Phases of the Moon
A diagram which demonstrates why the moon goes through phases.
Click on image for full size (13K GIF)
The diagram shows the Moon in different positions along its orbit around the Earth. The Sun is far away and acts like a light bulb in this picture. Half of the Moon is always reflecting light from the Sun (let's call it the light side), and half of the Moon is always in shadow (the dark side). But that's not all that is happening. Only half of the Moon is facing the Earth so that we can see it (let's call it the near side). The other half is facing away from the Earth (the far side). The phases occur because the near side isn't always the side reflecting light from the Sun.

When the Moon is between the Earth and the Sun (labeled 1), the side of the Moon facing the Earth is the dark side. The Moon cannot be seen. We call this the New Moon because it begins a new cycle of phases. When the Earth is between the Sun and the Moon (5), the side facing the Earth is the light side. We call this Full Moon, even though we can only see half the Moon at one time.

Halfway in between these times (3 & 7), only half of the near side of the moon is reflecting sunlight. So we can only see one-quarter of the Moon. We call these phases First and Third Quarters.

All the phases of the Moon have special names which indicate how much of the illuminated Moon can be seen from Earth, and whether this part is going to grow or shrink.

How Do the Phases Get Their Names?
The Phases of the Moon.
Click on image for full size (15K GIF)
When the Moon appears smaller than a quarter, we call it a crescent. When the Moon appears larger than a quarter, we call it gibbous. When the moon is getting bigger (phases New to Full) it is waxing. When it is getting smaller (phases Full to New) it is waning.

For example, if today the Moon were a waxing crescent, then we would only see a little piece of the half of the Moon that is reflecting sunlight. But tomorrow, that piece would be bigger. If today the Moon were a waning gibbous, then we would see a big piece of the half of the Moon that is reflecting sunlight. But tomorrow, that piece would be smaller.

There is an easy way to tell if a crescent Moon is growing or shrinking. If the crescent Moon looks like a "C" (C for collapsing!), it is shrinking.

Moon Phases Descriptions