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 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. We have put together a guide to some great ones you can see with a small telescope in our Fifty Fantastic Features pages.

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). You can read more about the Moon’s changing appearance here.

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.

Moon’s changing appearance

As the Moon orbits the Earth, we see its phase grow gradually from a fine crescent in the evening sky to a Full Moon, visible throughout the night. The phase then wanes to an early morning crescent, just before the date of New Moon when the Moon lies in the same general direction as the Sun and its invisible far side is illuminated.

Why we see phases of the Moon during its orbit. Credit: Yoko Kikuta

Throughout this period, as already explained, half of the Moon is always in sunlight. What produces the phases is its changing position relative to the Earth and the Sun.

Most of us are familiar with a phenomenon called “the new Moon in the old Moon’s arms” which can be seen when the Moon is a thin crescent. At such times, we can dimly see the whole of the Moon’s face, lit up by sunlight reflected back from the Earth.

Apart from the mountains and craters, there are large flat plains that together form the face of the Man in the Moon. These are called maria – meaning seas, despite the fact they are dry – and were formed when hot lava flooded through cracks in the surface caused by asteroid impacts.

Today, however, the Moon is mainly a solid, dead world, covered with a lunar soil, 10 metres or so deep, termed regolith. This was produced by pulverized rocks and dust from asteroid impacts over billions of years.

Surprisingly, Full Moon is not the best time to observe lunar features because they are lost in the glare of sunlight reflected directly back towards us. You need to watch when the dividing line between the Moon’s day and night sides, called the terminator, crosses the visible disk.

This is when lunar features cast strong shadows which clearly mark them out. Interestingly, the Moon’s rocks are actually dark, even if it does seem to shine so brightly in the night sky.

The Moon’s gravity is only a sixth as strong as here on Earth, yet is enough to cause the oceans’ tides. For Moon maps and a list of 50 suggested features that you can view for yourself, check out Skymania’s special Moon Guide.