The Moon: its size, composition, gravity and potential for life.

The Moon is Earth’s most familiar companion but it is still capable of guarding its own secrets. The Moon is the closest astronomical object to the Earth. With the Earth it forms what is almost a double planet as no other planet has a satellite as large in comparison to its own size.

The Moon has a diameter of 3476 km and orbits the Earth at a mean distance of 384,000 km. It orbits the Earth in 27 days and always keeps the same face pointed towards the Earth.

How was the Moon formed?

Despite our visits and observations we are still not sure how the Moon was formed. In recent years, the most widely-accepted theory is that a Mars-sized body struck the Earth early in its history. Part of the resulting debris then coalesced into the Moon.


The water on the side of the Earth closest to the Moon is pulled, by the Moon's gravitational force, more strongly than is the bulk of the Earth. The effect is to make bulges in the water on opposite sides of the Earth.

Gravity on the Moon

Although the Earth and Moon are both very massive, the Moon is smaller, so its force of gravity is less. Any object on the Moon will weigh about six times less than it does on Earth.

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 lunar surface and the ‘Man in the Moon’

The Moon's surface is characterised by light mountainous regions interspersed with dark maria or ‘seas’. 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 3000 million years ago.

Much of the Moon's surface is covered with craters. These are the result of impacts by meteorites. The largest are about 200 km in diameter, the smallest are only about a metre across. Most of these craters were formed between 3000 and 4000 million years ago.

Americans have landed on the Moon six times. The first time was in July 1969 and the last time was in December 1972. 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 brought back to Earth.

Once in a blue Moon

The phrase 'once in a blue Moon' is a familiar one meaning once in a very long interval of time.  A blue Moon, by folklore definition is said to be the second Full Moon of the month. However, researchers at Southwest Texas State University have been using historical documents to prove that a blue Moon was really a term used by the Maine Farmers’ Almanac to indicate the presence of a 13th Full Moon in a tropical year which usually had 12. A tropical year is a measurement of time from one winter solstice to the next.

The Royal Observatory is open daily from 10am

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