The planet Mercury
Three-color composites image of the Planet Mercury with peak sensitivity in the green part of the spectrum, roughly matching the human eye. (credit:NASA/Johns Hopkins University Applied Physics Laboratory/Arizona State University/Carnegie Institution of Washington) – click to enlarge

Mercury is the smallest planet in the Solar System and the closest to the Sun.

Mercury has a year of only 88 Earth days and, because it spins more slowly than the Earth, one day on Mercury takes up to two thirds of its year. During one of Mercury’s long days, the Sun makes a complete loop in the sky due to changes in the planet’s speed around the Sun.

Mercury is named after the messenger of the gods in Roman mythology, because it orbits the Sun so quickly and changes position in the sky faster than any of the other planets.

The surface of Mercury is covered in craters, as it has no atmosphere to protect it from meteorite impacts and no surface water, weather systems or volcanic activity to erode or cover them.

The surface of Mercury facing the Sun can reach temperatures of above 400 °C, while at night temperatures plummet to below -170 °C. This is because Mercury has no atmosphere to hold onto the heat from the Sun at night.


From Earth, because of its proximity to the Sun, Mercury can only be seen for about one hour before sunrise or after sunset, or during a total solar eclipse. Because, like Venus, Mercury’s orbit is inside that of the Earth, Mercury can be observed in transit as it passes across the disc of the Sun and exhibits phases similar to those of Venus or the Moon.

Warning: Never look through a telescope, or binoculars, pointed anywhere near the Sun, unless it is equipped with a specially designed solar filter. Also, do not look in the direction of the Sun without wearing eye protection such as safety glasses designed for observing a solar eclipse. Note that the Sun’s image can also be safely projected though a telescope onto a white screen behind the eyepiece, in order to observe transits of Mercury and Venus, although great care should be taken.

To find Mercury’s current position in the sky from your location, visit our Night Sky Simulator.

Mercury Stats:

  • Type: Terrestrial
  • Diameter: 4,878 km (0.38 Earths)
  • Mass: 3.3011×1023 (0.055 Earths)
  • Distance from Sun: 0.466 to 0.307 AU
  • Sidereal period (day length): 58.646 Earth days
  • Orbital period (year length): 87.969 Earth days or 0.241 years
  • Number of Moons: 0
  • Symbol: ☿ (HTML code: ☿)


Mercury was first visited by NASA’s Mariner 10 space probe, which made its closest approach on 16 March 1975.

The only other probe to visit Mercury was NASA’s MESSENGER (Mercury Surface, Space Environment, Geochemistry, and Ranging) mission, which entered orbit around Mercury on 18 March 2011 and impacted the planet’s surface on April 30, 2015.

On 20 October 2018, the BepiColombo mission was launched – a joint mission of the European Space Agency (ESA) and the Japan Aerospace Exploration Agency (JAXA). This mission includes two orbiters – The Mercury Planetary Orbiter (MPO), built by ESA, and the Mercury Magnetospheric Orbiter (MMO), built by JAXA – which are due to enter orbit around Mercury on 5 December 2025.

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Astronomy, Cosmology, Space and Astrophysics