Earth's Moon Homework #18

The Earth's moon shines so bright in the sky that it is something you can sometimes see during the day. But the Moon doesn't make its own light. The light you see coming from the Moon is reflected sunlight. The Moon is a sphere. When light shines on a sphere, the sphere is half lit and half dark. It doesn't matter where you position the sphere. It is always half lit and half dark.

The same is true for the Moon. It is always and half dark. The half that is lit is toward the Sun. The half that is dark is the side away from the Sun. It takes about four weeks for the Moon to orbit Earth. Look at the diagram below to think about where you are on Earth when you see the Moon during day or night.

The Moon doesn't always appear to be the same shape. That is because half of the Moon is always dark. The other half is lit by the Sun. As the Moon orbits the Earth, observers on Earth see different amounts of the lit half. The different shapes of the Moon are called phases. The phases change in a regular pattern as the Moon orbits Earth. The Moon completes its orbit and goes through its cycle phases in just over four weeks.

Four specific phases happen about one week apart. The new moon is invisible to us. It occurs when the Moon is between the Earth and the Sun, so we are looking at the dark side of the Moon. The full moon occurs when the Earth is between the Sun and the Moon, so we are looking at the lit side of the Moon. Halfway between the new Moon and full Moon is the first quarter Moon, so we see half lit and half dark. Half way between the full Moon and the new Moon is the third quarter Moon.

The Moon changes through its phases in a predictable pattern. The first appearance of the Moon after the new Moon is a thin crescent. The next day the crescent will be a little bigger. The Moon has moved in its orbit so that we can see a little bit more of the lit part, and a little less of the dark part. The crescent will get bigger each day until the first quarter moon. Getting bigger is called waxing. After the first quarter, the Moon continues waxing, but it is no longer a crescent Moon. It is a gibbous Moon. The gibbous Moon is nearly round on the side facing the Sun, with a bulge on the other side. The gibbous Moon waxes until it appears completely round. That is the full moon.

For the next two weeks the Moon is waning. Each day it appears to be a little bit smaller. The Moon has moved in its orbit so that we can see a little bit more of the dark part and a little bit less of the light part. The waning gibbous Moon becomes the third quarter Moon. Then the Moon becomes a waning crescent Moon. At the end of just over four weeks, the lunar cycle is complete and the Moon is new again.


Created with images by Lee_seonghak - "a total solar eclipse the night sky"

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