The Moon goes through basically eight phases as it orbits the Earth. These phases are a result of the Moon's alignment with both the Earth and the Sun. Before we go into the descriptions of the phases, we should define two terms:
The sidereal lunar cycle, or "sidereal month" is how long it takes the Earth, Moon, and a background star to line up. This period last about 27.3 days. After another 2.2 days, the Earth, Moon, and Sun are lined up in what is called the lunar month, or synodic month. This period is approximately 29.5 days. The difference between these periods is because as the Moon orbits the Earth, the Earth is also orbiting the Sun.
The first phase during the lunar month is called the "New Moon". Basically, it is when the Moon is between the Earth and the Sun. The unilluminated face of the Moon is pointed towards the Earth. However, the new Moon is not completely dark. Earthshine, reflection of sunlight off the Earth's surface can illuminate the surface of the Moon.
The next phase is the Waxing Crescent phase. Waxing in astronomy means that something is increasing in size. As the Moon moves around the Sun, a sliver of the Moon on the right limb is illuminated by the Sun as seen from the Earth. What we see is the crescent shape of the Moon.
The Moon is said to be aging as it goes from New Moon to Waxing Crescent. About a week after the New Moon, the Moon reaches the First Quarter phase because it has reached the end of one quarter of the lunar cycle. The entire right half of the Moon is illuminated as seen from the Earth.
The next phase is called Waxing Gibbous. The illuminated surface of the moon crosses the center of the moon and spills over into the left half of the Moon. The illuminated portion of the Moon resembles an oval with pointed ends.
Halfway through the lunar cycle, we hit the Full Moon. No, werewolves do not appear during the Full Moon. People do not get crazy during the Full Moon, though it may appear to be so. The Full Moon is the phase where the Earth is between the Sun and the Moon and from Earth we see the entire face of the Moon illuminated.
After the full Moon passes, the illuminated portion of the Moon begins to decrease, or wane. The next phase is the Waning Gibbous phase. The right limb has a darkened crescent shape, and the left half and a portion of the right half of the Moon are illuminated, opposite the Waxing Crescent phase.
After three weeks in the lunar cycle, the Moon reaches the Last Quarter or sometimes called Third Quarter. It is called this because either the Moon is at the beginning of the last week of the lunar cycle or the end of the third week of the lunar cycle.
In the final week before the Moon is "reborn" into the New Moon, the Moon goes through the Waning Crescent phase. Only a sliver on the left limb of the Moon is illuminated.
Here is an image showing the relative position of the phases with respect to the Earth and the Sun.
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