faraway Pluto.