Like the planets, we see the Moon because it reflects light from the Sun – it does not give out any light of its own – and during its orbit of the Earth, the angle between the Moon and the Sun varies widely as seen from the Earth, and so we see different amounts of the lunar surface, and as a result the Moon appears to change shape; these are the phases of the Moon.
The Moon takes about 28 days to orbit the Earth, which is roughly a month. This is where the term comes from, and more accurately we should call it a “moonth”!
It is also the time it takes to rotate once on its axis. As a result, the Moon only shows one side towards the Earth. If you were to walk around someone else, and kept looking at them all the time, they would only see your face, and never see the back of your head. However, during its orbit, it is obvious that half of the Moon is lit by the Sun, whether we can see all of that half or not. So the opposite side of the Moon to the half that faces the Earth should be referred to as “the far side of the Moon” and not “the dark side of the Moon”, as every part is lit up at some time.
The only “dark side of the Moon” is the album by Pink Floyd and a reference to the aliens’ landing site in “Close Encounters of the Third Kind”!
This image shows what happens when the Moon orbits the Earth. The Sun is to the left of the image, shining in the direction of the blue arrows.
Phases of the Moon
At New Moon, the Moon is between the Earth and the Sun. This means that the sunlit half of the Moon is the side facing away from the Earth.
After a few days, the Moon has moved around in its orbit. As it does so, it is still only the side facing the Sun that is illuminated, but we can start to see some of the portion that is lit. The Moon appears as a crescent, and as more of the Moon is lit up (as seen from Earth), it is said to be “waxing”. Once past Full Moon and the amount that we can see lit becomes less and less, the Moon is said to be “waning”.
Traditionally, when there are two Full Moons in the same calendar month, the second is called a “Blue Moon”. This does not happen very often, hence the phrase “Once in a Blue Moon”.
In 2012 this occurred on August 31.
Find out about eclipses of the Moon here: