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A diagram which demonstrates why the moon goes through phases.
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Phases of the Moon

The diagram shows the Moon in different positions along its orbit around the Earth. The Sun is far away and acts like a light bulb in this picture. Half of the Moon is always reflecting light from the Sun (let's call it the light side), and half of the Moon is always in shadow (the dark side). But that's not all that is happening. Only half of the Moon is facing the Earth so that we can see it (let's call it the near side). The other half is facing away from the Earth (the far side). The phases occur because the near side isn't always the side reflecting light from the Sun.

When the Moon is between the Earth and the Sun (labelled 1), the side of the Moon facing the Earth is the dark side. The Moon cannot be seen. We call this the New Moon because it begins a new cycle of phases. When the Earth is between the Sun and the Moon (5), the side facing the Earth is the light side. We call this Full Moon, even though we can only see half the Moon at one time.

Halfway in between these times (3 & 7), only half of the near side of the moon is reflecting sunlight. So we can only see one-quarter of the Moon. We call these phases First and Third Quarters.

All the phases of the Moon have special names which indicate how much of the illuminated Moon can be seen from Earth, and whether this part is going to grow or shrink.

Last modified October 19, 2005 by Randy Russell.

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The Summer 2010 issue of The Earth Scientist, available in our online store, includes articles on rivers and snow, classroom planetariums, satellites and oceanography, hands-on astronomy, and global warming.

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