Introductory Astronomy: Phases of the Moon

The Moon appears to go through phases. In other words, the amount of the Moon that we can see changes over time in a cyclic period that repeats itself approximately once a month. (The actual period of this cycle is approximately 29.5 Earth days.) The cause of these phases is the relative positions of the Sun, Earth, and Moon. As seen in the diagram, if the Sun is located off to the right of the picture, the Earth and Moon are illuminated as shown (the white areas being the lighted areas). Notice that no matter what phase the Moon is in, HALF of it is ALWAYS lit by the Sun. (Which half is always lit? The half that is facing the Sun.) The reason that we do not always see a Moon which is half lit is because of our position relative to the Moon and the Sun. As the Moon moves in its orbit, different portions of it appear (to us!) to be lit up as we look at it from Earth. This is why we see lunar phases.

For example, if the Moon is at position 1 in the diagram, the half of it that is lit by the sun is facing away from us, so we do not see the moon at all. This is called a new Moon. When the Moon is at position 3, we see half of the half of the Moon that is lit up. We call this a quarter Moon. The important point is that the moon doesn't change, nor does the amount of the Moon which is lit by the Sun. The only thing that changes is the position of the Moon relative to us and the Sun. This change in position causes the phases.

To see the current phase of the moon, click here

Waxing vs. Waning

The diagram above shows what the different phases of the Moon would look like as seen from Earth (note that the numbers below each phase correspond to the different positions of the Moon as seen in the first diagram). It appears that the Moon repeats certain phases: there are two crescent, gibbous, and half phases each month (each cycle). These phases are not exactly identical, however. Look closely at the diagram. You will notice that during phases 1 through 5, the amount of lighted area increases over time from right to left. When this occurs, the Moon is said to be waxing. During phases 5 through 8, the amount of light area decreases (or the darkened area increases) from right to left. When this occurs, the Moon is said to be waning. Therefore you can tell if the Moon is waxing or waning based on whether the right side of the Moon is dark or light. (Of course, this only works in the Northern Hemisphere. In the Southern Hemisphere the effect is just the opposite!) Astronomers use this to distinguish between the repeated phases of the moon by referring to the waxing or waning crescent, gibbous, and half phases.

Telling Time

The phase of the Moon can tell you the time of day. For example, because a full Moon is seen when the Moon is on the opposite side of the Earth from the Sun, an observer on the Earth will see the Moon rise just as the Sun sets. The next day, the Moon will rise approximately one hour later (because the Moon will have moved farther along on its orbit). This means that a waning half Moon will rise at midnight, a new moon at dawn, etc...

Phases of Other Planets

Another interesting point about phases is that they are not restricted to the Moon. We can observe phases of Venus and Mercury as well. (We do not observe phases of the outer planets because their orbits lie outside the orbit of the Earth and therefore they can never be between the Sun and the Earth.) There are two differences between the phases of the Moon and the phases of Venus and Mercury, however. The first is that we do not see all of the phases of Venus and Mercury. Think for example about the full phase. This occurs when the Sun lights the side of the Moon which faces the Earth (i.e., when the half of the Moon that is lit IS the side of the Moon which faces the Earth). Clearly Venus and Mercury cannot be on the opposite side of the Earth from the Sun because their orbits lie inside the orbit of the Earth. Therefore, for the Sun to light the half of Venus or Mercury which faces the Earth, these planets would have to be on the other side of the Sun from the Earth. In order to see this phase for these planets, we would have to be able to look through the Sun (which we can't). The second difference is that Venus and Mercury appear to change size as they cycle through their phases. (Actually, only Venus is really close enough to the Earth for this to be noticeable.) The reason for the apparent change in size is that the distance between the Earth and Venus changes as they orbit the Sun. So, Venus looks smaller when it is in its crescent phase than when it is in its gibbous phase.