The innermost planet in the Solar System is a dense, heavily cratered world that takes about 59 Earth days to fully rotate on its own axis as it travels on its 88-day journey around the Sun.
Find out more about the other planets in the Solar System
Photo: Mercury taken by the Messenger probe (NASA/JHU Applied Physics Laboratory/Carnegie Institution)
The closest planet to the Sun is hard to spot.
Learn how to remember all the planets of the Solar System in order.
Here’s an animated guide to remembering all the planets of the Solar System in order, using an easy mnemonic trick.
Enjoy Astronomer Marek Kukula's guide to the Solar System.
If Jupiter were much larger it would be a star in its own right! Enjoy Astronomer Marek Kukula's eloquent guide to the Sun, the planets and the outer reaches of the Solar System.
Tortured Mercury suffers the biggest temperature swings of all the planets.
Professor Brian Cox looks at Mercury, the smallest planet in the Solar System, and explains why it lost its atmosphere.
Sir Patrick Moore has doubts about Lowell's powers of observation.
Sir Patrick Moore describes how Percival Lowell mapped Mercury. Though he recognises that Lowell made many contributions to astronomy, Sir Patrick notes that his maps of other planets were not accurate.
BBC News's Christine McGourty reports on Messenger's mission.
In a 14 January 2008 BBC News report, science correspondent Christine McGourty reports on Messenger's mission at Mercury as the spacecraft prepares for its first flyby of the innermost planet in the Solar System. She speaks with Alison Boyle from London's Science Museum.
Mercury is the smallest and closest to the Sun of the eight planets in the Solar System,[a] with an orbital period of about 88 Earth days. Seen from Earth, it appears to move around its orbit in about 116 days, which is much faster than any other planet. It has no known natural satellites.[b] The planet is named after the Roman deity Mercury, the messenger to the gods.
Because it has almost no atmosphere to retain heat, 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). Mercury's axis has the smallest tilt of any of the Solar System's planets (about 1⁄30 of a degree), but it has the largest orbital eccentricity.[a] As such it does not experience seasons in the same way as most other planets such as Earth. At aphelion, Mercury is about 1.5 times as far from the Sun as it is at perihelion. Mercury's surface is heavily cratered and similar in appearance to the Moon, indicating that it has been geologically inactive for billions of years.
Mercury is gravitationally locked and rotates in a way that is unique in the Solar System. As seen relative to the fixed stars, it rotates exactly three times for every two revolutions[c] it makes around its orbit. 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 which 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.