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 off in the
distance, lighting the Earth-Moon system. At any position, half of
the Moon is illuminated by the Sun (the light side of the Moon) and
half is not (the dark side). Also, half of the Moon is visible to the
Earth (the near side of the Moon) and half is not (the far side). As
the Moon moves around the Earth, we can see different fractions of the
illuminated half of the Moon.
When the Moon is between the Earth and the Sun (1), the near side of
the Moon is the dark side. The Moon cannot be seen. We call this New
Moon, the beginning of a new cycle of lunar phases. When the Earth is
between the Sun and the Moon (5), the near side is the light side. We
call this Full Moon, even though we only see half the Moon.
Halfway in between these times (3 & 7), only half of the near side of
the moon is illuminated by the Sun. 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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