# Mercury (planet)

By [Glory](https://paragraph.com/@heardless) · 2023-01-21

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**Mercury** is the smallest [planet](https://en.wikipedia.org/wiki/Planet) in the [Solar System](https://en.wikipedia.org/wiki/Solar_System) and the closest to the [Sun](https://en.wikipedia.org/wiki/Sun). Its orbit around the Sun takes 87.97 Earth days, the shortest of all the Sun's planets. It is named after the Roman god _Mercurius_ ([Mercury](https://en.wikipedia.org/wiki/Mercury_\(mythology\))), god of commerce, messenger of the gods, and mediator between gods and mortals, corresponding to the Greek god [Hermes](https://en.wikipedia.org/wiki/Hermes) (Ἑρμῆς). Like [Venus](https://en.wikipedia.org/wiki/Venus), Mercury orbits the Sun within [Earth's orbit](https://en.wikipedia.org/wiki/Earth%27s_orbit) as an [inferior planet](https://en.wikipedia.org/wiki/Inferior_and_superior_planets); its [apparent distance](https://en.wikipedia.org/wiki/Apparent_distance) from the Sun as viewed from Earth never exceeds 28°. This proximity to the Sun means the planet can only be seen near the western horizon [after sunset](https://en.wikipedia.org/wiki/Dusk) or the eastern horizon [before sunrise](https://en.wikipedia.org/wiki/Dawn), usually in [twilight](https://en.wikipedia.org/wiki/Twilight). At this time, it may appear as a bright star-like object, but is more difficult to observe than Venus. From Earth, the planet telescopically displays the complete range of [phases](https://en.wikipedia.org/wiki/Planetary_phase), similar to Venus and the [Moon](https://en.wikipedia.org/wiki/Moon), which recurs over its [synodic period](https://en.wikipedia.org/wiki/Synodic_period) of approximately 116 days. Due to its synodic proximity to Earth, Mercury is most often the closest planet to Earth, with Venus periodically taking this role.[\[18\]](https://en.wikipedia.org/wiki/Mercury_\(planet\)#cite_note-AIP_Publishing_2019_p.-18)[\[19\]](https://en.wikipedia.org/wiki/Mercury_\(planet\)#cite_note-MoreOrLess-19)

Mercury rotates in a way that is unique in the Solar System. It is [tidally locked](https://en.wikipedia.org/wiki/Tidal_locking) with the Sun in a 3:2 [spin–orbit resonance](https://en.wikipedia.org/wiki/Orbital_resonance),[\[20\]](https://en.wikipedia.org/wiki/Mercury_\(planet\)#cite_note-20) meaning that relative to the [fixed stars](https://en.wikipedia.org/wiki/Fixed_stars), it rotates on its axis exactly three times for every two revolutions it makes around the Sun.[\[a\]](https://en.wikipedia.org/wiki/Mercury_\(planet\)#cite_note-rotation/revolution-21)[\[21\]](https://en.wikipedia.org/wiki/Mercury_\(planet\)#cite_note-orbit-22) As seen from the Sun, in a [frame of reference](https://en.wikipedia.org/wiki/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 Mercurian years.

Mercury's axis has the smallest [tilt](https://en.wikipedia.org/wiki/Axial_tilt) of any of the Solar System's planets (about 1⁄30 degree). Its [orbital eccentricity](https://en.wikipedia.org/wiki/Orbital_eccentricity) is the largest of all known planets in the Solar System;[\[b\]](https://en.wikipedia.org/wiki/Mercury_\(planet\)#cite_note-dwarf_planet-23) at [perihelion](https://en.wikipedia.org/wiki/Perihelion), Mercury's distance from the Sun is only about two-thirds (or 66%) of its distance at aphelion. Mercury's surface appears heavily cratered and is similar in appearance to the Moon's, indicating that it has been geologically inactive for billions of years. Having almost no atmosphere to retain heat, it has surface temperatures that vary diurnally more than on any other planet in the Solar System, ranging from 100 K (−173 °C; −280 °F) at night to 700 K (427 °C; 800 °F) during the day across the equatorial regions.[\[22\]](https://en.wikipedia.org/wiki/Mercury_\(planet\)#cite_note-:0-24) The polar regions are constantly below 180 K (−93 °C; −136 °F). The planet has no [natural satellites](https://en.wikipedia.org/wiki/Natural_satellite).

Two spacecraft have visited Mercury: [_Mariner 10_](https://en.wikipedia.org/wiki/Mariner_10) flew by in 1974 and 1975; and [_MESSENGER_](https://en.wikipedia.org/wiki/MESSENGER), launched in 2004, orbited Mercury over 4,000 times in four years before exhausting its fuel and crashing into the planet's surface on April 30, 2015.[\[23\]](https://en.wikipedia.org/wiki/Mercury_\(planet\)#cite_note-25)[\[24\]](https://en.wikipedia.org/wiki/Mercury_\(planet\)#cite_note-EclipseReboost-26)[\[25\]](https://en.wikipedia.org/wiki/Mercury_\(planet\)#cite_note-Pressurant2015-27) The [_BepiColombo_](https://en.wikipedia.org/wiki/BepiColombo) spacecraft is planned to arrive at Mercury in 2025

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*Originally published on [Glory](https://paragraph.com/@heardless/mercury-planet-2)*