Mercury |
Smallest and closest to the Sun of the eight planets in the Solar System. |
Orbital period |
88 Earth days |
Satellites |
It has no known natural satellites. |
Etymology |
Named after the Roman deity Mercury, the messenger to the gods. |
Atmosphere |
it has almost no atmosphere. |
Temperature |
Mercury's surface experiences the greatest temperature variation of all the planets, ranging from 100 K (−173 °C; −280 °F) at night to 700 K (427 °C; 800 °F) during the day at some equatorial regions. The poles are constantly below 180 K (−93 °C; −136 °F). |
Tilt |
Mercury's axis has the smallest tilt(about 1⁄30 of a degree) of any of the Solar System's planets. |
Rotation |
It rotates in a unique way in the Solar System. As seen relative to the fixed stars, it rotates on its axis exactly three times for every two revolutions. As seen from the Sun, in a frame of reference that rotates with the orbital motion, it appears to rotate only once every two Mercurian years. An observer on Mercury would therefore see only one day every two years. |
- Because Mercury moves in an orbit around the Sun that lies within Earth's orbit (as does Venus), it can appear in Earth's sky in the morning or the evening, but not in the middle of the night.
- Also, like Venus and the Moon, it displays a complete range of phases as it moves around its orbit relative to Earth.
- Although Mercury can appear as a bright object when viewed from Earth, its proximity to the Sun makes it more difficult to see than Venus.
- Two spacecraft have visited Mercury: Mariner 10 flew by in the 1970s and MESSENGER, launched in 2004, remains in orbit.