Phases of the Moon

Describing the phases of the Moon

The Moon's appearance changes in a regular way, moving through a sequence of phases in one lunar month.

The Moon orbits the Earth once every 27.3 days. This means that its position relative to someone standing on the Earth at midnight will change over the course of a month. The phases of the Moon are caused by the changing position of the Moon relative to the Sun and to an observer on the Earth.

Explaining the phases of the Moon

The term New Moon is slightly confusing because what it actually means is that the Moon is not visible at all as it is on the other side of the Earth from an observer at midnight.

Why, you might ask, is it not visible during the day?

For two reasons:

• The unlit face is facing towards us.
• The brightness of sunlight is so high that it makes it very difficult to see other objects unless they are very bright.

Imagine somebody standing on the Earth at midnight. If the Moon is in the position shown in screen 1 of the interactive, the whole of the illuminated half of the Moon will be visible to the observer: a full Moon. Harder to explain though, is why the Moon appears to wax and wane. This requires the ability to imagine what an observer on the Earth would see if the Moon was, for example, in either of the following positions:

With the Moon as it is on step 3 of pane 1 of the interactive, the observer on the Earth at midnight sees the illuminated face sideways on and observes a half Moon (although, if you think about it, just one quarter of the Moon's surface is visible to the observer). This is the First Quarter.

As the Moon moves between this position and the full Moon position (imagine the Moon orbiting the Earth in an anti-clockwise direction as seen from above), more of the illuminated half will be seen and it is said to wax (waxing gibbous).

As the Moon moves between the full Moon position and where it is on step 7 of pane 1 of the interactive, less of the illuminated half will be seen and it is said to wane (waning gibbous) until in this position a half Moon is seen (last quarter).

The lunar month, an issue with the diagrams and the far side

As the Moon moves on from this position towards the New Moon position, even less of the illuminated hemisphere will be visible from the Earth and it is said to wane (waning crescent).

The part of the Moon that we can see is the fraction of the side that is lit up, which is visible from Earth. Half of the Moon is always lit up, but how much of this we see depends on where the Moon is in its orbit. As the Moon travels around the Earth, the fraction first grows larger, until the Moon is full (directly opposite the Sun). The fraction then grows smaller again, until the Moon is on the same side of the Earth as the Sun.

In the course of one night, the 24 hour spinning motion of the Earth takes us past one of these distinctive phases of the Moon, and as the month progresses (and the Moon moves farther around its orbit of the Earth) the observed phase gradually changes.

These simple diagrams raise a question. Why is the full Moon not in the shadow of the Earth? In other words, when the Moon travels around to the opposite side of the Earth to the Sun, why does the Earth not block the light from the Sun?

The answer to this is that the Moon does not orbit the Earth in the same plane as the Earth orbits the Sun (the plane of the ecliptic). The consequence is that the Earth rarely stops light from reaching the Moon. When it does a lunar eclipse results.

The slightly odd thing about the Moon is that it rotates once on its axis in exactly the same time (27.3 days) that it takes to orbit the Earth. The result is that it always keeps the same face towards us so that we never see the other side of the Moon. The effect is not so strange as it may seem. It is caused by the interaction between the Earth, the Moon and the tides. The Moon did not always spin at this rate.

The dark side of the Moon is not totally dark as it is illuminated by reflected light. It has been observed by satellites and space probes and by astronauts who have orbited the Moon.