As you’ve probably noticed, the Moon looks different every day. Sometimes we see a new moon, when the Moon is enshrouded in shadow, and other times we see a full moon, when the entire face of the Moon is illuminated. During the first half of a lunar month, when the amount of illumination on the Moon is increasing, astronomers call this a “waxing moon”.
To understand why the brightness of the Moon changes, think about how the Moon orbits the Earth. When the Moon is in between the Earth and the Sun, the side of the Moon facing away from the Earth is fully illuminated, and the side we can see is shrouded in darkness. But the Moon doesn’t stay still, it’s constantly orbiting the Earth, taking 29.53 days to complete the cycle from new moon to full moon and then back again to new moon. As the Moon orbits the Earth, the angle between the Moon and the Sun increases; and this is why you get a waxing moon.
At the beginning, the angle between the Moon and the Sun is 0-degrees. And then over the next 2 weeks, that angle increases. After a week, the angle between the Moon and the Sun is 90-degrees and continuing to increase to 180-degrees, when the Sun and Moon are on opposite sides of the Earth.
When the Moon is halfway illuminated, astronomers call it a first quarter moon. In between the first quarter moon and a full moon, astronomers call the Moon’s phase a waxing gibbous moon.
The reverse of a waxing moon is called a waning moon. This is when the Moon is decreasing in brightness from a full moon back to a new moon.
We have an article about all the phases of the Moon here on Universe Today.
You can listen to a very interesting podcast about the formation of the Moon from Astronomy Cast, Episode 17: Where Did the Moon Come From?