From Earth, the Moon appears to go through "phases" as light from the Sun reflects off its surface. At times, it appears fully illuminated, and at other times, it is almost or entirely invisible in the sky. This allows the Moon to pass through determined cycles during each month, as its position around Earth determines how much light is reflected. This change is not due to Earth-Shadow, but rather due to the position of the Moon relative to the Sun; since the Moon always presents the same face towards the Sun, the view from Earth sometimes includes the Dark Side of the Moon, which is almost invisible.
The phases of the Moon are as follows:
- New Moon: The moon is almost entirely obscured, as it is between the Sun and Earth, and Sunlight reflects in the wrong direction.
- Waxing Crescent: The first crescent of the Moon becomes visible as Sunlight reflects off it at an angle.
- First Quarter: about half of the Moon is visible.
- Waxing Gibbous: The Moon becomes more visible as more of its reflective side is exposed.
- Full Moon: The Moon's full surface is visible as the Earth is between the Sun and the Moon; maximum Sunlight is reflected.
At this point, the Moon goes through the opposite cycle:
- Waning Gibbous
- Last Quarter
- Waning Crescent
- New Moon
This phase cycle occurs fully once per month, and can be used to estimate the time of month. Sometimes, the movement of the Sun and Earth interfere with the phases, such as in the case of a Lunar Eclipse.
A phase of the moon is defined by the shape of the illuminated portion of the moon, as it appears to observers on Earth. The moon's appearance changes throughout the month, as it travels on its orbit around the Earth. The relative positions of the sun, Earth, and moon determine which portion of the moon's sunlit surface can be seen.
When the moon is between the Earth and the sun, its daylight side is turned away from the Earth, so the moon can't be seen at all. This is called the "new moon." As the moon continues its revolution around the Earth, a slice of its surface becomes visible. This is called the "waxing crescent phase. About a week after the new moon, half the moon is visible. T his is the "first quarter" phase.
During the following week, more than half the surface of the moon can be seen; this is called the "waxing gibbous" phase. Finally, about two weeks after the new moon, the moon and sun are on opposite sides of the Earth. The side of the moon facing the sun is also facing the Earth, and the moon's entire illuminated side is seen as a "full moon."
For the rest of the month, the moon goes through the reverse process, appearing as a progressively smaller portion of a circle. During the third week the moon is in the "waning gibbous" phase; at the end of that week it appears as a half-circle and is called a "third quarter" moon. It then enters the "waning crescent" phase. Less and less of the moon is visible each day until a new moon occurs again.
Sources: Abell, George O. Realm of the Universe, 5th ed., pp. 72-73; Menzel, Donald H., and Jay M. Pasachoff. A Field Guide to Stars and Planets, pp. 318-22.