Glossary term: Mercury

Description: Mercury is the closest planet to the Sun and the smallest of the eight major planets in the Solar System. It is a rocky, terrestrial planet with a radius of about 2500 kilometers (km), which is slightly larger than the Earth's Moon. It has a mass of 0.055 times the mass of the Earth. Due to its close distance, the Sun is seven times brighter seen from Mercury, and Mercury's surface is much affected by the solar wind. Its very thin exosphere is made of material produced by this interaction, plus matter blasted off the surface due to frequent falling objects. The thin atmosphere cannot keep the planet's temperature, so the surface is extremely cold (-180 degrees Celsius) during the night, and extremely hot (400 degrees Celsius) during the day, and thus very dry.

Its typical distance from the Sun is about 58 million km, about 0.39 astronomical units (Earth–Sun distances), taking a little under 88 days to complete one orbit. Mercury has no moons known to orbit it.

As Mercury orbits the Sun closer than the Earth, it always appears close to the Sun in the sky. Mercury is named after the speedy Roman messenger god, derived from its fast motion across the sky. Two space probes (Mariner 10 and MESSENGER) have visited Mercury, with the BepiColombo mission due to arrive in the late 2020s.

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Term and definition status: This term and its definition have been approved by a research astronomer and a teacher

The OAE Multilingual Glossary is a project of the IAU Office of Astronomy for Education (OAE) in collaboration with the IAU Office of Astronomy Outreach (OAO). The terms and definitions were chosen, written and reviewed by a collective effort from the OAE, the OAE Centers and Nodes, the OAE National Astronomy Education Coordinators (NAECs) and other volunteers. You can find a full list of credits here. All glossary terms and their definitions are released under a Creative Commons CC BY-4.0 license and should be credited to "IAU OAE".

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The planet Mercury covered by many craters


Caption: This image is a composite of a picture mosaic of the planet Mercury's surface obtained by the MESSENGER (Mercury Surface, Space Environment, Geochemistry and Ranging) space probe. MESSENGER was launched by NASA in 2004 and explored Mercury from 2011 to 2015.
Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington credit link

License: PD Public Domain icons

The crescent Moon sits to the left of two bright planets. On the right side the Pleiades star cluster can be seen.

Moon-Mercury-Pleiades Conjunction

Caption: Honourable mention in the 2022 IAU OAE Astrophotography Contest, category Still images of celestial patterns.   This photograph shows the young lunar crescent, some of the nine brightest stars of the Pleiades (with one behind a cloud) on the right, and the planet Mercury, looking slightly red, in the middle of the image. This picture is rather suggestive of the idea that the Pleiades might possibly consist of seven stars. However, the viewer is misled by the clouds; five of the stars form a tiny chariot, one is next to the handle, and three are at the other end of the quadrilateral. Eight stars would be clearly visible if there were no clouds. This configuration of the young Moon next to the Pleiades is visible only in the northern hemisphere spring. Thus it was used by the ancient Babylonians to determine the second month of their year and to judge whether or not an intercalary month was necessary. At least as early as the second millennium before the common era, the Babylonians used several asterisms for each month, with another one of them reappearing every five days after invisibility during daylight. To determine the necessity of intercalation in order to synchronise the solar and the lunar year, the Babylonians used several asterisms, not only the Pleiades. For instance, they also made use of the bright stars Arcturus and Sirius, and they observed a configuration with the Moon as well as heliacal phenomena. The modern Jewish and modern Islamic traditions still make use of some of the Babylonian astronomical rules. However, given that the constellations have shifted as a result of precession, and the fact that nowadays we also have computational means to calculate our calendars, this configuration of the small crescent Moon and the Pleiades is less useful, though it remains exceptionally beautiful. Thus the ancient Babylonian and middle Babylonian tradition survives only rudimentarily. Furthermore, it is unlikely that it is depicted in the Nebra Disc from Bronze Age Europe, as has long been claimed. This image was taken on Elba Island, Italy, in May 2022.
Credit: Giulio Colombo/ IAU OAE

License: CC-BY-4.0 Creative Commons Attribution 4.0 International (CC BY 4.0) icons

Mercury appears smaller and fainter than Venus. The bottom right of Venus is an illuminated crescent.

Venus and Mercury Trails

Caption: In this composite image, both Mercury (left) and Venus (right) can be seen heading into the sunset. The phases of each are beautifully captured as they descend. Not all planets or moons in the Solar System show phases as viewed from Earth. This phenomenon occurs because the orbits of Venus and Mercury are positioned between Earth’s orbit and the Sun, sometimes allowing us to see only part of the illuminated portion of each planet. These phases are similar to the phases we see of our own Moon.
Credit: Marcella Giulia Pace (CC BY 4.0)

License: CC-BY-4.0 Creative Commons Attribution 4.0 International (CC BY 4.0) icons