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 Earthdays. Seen from Earth, it appears to move around its orbit in about 116 days, which is much faster than any other planet. This rapid motion may have led to it being named after the Roman deity Mercury, 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 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[b] 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’s orbit lies within Earth’s orbit (as does Venus‘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 of phases as it moves around its orbit relative to 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. Two spacecraft have visited Mercury: Mariner 10 flew by in the 1970s and MESSENGER, launched in 2004, remains in orbit.
Mercury in Babylonian Astronomy
The earliest known recorded observations of Mercury are from the Mul.Apin tablets. These observations were most likely made by an Assyrian astronomer around the 14th century BC. The cuneiform name used to designate Mercury on the Mul.Apin tablets is transcribed as Udu.Idim.Gu\u4.Ud (“the jumping planet”).[f] Babylonian records of Mercury date back to the 1st millennium BC. The Babylonians called the planet Nabu after the messenger to the gods in their mythology.
Mercury in Greek Astonomy
The ancient Greeks of Hesiod‘s time knew the planet as Στίλβων (Stilbon), meaning “the gleaming”, and Ἑρμάων (Hermaon). Later Greeks called the planet Apollowhen it was visible in the morning sky, and Hermes when visible in the evening. Around the 4th century BC, Greek astronomers came to understand that the two names referred to the same body, Hermes (Ἑρμής: Hermēs), a planetary name that is retained in modern Greek (Ερμής: Ermis). The Romans named the planet after the swift-footed Roman messenger god, Mercury (Latin Mercurius), which they equated with the Greek Hermes, because it moves across the sky faster than any other planet. The astronomical symbol for Mercury is a stylized version of Hermes’ caduceus.
The Roman-Egyptian astronomer Ptolemy wrote about the possibility of planetary transits across the face of the Sun in his work Planetary Hypotheses. He suggested that no transits had been observed either because planets such as Mercury were too small to see, or because the transits were too infrequent.
Mercury in Chineese Astrology
In ancient China, Mercury was known as Chen Xing (辰星), the Hour Star. It was associated with the direction north and the phase of water in the Wu Xing.Modern Chinese, Korean, Japanese and Vietnamese cultures refer to the planet literally as the “water star” (水星), based on the Five elements. Hindu mythologyused the name Budha for Mercury, and this god was thought to preside over Wednesday.
Mercury in Norse Astrology
Mercury in Mayan Astrology
Mercury in Ancient Indian Astronomy
The ancient association of Mercury with Wednesday is still visible in the names of Wednesday in various modern languages of Latin descent, e.g. mercredi in French, miércoles in Spanish, or miercuri in Romanian. The names of the days of the weekwere, in classical times, all related to the names of the seven bodies that were then considered to be planets.
In ancient Indian astronomy, the Surya Siddhanta, an Indian astronomical text of the 5th century, estimates the diameter of Mercury as 4,841 kilometres (3,008 mi), an error of less than 1% from the currently accepted diameter of 4,880 kilometres (3,032 mi). This estimate was based upon an inaccurate guess of the planet’s angular diameter as 3.0 arcminutes (50 millidegrees).
Mercury in Medieval Islamic Astronomy
In medieval Islamic astronomy, the Andalusian astronomer Abū Ishāq Ibrāhīm al-Zarqālī in the 11th century described the deferent of Mercury’s geocentric orbit as being oval, like an egg or a pignon, although this insight did not influence his astronomical theory or his astronomical calculations. In the 12th century, Ibn Bajjah observed “two planets as black spots on the face of the Sun”, which was later suggested as the transit of Mercury and/or Venus by the Maragha astronomer Qotb al-Din Shirazi in the 13th century. (Note that most such medieval reports of transits were later taken as observations of sunspots.)
In India, the Kerala school astronomer Nilakantha Somayaji in the 15th century developed a partially heliocentric planetary model in which Mercury orbits the Sun, which in turn orbits Earth, similar to the Tychonic system later proposed by Tycho Brahe in the late 16th century.