Glossary term: Moon

Description: The Moon is a celestial body that is not luminous on its own, but reflects the sunlight falling on it. This produces the Moon's characteristic phases. The Moon is the only major natural satellite of Earth, and ranks fifth among the natural satellites of the Solar System in terms of size and mass. "Moon" is capitalized to distinguish it from other natural satellites, or moons, in the Solar System and beyond. Compared with other Solar System moons, the Moon has the largest size relative to the size of the planet it orbits. The Moon follows an elliptical orbit around Earth, at an average distance from Earth of 384,000 kilometers (km). It has no atmosphere and is composed of similar materials to the Earth with an iron-rich core and rocky outer layers. The similarity is no accident: To the best of our knowledge, the Moon formed from the debris of the collision between Earth and a Mars-sized planet around 4.5 billion years ago; most of its material stems from the original Earth's mantle. 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 3.79 x 10⁷ square kilometers, its volume is 2.20 x 10¹⁰ cubic kilometers, and its mass is 7.35 x 10²² kilograms (kg). The exact value of the Moon's orbital period around Earth depends on the frame of reference: relative to the distant stars, it completes one orbit every 27.3 days ("sidereal period"). For an observer on Earth, the time between two new moons is 29.5 days ("synodic period").

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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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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

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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

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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

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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

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Full moon

Caption: The image shows the nearly full Moon observed with a small telescope and a DSLR camera.
Credit: Luc Viatour credit link

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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

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The bright Moon illuminates a beach. Three bright planets form a line below and to the right of the Moon.

To guard the Stars and the Sea Together

Caption: Winner in the 2022 IAU OAE Astrophotography Contest, category Still images of celestial patterns. This image composition is amazing. In the far background of the landscape we see a chain of mountains that seems to mirror the structure of the Milky Way in the sky above. The strong daylight-like colours of the landscape are caused by the Moon, the bright light at the top of the image. Taken in Kinabalu, Malaysia, in February 2019, this image shows the alignment of planets and the Moon, conveying the idea of the ecliptic as the central line of the Zodiac, the plane within which all planets orbit the Sun. The ecliptic is the central line of the Zodiac, so the region of about five to 10 degrees either side of the ecliptic is where the constellations of the Zodiac are located. Starting from the horizon towards the bottom left of the image we can see the planets Venus, Saturn and Jupiter. The planets have different cultural significance for people around the world, and are deeply embedded in social, religious and practical aspects of life. For example, Wardaman traditions of Indigenous Australians associate the planets with ancestor spirits who traverse the Celestial Road (ecliptic). The appearance and disappearance of planets in the sky are associated with various ceremonies. For example, when Venus starts being the “Morning star” after having been the “Evening star”, this marks the Banumbirr ceremony for the Yolnu people of Arnhem Land, in Australia. The image also shows the constellations Scorpius, Aquila, Lupus and Triangulum Australe, the asterism of the Teapot, and the two pointer stars Alpha and Beta Centauri. The constellations, asterisms and individual stars within them have significance in many different cultures. Malaysia, being close to the equator, has had connections to the north as well as to the south and almost the whole sky is visible over the course of the year. The star Antares is seen by the Kokatha people of the Western Desert as Kogolongo, the red tailed black cockatoo, while the Boorong refer to it as Djuit, the red-rumped parrot. The two stars which form the stinger of Scorpius (Shaula and Lesath), are called Karik Karik, the Australian Kestrel.
Credit: Likai Lin/IAU OAE

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Three bright objects in the sky with one top left, one in the middle and one lower centre.

Jupiter, Venus, Moon Conjunction

Caption: Captured with a smartphone in February 2023, over the skies of Luzerne County, Pennsylvania, USA, this photograph offers a glimpse into a conjunction, an enthralling astronomical phenomenon that occurs when two or more celestial objects are seen in close proximity in the sky from our perspective, despite the objects not being physically near to each other. In this image, the brilliance of Jupiter (top), the allure of Venus (middle), and the familiar glow of our Moon (bottom) dance together against a backdrop of delicate clouds and a treeline silhouette, making it a moment worth treasuring.
Credit: Joslynn Appel/IAU OAE (CC BY 4.0)

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The face of the full moon with its dark mare and lighter highlands

Full Flower Moon 2023

Caption: The Flower Moon, named for the bloom of flowers in North America, casts a radiant glow over the night sky in Luzerne County, Pennsylvania, USA, in May 2023. Despite initial cloud cover, the moon's luminosity broke through around 23:00, creating an enchanting spectacle. This captivating smartphone-captured image not only commemorates the celestial event but also emphasises the significance of safeguarding our dark skies. The moon’s brilliance illuminates the intricacies of Earth’s atmosphere, creating an atmospheric drama heightened by the presence of scattered clouds. Preserving these pristine, unblemished skies is vital in retaining our connection to celestial wonders.
Credit: Joslynn Appel/IAU OAE (CC BY 4.0)

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Lunar rainbow and lunar Brocken spectre, by Kouji Ohnishi, Japan

Caption: Third place in the 2021 IAU OAE Astrophotography Contest, category Sun/Moon haloes. This stunning photograph of the lunar rainbow and the lunar Brocken Spectre amidst the night sky was captured from Mount Tsubakuro located in Japan’s Hida Mountains in Nagano. Both these atmospheric occurrences are due to the moon’s light being reflected and refracted from water droplets. A lunar rainbow or a moonbow is a rare phenomenon that occurs with the right settings of a bright full moon which is less than 42° high, rain on the opposite side of the moon and a dark night sky. The Brocken Spectre is named after the highest peak of the Harz mountain range in Germany, where it was first recorded. Here it is seen from the summit as a magnified shadow of the observer cast onto the cloud surrounded by a glory consisting of concentric circles centered at the point directly opposite the bright moon in the background.
Credit: Kouji Ohnishi/IAU OAE

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License: CC-BY-4.0 Creative Commons Attribution 4.0 International (CC BY 4.0) icons
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astroEDU educational activity (links to astroEDU website)
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astroEDU educational activity (links to astroEDU website)
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