The Moon

The Moon is small and, on the cosmic scale of things, a rather insignificant world. But as our partner in space, it dominates our skies. Its diameter is only 3,475 km, but we see it in detail impossible with any other object in the universe.

The Moon
The Moon in its waning stages. Credit: Paul Sutherland

Our natural satellite lies close to us, at an average distance of just 384,400 km. It means we can view its mountains, valleys, craters and other formations in great detail.

The Moon is also the only world other than the Earth that humans have visited – and there are plans for them to go back before too long.

Some features on the Moon can be seen with the naked eye, but even binoculars are enough to reveal the spectacular craters that have left their mark.

Curiously, we always see the same face of the Moon, because it makes exactly one rotation for every orbit around the Earth. There is a gentle rocking effect, called libration, which in turn brings features on different regions of the far side into view, but more than four tenths remains permanently hidden. The Moon was “locked” to face us by the Earth’s gravitational pull. It was only thanks to the work of space probes that we know what is on the far side.

There is plenty to fascinate observers on the side of the Moon we can see. Just don’t expect it to change very much, or indeed at all! The Moon is a sterile body and has no atmosphere to speak of to weather it (though water ice has collected in craters in permanent shadow near the south pole). Features, including the footprints of the first astronauts to visit it, will remain for millions of years.

Despite this lack of physical change, lunar features do change their appearance from night to night. This is because the sunlight that illuminates the surface falls at a different angle as the Moon follows its orbit around the Earth. (More accurately, the two worlds rotate around their centre of gravity, but this point lies beneath the Earth’s surface).

Lunar features each spend around two weeks in daylight and then the next two in darkness as the Sun appears to rise over their local horizon before crossing the sky and setting to begin a long night. This means there is no “dark side” of the Moon – sorry, Pink Floyd fans – because generally, all parts get sunlight and darkness in equal measure. I say generally because there are areas inside some craters near the Moon’s poles which never see sunlight. It is believed there might even be water ice there, delivered by comets that hit the Moon aeons ago.

Temperatures are extreme on the Moon, ranging from drop suddenly from more than 100 C in direct sunlight to less than -170 C when darkness falls.

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