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The Moon

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The Moon is the Earth’s closest neighbour in space and can be the brightest object in the sky other than the Sun. The distance from the centre of the Moon to the centre of the Earth is less than one hundredth the distance to the nearest planet, Venus. As the Moon’s orbit is slightly elliptical, the Earth–Moon distance varies from 356400km to 406700km. The Moon orbits the Earth every 27.3 days but the interval between Full Moons is two days longer than this because the Earth is orbiting the Sun. The Moon always keeps one side facing towards Earth, apart from slight “wobbles”, called “librations”, which cause 59% of the Moon’s surface to be visible from the Earth at some time.

The Moon is 3476km in diameter, roughly one quarter of that of the Earth. This makes it bigger than Pluto and 71% the size of Mercury. As a proportion of its parent planet our Moon is larger than any other moon in the solar system, with the exception of Pluto's moon, Charon, and some Edgeworth–Kuiper Belt objects. For this reason, astronomers sometimes describe the Earth and the Moon as a “double planet”. However, the two worlds could hardly be more different. The Moon has a dry, grey, rocky surface and no atmosphere.

The age of the Solar System is 4.567 billion years and the age of the Earth–Moon system is about 4.5 billion years.
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This amazing mosaic image was created by UK amateur astronomer Ken Florey. Using a 4inch refracting telescope, Ken photographed the surface near the terminator (the line dividing the sunlit from the night-time surface of the Moon) over the whole range of Moon phases. Near the terminator, the “dawn shadow” on the Moon greatly accentuates the topography. Ken then combined the images digitally.














Find out more about:
·          Lunar Exploration
·          Experiments on the Moon
·          After the Apollo Missions
·          Moon Rocks
·          Names for the Full Moon


RASlogo.JPGThis is an online edition of one of a series of leaflets prepared by the Education Committee of the Royal Astronomical Society. The first edition was produced with the aid of a grant from COPUS, the Committee on the Public Understanding of Science.