A diagram which demonstrates why the moon goes through phases.
Click on image for full size
Phases of the Moon
The diagram shows the Earth, and 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 reflecting light from the Sun (the light side of
the Moon) and half is in shadow (the dark side). Also, half of the
Moon can be seen from Earth (the near side of the Moon) and half
can not (the far side). As the Moon moves around the Earth, the dark
side and the far side overlap and produce the phases of the Moon as we
are familiar with them.
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 the 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.
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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