Mercury is the smallest and closest to the Sun of the eight planets in the Solar System, with an orbital period of about 88 Earth days. Seen from the 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.
In Western astrology, Mercury is the ruling planet of Gemini and Virgo. That is, the supposed astrological influence of the planet was greatest when it was observed in these constellations.
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 week were, in classical times, all related to the names of the seven bodies that were then considered to be planets.
Since Mercury 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. Mercury’s surface is heavily cratered and similar in appearance to Earth’s Moon, indicating that it has been geologically inactive for billions of years.
Mercury’s period of revolution around the Sun is 88 days. It therefore makes about 4.15 revolutions around the Sun in one Earth-year.
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 the fixed stars, it rotates exactly three times for every two revolutions 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.
One year passes during each night, while the Sun is below the horizon, so the surface temperature is very low. During the other year of each day, the Sun appears to move slowly in the sky from the eastern to the western horizon, while the planet makes a complete revolution around the Sun, passing through both perihelion and aphelion.
Mercury is seen most easily when it is close to greatest elongation, which means that its angular separation from the Sun is greatest. It can be near greatest western elongation, which means it is west of the Sun in the sky, so it is visible soon before sunrise, or greatest eastern elongation, which means it is visible soon after sunset. However, the exact dates of the greatest elongations are not the best ones on which to try to see Mercury. The phase of the planet greatly affects its apparent brightness.
Mercury is technically brightest as seen from Earth when it is at a full phase. Although the planet is farthest away from Earth when it is full the greater illuminated area that is visible and the opposition brightness surge more than compensates for the distance. The opposite is true for Venus, which appears brightest when it is a crescent, because it is much closer to Earth than when gibbous.
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 the Earth. While 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.
Mercury is one of four terrestrial planets in the Solar System, and is a rocky body like the Earth. It is the smallest planet in the Solar System, with an equatorial radius of 2,439.7 km. Mercury is even smaller—albeit more massive—than the largest natural satellites in the Solar System, Ganymede and Titan.
One distinctive feature of Mercury’s surface is the presence of numerous narrow ridges, extending up to several hundred kilometers in length. It is believed that these were formed as Mercury’s core and mantle cooled and contracted at a time when the crust had already solidified.
Mercury’s surface is very similar in appearance to that of the Moon, showing extensive mare-like plains and heavy cratering, indicating that it has been geologically inactive for billions of years.
Mercury was heavily bombarded by comets and asteroids during and shortly following its formation 4.6 billion years ago, as well as during a possibly separate subsequent episode called the late heavy bombardment that came to an end 3.8 billion years ago. During this time the planet was volcanically active; basins such as the Caloris Basin were filled by magma, which produced smooth plains similar to the maria found on the Moon.
Mercury’s surface is more heterogeneous than either Mars’ or the Moon’s, both of which contain significant stretches of similar geology, such as maria and plateaus.
One unusual feature of the planet’s surface is the numerous compression folds, or rupes, which crisscross the plains. As the planet’s interior cooled, it may have contracted and its surface began to deform, creating these features. The folds can be seen on top of other features, such as craters and smoother plains, indicating that the folds are more recent. Mercury’s surface is flexed by significant tidal bulges raised by the Sun—the Sun’s tides on Mercury are about 17 times stronger than the Moon’s on Earth.
The earliest known recorded observations of Mercury seem to be 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”). 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.
The ancient Greeks of Hesiod’s time knew the planet as Στίλβων (Stilbon), meaning “the gleaming”, and Ἑρμάων (Hermaon). Later Greeks called the planet Apollo when 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 (Ερμής: Ermis), a planetary name which is retained in modern Greek. 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.
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 mythology used the name Budha for Mercury, and this god was thought to preside over Wednesday. The god Odin (or Woden) of Germanic paganism was associated with the planet Mercury and Wednesday. The Maya may have represented Mercury as an owl (or possibly four owls; two for the morning aspect and two for the evening) that served as a messenger to the underworld.