Why Does the Moon Appear to Change Shape?
The Moon orbits around the Earth every 29 Ĺ Earth days. At all times, half of the Moon is lit by the Sun. The other half of the Moon facing away from the Sun is in darkness. As the Moon orbits around the Earth we can see more and more of its lit side. This process slowly changes. These changes are called the phases of the Moon.
Phases of the Moon
When the Moon is between the Earth and the Sun, it is called the new moon. The new moon rises and sets about the same time as the Sun does, but we canít see it because the side that is being lit by the Sun is the side we cannot see from Earth.
After about one week after the new moon, the moon looks like a half-circle, and it is called the first quarter because the moon has completed one-quarter of its orbit around the Earth. Half of the Moonís sunlit side is now visible from Earth. The first quarter moon rises at about noon and sets at about midnight.
One week after the first quarter moon, the Moon has moved to a point where the Earth is between the Moon and the Sun. We can now see the entire sunlit side of the Moon. The full moon rises as the Sun sets and sets as the Sun rises.
One week after the full moon, the Moon again looks like a half-circle, and it is called the last quarter because the Moon has completed all but the last quarter of its orbit around the Earth. Half of the Moonís sunlit side is again visible from Earth. The last quarter moon rises at about midnight and sets at about noon.
Great Moon Phase Videos
We found some incredible videos to help you better understand moon phases. Click on the images below to open up another window to view the videos. When you're finished, just close the window and you'll come right back here!
Waxing is a word meaning "growing." A waxing moon occurs between a new moon and a full moon. The amount of the lit surface we can see is growing, heading to the west side of the Moon.
Waning is a word meaning "shrinking." A waning moon occurs between a full moon and a new moon. The amount of the lit surface we can see is shrinking, heading to the east side of the Moon.
A lunar eclipse (when the Moon is blocked by the Earthís shadow) can only happen when there is a full moon. This is the only time when Earth is between the Moon and the Sun. A lunar eclipse does not occur every time there is a full moon, though, because the Moonís orbit takes it higher and lower than Earthís orbit around the Sun. This means the orbits are not exactly in line and the Earthís shadow isnít on the Moon.
A solar eclipse (when the Sun is blocked by the Moon) can only occur when there is a new moon. This is the only time when the Moon is between the Sun and the Earth. A solar eclipse doesnít occur every time there is a new moon, though, because the Moonís orbit takes it higher and lower then the Earthís orbit around the Sun. This means the orbits are not exactly in line and the Moon isnít exactly in-between the Sun and the Moon.
Does the moon make its own light? No, because it is not a star. Then why can you read by the moonlight? Well, this is because the Moon reflects the Sunís light, but it only reflects about 7% of the light that falls on it. Earth reflects about 39% of the light that falls on it.
If we could fill the sky with full moons (which would take about 105,000 full moons!), it would still be only ľ as bright as the Sun is. The Sun is 465,000 times brighter than a single full moon!
Unless otherwise noted, all images courtesy of NASA. Permission for use at http://www.nasa.gov/gallery/photo/guideline.html.
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