Optics

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Shadows, lunar eclipses and phases of the moon

The Moon and eclipses

1) Motions of the Moon

The Moon revolves around Earth in about twenty eight days.
In the same time, The Moon also rotates in very peculiar way: The Moon spins on its axis showing always the same side to the Sun, and keeping the other side in the shadow: they are calles the " bright side " and the " dark side ".
Depending on the position of the Moon, we can see various parts of the bright side of the moon.
The different aspects the Moon takes during its revolution around the Earth are called "phases of the Moon"
These are the main phases of the Moon:
- When all the bright side can be seen: full Moon.
- When the bright side can't be seen at all: new Moon.
- When we can see only half of the bright side: Quarter of Moon.

2) Eclipses

The Moon ceases to be lighted by the Sun when it penetrates in the shadow cone of the Earth: this an eclipse of Moon

There are two kind of Moon eclipse:
- If The whole Moon penetrates in the shadow of Earth then it is a total eclipse. The bright side of the Moon can't be seen at all.
- If only a part of the Moon penetrates in the shadow of Earth then it is a partial eclipse. Only a part of the bright side disappears during this eclipse.

A Moon eclipse can happen only if the Sun, the Earth and the Moon are aligned. A lunar eclipse is therefore possible only at the full Moon. Nevertheless there is no lunar eclipse at every lunar cycle because the revolution plane of the Moon does not coincide completely with that of Earth

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