Mercuryis the smallest and closest to theSunof the eightplanetsin theSolar System,with anorbital periodof about 88Earthdays. Seen from the Earth, it appears to move around itsorbitin about 116 days, which is much faster than any other planet. This rapid motion may have led to it being named after theRoman deityMercury, the fast-flying 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 from100 K(173C; 280F)at night to700 K(427C; 800F)during the day at some equatorial regions. The poles are constantly below180 K(93C; 136F). Mercury's axis has the smallesttiltof any of the Solar System's planets (about1⁄30of a degree), but it has the largestorbital eccentricity.Ataphelion, Mercury is about 1.5 times as far from the Sun as it is atperihelion. Mercury's surface is heavily cratered and similar in appearance to theMoon, indicating that it has been geologically inactive for billions of years.
Mercury does not experience seasons in the same way as most other planets, such as the Earth. It is locked so it rotates in a way that is unique in the Solar System. As seen relative to thefixed stars, it rotates exactly three times for every two revolutionsit makes around its orbit. As seen from the Sun, in aframe of referencethat 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's orbit lies within Earth's orbit (as doesVenus's), 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 ofphasesas it moves around its orbit relative to the Earth. Although Mercury can appear as a very bright object when viewed from Earth, its proximity to the Sun makes it more difficult to see than Venus.