Phases
of the Moon
From the Earth, the Moon appears to undergo cyclic changes in its shape
or appearance. This phenomenon occurs because the Moon orbits the Earth
once every 29 1/2 days on a plane very close to that of the Earth's
orbit around the Sun. We can see the Moon because it reflects light
from the Sun back to us here on Earth.
As it moves through its orbit the Moon reflects light from the Sun
at varying angles according to the Moon's position in its orbit around
the Earth. To an observer on Earth this seems to make the portion of
the Moon lit by sunlight constantly change throughout the Moon's 29
1/2 day orbit.
The same face of the Moon is always facing the Earth and over the
full orbit of the Moon the night/day dividing line shadow slowly passes
over the surface of the Moon. Because the Moon always has the same part
facing the Earth this means the Moon rotates on it own axis once for
every orbit, ie. the lunar day is equivalent to 29 1/2 Earth days.
New Moon
When the Moon is positioned between the Earth and
the Sun we see virtually no reflected light from the Moon. This is known
as the "New Moon" and the Moon appears very close to the Sun
in the sky at this time of the month. To some ancient peoples the Moon
seemed to be swallowed up by the Sun and was born again as a "New
Moon".
Full Moon
As the Moon progresses in its orbit around the Earth from the New Moon,
more of the lit portion becomes visible from Earth until, 15 days later,
where a point is reached in the sky directly opposite the Sun where
the whole disc of the Moon can be seen; a "Full Moon". The
other half of the Moon's orbit causes the lit portion to decrease until
the "New Moon" is once again reached.
Crescents and Quarter Moons
The phases in between the "Full Moon" and "New Moon"
are known as "Crescents" and "Quarter Moons". The
"Phases of the Moon" animation shows a complete orbit with
the phases of the Moon shown against the relative positions of the Earth,
Sun and Moon.
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