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Have you ever heard the term “gibbous moon”, and wondered what it means? A gibbous moon is one of the phases of the Moon, when the size of the illuminated portion is greater than half but not a full Moon.
The period between a first quarter moon and a full moon is known as a waxing gibbous moon, because the illuminated region of the Moon is increasing from day to day. After it becomes a full moon, but hasn’t reached the last quarter, the Moon is called a waning gibbous moon.
Think for a little bit about the position of the Moon as it orbits around the Earth. When the Moon is in between the Earth and the Sun, and the three objects are lined up perfectly, the angle between the Moon and the Sun is 0-degrees. But the Moon is orbiting the Earth. So when it reaches the first quarter, we see half the Moon illuminated; this is because the Moon is forming a 90-degree angle with the Sun. Above this angle, the Moon is considered to be a waxing gibbous moon. And then after it reaches 180-degrees, when the Moon and the Sun are on opposite sides of the Earth, it becomes a waning gibbous moon.
We have done many stories on Universe Today about the Moon. Here’s one about the phases of the Moon.
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?