Written By John Bradley
The Moon is Earth’s only permanent natural satellite. It is the fifth largest natural satellite in the Solar System, and the largest among planetary satellites relative to the size of the planet that it orbits.
The Moon is a dusty ball of rock, measuring 3,476km in diameter – that’s roughly a quarter of the size of Earth. Its surface is home to mountains, huge craters and flat planes called ‘seas’ made of hardened lava.
The Moon always shows Earth the same face. The Moon is in synchronous rotation with Earth. Its near side is marked by large dark plains (volcanic ‘maria’) that fill the spaces between the bright ancient crustal highlands and the prominent impact craters.
The Moon’s surface is actually dark. Although compared to the night sky it appears very bright, with a reflectance just slightly higher than that of worn asphalt. Its gravitational influence produces the ocean tides, body tides, and the slight lengthening of the day.
The Moon was made when a proto-planet smashed into Earth. The most widely-accepted explanation is that the Moon was created when a planet the size of Mars slammed into Earth, shortly after the solar system began forming about 4.5 billion years ago.
Everyone knows that the Moon is partly responsible for causing the tides of our oceans and seas on Earth, with the Sun also having an effect. However, as the Moon orbits the Earth it also causes a tide of rock to rise and fall in the same way as it does with the water. The effect is not as dramatic as with the oceans, but nevertheless it is a measurable effect, with the solid surface of the Earth moving by several centimetres with each tide.
The Moon has quakes, too. They’re not called earthquakes but moonquakes. They are caused by the gravitational influence of the Earth. Unlike quakes on Earth that last only a few minutes at most, moonquakes can last up to half an hour. They are much weaker than earthquakes though.
There is water on the Moon! This is in the form of ice trapped within dust and minerals on and under the surface. It has been detected on areas of the lunar surface that are in permanent shadow and are therefore very cold, enabling the ice to survive. The water on the Moon was likely delivered to the surface by comets.
