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The Moon is always half illuminated by the Sun. But from our perspective here on Earth, the amount of the Moon illuminated by the Sun changes over the course of a lunar month. The Moon starts out as a new moon, increases in brightness until it becomes a full moon, and then decreases in brightness again until it’s a new moon. When the Moon is decreasing in brightness, it’s said to be a “waning moon”.
To understand this process, think about the geometry of the Moon and the Sun compared to the Earth. When the Sun, Moon and Earth are perfectly lined up, the angle between the Sun and the Moon is 0-degrees. At this point, the side of the Moon facing the Sun is fully illuminated, and the side facing the Earth is enshrouded in darkness. We call this a new moon.
Over the course of the lunar month, the phase of the moon changes, because the angle between the Moon and the Sun is increasing from our perspective. A week into the lunar cycle, and the Moon and Sun are separated by 90-degrees. And then, when the Moon and Sun are on opposite sides of the Earth, they’re at 180-degrees – this is a full moon.
When the Moon starts to decrease its angle again, going from 180-degrees back down to 0-degrees, astronomers say that it’s a waning moon. In other words, when the Moon is waning, it will have less and less illumination every night until it’s a new moon.
When the Moon is no longer full, but it hasn’t reached a quarter moon, when it’s half illuminated from our perspective, we say that it’s a waning gibbous moon.
The reverse of a waning moon is called a waxing moon. This is when the Moon is increasing in brightness from a new moon to a full moon.
We have an article on Universe Today that describes all the phases of the Moon.
Want to know when the next waning gibbous moon is going to happen? NASA has a list of moon phases for a period of 6000 years.
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?