A diagram which demonstrates why the moon goes through phases.
Click on image for full size
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.
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