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Royal Greenwich Observatory
Information Leaflet No. 29: 'The Moon'.
The Moon is the closest astronomical object to the Earth. With the Earth, it forms what is almost a double planet, for no other planet has a satellite that is as large in comparison to the size of the planet.
The Moon has a diameter of 3,476 km and orbits the Earth at a mean distance
of 384,000 km.
It orbits the Earth in 27.322 days, and always keeps the same face pointed towards the Earth.
The Moon shines by reflecting the light from the Sun and shows the
characteristic phases during each orbit of the Earth. (See Phases of the
Near New Moon, when the sunlit portion of the Moon is small, the phenomenon of 'the old Moon in the young Moon's arms' is often seen. This is caused by sunlight being reflected towards the Moon by the Earth and being reflected back again to the Earth. We are seeing Earthshine, the equivalent of moonlight on the Earth.
The orbital plane of the Moon is inclined to that of the Earth about the Sun and so eclipses are only seen when New Moon or Full Moon occur when the Moon is near to the crossing points of these planes (See Eclipses).
The gravitational pull of the Moon and the Sun are responsible for the Tides (See The Tides).
The Moon has no atmosphere. Any early atmosphere that the Moon might have
had, has escaped from the Moon's feeble gravitational pull. This is only
one sixth that at the surface of the Earth.
Because of the lack of any atmosphere, the temperature of the Moon's surface varies between -180°C and +110°C.
The Moon offers little protection from the solar wind, cosmic rays or micrometeorites, and so it is not surprising that there is no form of life on the Moon.
The Moon's surface is characterised by light mountainous regions
interspersed with dark maria. The 'Man in the Moon' is formed from patches
of these two types of terrain. The maria are vast impact basins which have
been filled with basaltic rocks some 3,000 million years ago.
Much of the Moon's surface is covered with craters. These are the result of impacts by meteors. The largest are about 200 km in diameter, the smallest are only about a metre across. Most of these craters were formed between 3,000 and 4,000 million years ago.
Much of our knowledge of the structure of the Lunar surface and the geology of the Moon comes from the landings of the Apollo series and the samples of Lunar material which were brought back to Earth. Despite this, we are still not sure how the Moon was formed. The most likely theory is that the Earth and Moon were formed together as a 'double planet'.
The Moon is probably the most satisfying object to look at through any
telescope. The craters and mountains can be seen with even a small
The best place to look is near the terminator, where the Sun is either rising or setting in the Moon. Here the shadows cast by mountains and crater walls are longest and can give very dramatic views. After as short a time as an hour, changes in the shadows can be seen, as the sunlight reaches or leaves peaks near the terminator.
Many amateur astronomers look for 'transient Lunar phenomena'. These are outbursts of some kind, which give rise to short-lived colour or brightness changes in small areas. It is not clear how many of these are real or what causes them.
Produced by the Information Services Department of the Royal Greenwich Observatory.
PJA Thu Nov 25 10:44:28 GMT 1993
See also Young Volcanoes on the Moon (Science@NASA, November 24, 2014)
Updated: May 5 '97, June 24 '14, November 25 '14
Best seen with Font Verdana.
See About the Web Pages of Observatorio ARVAL.
For some illustrative images, link to:
Galileo To Jupiter in ARVAL Gallery
(See slides 8, 13 and 14)
See also the ARVAL Moon Map, and in ARVAL Gallery, see the Total Lunar Eclipses.
For some illustrative images and excellent texts, link to: The Moon in Calvin J. Hamilton's Views of the Solar System
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