Why Does the Moon Have Phases

Every 29 days the moon goes through a transition from a full moon, to a quarter to a new moon, back to a quarter, and then finally a full moon once again. This transformation is beautiful to observe. My favorite phase is when the moon is just a slight sliver, a Cheshire cat smile. Figure 1 shows several of the phases of the moon.

One of the major misconceptions about the phases of the moon is that they are caused by the shadow of the earth. This is not true! The Earth's shadow does play a role in eclipses, but we will get to that at the end of this article.

First lets talk about why the moon is bright. The moon is bright because the sun is shining on it. This is the same reason the earth is bright during the day. The tricky part for us to understand is that half of the moon is always illuminated by the sun. The side that is facing the sun. The other half of the moon is dark. This is also true on the earth. It is always day on one half of the earth, while it is night on the other half.

So if half of the moon is always illuminated by the sun, why don't we see it as always full? The reason is that moon is orbiting the earth. Every 29 days the moon makes a complete orbit around the earth. For part of its orbit the moon is between the earth and the sun. At other times the moon is further from the sun than the earth. As a result we are seeing the moon's illuminated portion from different angles. This causes the phases of the moon.

I have drawn a diagram of the moons orbit in Figure 2. In this diagram you can see that the side of the moon facing the sun is always illuminated. But the half of the moon facing the earth is illuminated by different degrees.

So based on Figure 2 can you see when the moon is full (as seen by earth)? Position A, B, C, or D in Figure 2?

If you guessed position A, when the earth is between the moon and the sun, you were right! When the moon is at this point in its orbit, it's illuminated side faces the earth. It appears as a full moon to us on earth.

So when is it a new moon?

If you said position C, then you are right again! Can you tell me why? You can email me to check your answer.