The Phases of the Moon

Most people have looked up at the night sky and seen the moon. Sometimes, it looks like this:

Other times, it looks like this:

Sometimes, it even looks like it's not there!

Why does the moon behave so? What makes it look sometimes like a crescent, other times like a semi-round circle, and other times like a full circle or a dark spot? In this post I'll explain why the moon has phases, hopefully in a fashion that is clear and understandable.

To start off, we need a sense of our bearings. Below is a diagram showing the moon's position with respect to Earth:

The moon orbits around the Earth - every 27 days, it completes one revolution around the Earth and returns (more or less) to its original position relative to the Earth. By relative, I mean that from the Earth's viewpoint, the Moon is in its old position. Remember that the Earth rotates around the Sun, so Earth travels in an orbit around the Sun, with the Moon orbiting around the Earth at the same time.

What does this mean? Let's think about the we can see the Moon at night, first of all. The Moon is not a star - it doesn't produce light like the Sun (or else the night sky would be very bright!). Instead, the Moon reflects light produced from the Sun.

Now, imagine that the Moon is at the part of its orbit where it lies on a line in between the Sun and the Earth:

We can see that sunlight will strike the Moon's surface and reflect, indicated by the lighter shade of grey in the above diagram. However, we are located on Earth! As a result, the reflected moonlight will bounce back in the Sun's direction instead of towards us at the Earth. We can only see the dark side of the moon - in other words, nothing at all. This is called the new moon phase, where the moon is dark and we can't see it at night.

Armed with this knowledge, the other phases of the moon are relatively easy to understand. After the new moon phase comes the crescent moon phase. The moon then looks like a crescent in the sky - the moon isn't changing shape, however. Instead, the half of it that is illuminated shows up as a crescent from our point of view on Earth:

Next comes the full moon. As its name suggests, the full moon is when the moon is completely lit up - in other words, the half that is illuminated by the Sun is completely visible by us:

With knowledge of these three phases, we can understand the eight phases of the moon. A full chart is provided below:

Physically speaking, the phases are not particularly different from each other. At all times, about half the moon is lit up - the only thing changing is the fraction of the half that we can see from Earth. For example, the waxing crescent and waning gibbous phases are like inverses of each other - the half-circle shift in the position of the moon is what makes them look different.

So that's how the phases of the moon work. You may have noticed a few inconsistencies, though. For example: why can we even see the full moon? If the full moon occurs when the Earth is between it and the Sun, shouldn't the Earth block the light from the Sun, preventing it from reflecting it off the moon in the first place?

That's absolutely correct, and this happens during a lunar eclipse, when the full moon passes into Earth's shadow:

You may remark that lunar eclipses don't happen every 27 days! That's because the moon does not lie exactly on the same plane that the Earth and the Sun occupy. Instead, it lies on a slightly tilted plane:

The Earth doesn't orbit the Sun on the same plane as the moon orbits the Earth. Because of this, the full moon can occur. Every so often, however, the orbit of the moon lines up with the orbit of the Earth in just the right way so that the full moon happens when the Earth, Sun, and moon are all in the same plane. That's when we get a lunar eclipse.

As you can see, the phases of the moon are actually very simple if you take some time to think about it. With the simple step of abstracting your viewpoint to a spot outside of Earth, the phenomenon of moon phases becomes easy to understand.