New Moon Full Moon Crescents and Quarter Moons

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.

Phases of the Moon

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