Mercur is the smawest an closest tae the Sun o the aicht planets in the Solar Seestem,[lower-alpha 1] wi an orbital period o aboot 88 Yird days. Seen frae the Yird, it appears tae move aroond its orbit in aboot 116 days, which is much faster than ony ither planet. This rapid motion mey hae led tae it bein named efter the Roman deityMercur, the fast-flyin messenger tae the gods. Acause it haes almaist na atmosphere tae retain heat, Mercury's surface experiences the greatest temperatur variation o aw the planets, rangin frae 100 K (−173 °C; −280 °F) at nicht tae 700 K (427 °C; 800 °F) durin the day at some equatorial regions. The poles are constantly belaw 180 K (−93 °C; −136 °F). Mercury's axis haes the smawest tilt o ony o the Solar Seestem's planets (aboot 1⁄30 o a degree), but it haes the lairgest orbital eccentricity.[lower-alpha 1] At aphelion, Mercur is aboot 1.5 times as far frae the Sun as it is at perihelion. Mercur's surface is hivily cratered an similar in appearance tae the Muin, indicatin that it haes been geologically inactive for billions o years.
Mercur daes nae experience saisons in the same way as maist ither planets, such as the Yird. It is locked so it rotates in a way that is unique in the Solar Seestem. As seen relative tae the fixed starns, it rotates exactly three times for every twa revolutions[lower-alpha 2] it maks aroond its orbit. As seen frae the Sun, in a frame o reference that rotates wi the orbital motion, it appears tae rotate anly ance every twa Mercurian years. An observer on Mercur would tharefore see anly ane day every twa years.
Acause Mercur's orbit lies within Yird's orbit (as daes Venus's), it can appear in Yird's sky in the mornin or the evenin, but nae in the middle o the nicht. An aa, like Venus an the Muin, it displays a complete range o phases as it moves aroond its orbit relative tae the Yird. Although Mercur can appear as a very bricht object when viewed frae Earth, its proximity tae the Sun maks it mair difficult tae see than Venus.
↑ 1.01.1Pluto was considered a planet from its discovery in 1930 to 2006, but after that it has been classified as a dwarf planet. Pluto's orbital eccentricity is greater than that of Mercury. Pluto is also smaller than Mercury, but was assumed to be larger until 1976.
↑In astronomy, the words "rotation" and "revolution" have different meanings. "Rotation" is the turning of a body about an axis which passes through the body, as in "The Earth rotates once a day.". "Revolution" is motion around a centre which is external to the body, usually in orbit, as in "The Earth takes a year for each revolution around the Sun.". The verbs "rotate" and "revolve" mean doing rotation and revolution, respectively.