Glossary term: Moon

Description: The Moon is a celestial body that is not luminous on its own but reflects the sunlight falling on it. It is the only natural satellite of Earth, and it ranks fifth among the natural satellites of the Solar System in terms of size and mass; it has the largest ratio of size to the size of the planet that it orbits. It has an elliptical orbit around Earth. The Moon has no atmosphere and is composed of similar materials to the Earth with an iron-rich core and rocky outer layers. The surface of the Moon has dark areas known as mare, lighter highlands and is pockmarked with craters. The surface area of the Moon is 3.793 x 10⁷ square kilometers, its size is 2.1958 x 10¹⁰ cubic meters, and its mass is 7.3477 x 10²² kilograms. It goes around Earth at a distance of about 385,000 kilometers. The Moon's orbit around Earth has a sidereal period of 27.3 days and a synodic period (the time between two new moons) of 29.5 days.

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Term and definition status: This term and its definition is still awaiting approval

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".

Related Media

An astronaut in a white spacesuit stands on the grey lunar surface with a piece of equipment in-front of a lunar lander

Apollo 11 lunar activity

Caption: NASA astronaut Edwin 'Buzz' Aldrin installs a seismometer in front of the Apollo 11 Eagle lunar lander during an extravehicular activity (EVA) on the Moon. Neil Armstrong shot this photo during the first human mission to the surface of the Moon in 1969.
Credit: NASA/Project Apollo Archive credit link
License: PD Public Domain icons
A halo is an optical phenomenon where circles or arcs of light are visible in the sky. Halos are caused  a bright object such

Selene meets the Moon, by Sheila Wiwchar, Canada

Caption: First place in the 2021 IAU OAE Astrophotography Contest, category Sun/Moon haloes. Photographed at Kaleida, Manitoba in Canada, this fisheye image beautifully captures the rare optical phenomena encircling the moon known as the “22° halo” and the horizontal white circle passing through the moon called the “paraselenic circle”. The white band circling the whole sky at the same altitude as the moon is named after Selene, the ancient Greek Titan, famously called the goddess of the moon. The more commonly observed counterpart produced by the sun is known as the parhelic circle, named after the Greek god of the sun, Helios. Both the 22° halo and the paraselenic circle are produced due to reflection of the moonlight from near vertical surfaces of ice crystals. Parts closer to the moon are caused due to external reflections, whereas those further away are created due to internal reflections. The constellation of the big dipper at the center makes this image even more spectacular. Can you spot it?
Credit: Sheila Wiwchar/IAU OAE
License: CC-BY-4.0 Creative Comments Attribution 4.0 International (CC BY 4.0) icons
A composite of images of the Moon. Along the sequence of images, the Moon's colour changes from grey to red and back to grey.

Red Moon, by Daniel Henrion, France

Caption: Third place in the 2021 IAU OAE Astrophotography Contest, category Total lunar eclipse. Time-lapse images of a total supermoon lunar eclipse that took place on 28 September 2015. The photos show the Moon during the time it moved through the Earth´s umbra: Earth´s umbra touched the Moon´s outer limb at 1.07 a.m. UTC (upper left corner) and left the Moon´s surface at 4.27 a.m. UTC (lower right corner).
Credit: Daniel Henrion/IAU OAE
License: CC-BY-4.0 Creative Comments Attribution 4.0 International (CC BY 4.0) icons
The Moon behind jagged, snowy mountains. The lower two thirds of the Moon are darker and redder than the upper third

The eclipsed Moon sets near the Rochetta di Prendera, Dolomiti Unesco, by Alessandra Masi, Italy

Caption: First place in the 2021 IAU OAE Astrophotography Contest, category Total lunar eclipse. A lunar eclipse occurs when the full moon moves into the Earth`s shadow. This is the case when the Sun, Earth and Moon are exactly or very closely aligned. The reddish light reflected from the lunar surface is caused by sunlight that has been refracted on its way through Earth´s atmosphere in the direction of the Moon. It appears reddish because of the Rayleigh scattering of bluer light. The round shape of the Earth's shadow visible on the lunar surface was a proof for Aristotle that the Earth must be a sphere. This photo shows the eclipsed Moon that sets near the Rochetta di Prendera, Dolomiti Unesco, Italy, on 21 January 2019.
Credit: Alessandra Masi/IAU OAE
License: CC-BY-4.0 Creative Comments Attribution 4.0 International (CC BY 4.0) icons
The nearly full Moon with craters, light highlands and dark plains

Full moon

Caption: The image shows the nearly full Moon observed with a small telescope and a DSLR camera.
Credit: Luc Viatour credit link
License: CC-BY-SA-3.0 Creative Comments Attribution-ShareAlike 3.0 Unported icons