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.