Mercury is the smallest planetin the Solar System and the closest to the Sun. Its orbit around theSun takes 87.97 Earth days, the shortest of all the Sun's planets. Itis named after the Roman god Mercurius (Mercury), god of commerce,messenger of the gods, and mediator between gods and mortals,corresponding to the Greek god Hermes (Ἑρμῆς). Like Venus,Mercury orbits the Sun within Earth's orbit as an inferior planet,and its apparent distance from the Sun as viewed from Earth neverexceeds 28°. This proximity to the Sun means the planet can only beseen near the western horizon after sunset or the eastern horizonbefore sunrise, usually in twilight. At this time, it may appear as abright star-like object, but is more difficult to observe than Venus.From Earth, the planet telescopically displays the complete range ofphases, similar to Venus and the Moon, which recurs over its synodicperiod of approximately 116 days.
Mercury rotates in a way that is uniquein the Solar System. It is tidally locked with the Sun in a 3:2spin–orbit resonance, meaning that relative to the fixed stars, itrotates on its axis exactly three times for every two revolutions itmakes around the Sun. As seen from the Sun, in a frame of referencethat rotates with the orbital motion, it appears to rotate only onceevery two Mercurian years. An observer on Mercury would therefore seeonly one day every two Mercurian years.
Mercury's axis has the smallest tilt ofany of the Solar System's planets (about 1⁄30 degree). Its orbitaleccentricity is the largest of all known planets in the Solar System;at perihelion, Mercury's distance from the Sun is only abouttwo-thirds (or 66%) of its distance at aphelion. Mercury's surfaceappears heavily cratered and is similar in appearance to the Moon's,indicating that it has been geologically inactive for billions ofyears. Having almost no atmosphere to retain heat, it has surfacetemperatures that vary diurnally more than on any other planet in theSolar System, ranging from 100 K (−173 °C; −280 °F) at night to700 K (427 °C; 800 °F) during the day across the equatorialregions. The polar regions are constantly below 180 K (−93 °C;−136 °F). The planet has no known natural satellites.
Two spacecraft have visited Mercury:Mariner 10 flew by in 1974 and 1975; and MESSENGER, launched in 2004,orbited Mercury over 4,000 times in four years before exhausting itsfuel and crashing into the planet's surface on April 30, 2015. TheBepiColombo spacecraft is planned to arrive at Mercury in 2025.
Nomenclature
The ancients knew Mercury by differentnames depending on whether it was an evening star or a morning star.By about 350 BC, the ancient Greeks had realized the two stars wereone. They knew the planet as Στίλβων Stilbōn, meaning"twinkling", and Ἑρμής Hermēs, for itsfleeting motion, a name that is retained in modern Greek (ΕρμήςErmis). The Romans named the planet after the swift-footed Romanmessenger god, Mercury (Latin Mercurius), which they equated with theGreek Hermes, because it moves across the sky faster than any otherplanet. The astronomical symbol for Mercury is a stylized version ofHermes' caduceus; a Christian cross was added in the 16th century: ☿.
Physical characteristics
Mercury is one of four terrestrialplanets in the Solar System, and is a rocky body like Earth. It isthe smallest planet in the Solar System, with an equatorial radius of2,439.7 kilometers (1,516.0 mi). Mercury is also smaller—albeitmore massive—than the largest natural satellites in the SolarSystem, Ganymede and Titan. Mercury consists of approximately 70%metallic and 30% silicate material.
