inferior planet

inferior planet, planet whose orbit lies inside that of the earth. There are two inferior planets, Mercury and Venus. They always seem to be close to the sun in the sky; the greatest elongation of Mercury is 28°, and that of Venus, 47°. For this reason, neither can be seen for more than two or three hours after sunset or before sunrise.

The terms "inferior planet" and "superior planet" were originally used in the Ptolemaic cosmology to differentiate those planets (Mercury and Venus) that were between the stationary Earth and the orbiting Sun from those planets (Mars, Jupiter, and Saturn), which lay beyond the Sun's orbit.

In the 16th century, the terms were modified by Copernicus, who rejected Ptolemy's geocentric model, to distinguish a planet's orbit's size in relation to the Earth's.

• "Inferior planet" is used in reference to Mercury and Venus, which are closer to the Sun than the Earth is.
• "Superior planet" is used in reference to Mars, Jupiter, Saturn, Uranus, Neptune, and all the known minor and dwarf planets, including Ceres and Pluto, which are farther from the Sun than the Earth is.

The terms are sometimes used more generally: for instance, the Earth is an inferior planet as seen from Mars.

This classification should not be confused with the terms inner and outer planet, which designate those planets which lie inside the asteroid belt and those that lie outside it, respectively. "Inferior planet" should also not be confused with the very different minor planet or dwarf planet.