Phases of the Moon

While the moon is a constant object in the sky, its orbit around the Earth causes only portions of it to stay visible at any given time. These periods of time are called "phases"; these phases come and go in a cycle that takes approximately 27.3 days to complete. There are a total of eight Moon phases from one New Moon to another, but they are split into two categories: waxing and waning.

Waxing phases

When one says that the Moon is "waxing," what is meant is that more and more of the Moon is growing visible, resulting in it looking significantly brighter. This phase starts with a New Moon, meaning that the Moon is not reflecting much or any sunlight, giving off the appearance of simply not existing. As it starts to brighten, it enters the Waxing Crescent Phase until it reaches the First Quarter, when exactly half of the Moon is visible. From this point until the entire face of the Moon facing Earth is visible, it is referred to as Waxing Gibbous.

Waning phases

The Moon begins waning only after growing into a Full Moon. The nomenclature for these periods mirrors that of the waxing phases, but in reverse. The progressive darkening of the Moon results in the Waxing Gibbous phase. This phase ends when half of the Moon (the opposite half as during First Quarter) is visible; this is called the Last Quarter. From here, the Moon returns to an apparent crescent shape, the Waxing Crescent. It remains in this phase until it darkens completely, growing into a Full Moon again to complete the cycle.

Using these guidelines, it is possible to tell within which phase the Moon lies. While the cycle does not quite match with a month, it is always the same length of time, making it a reliable natural clock of sorts. With practice, one can even estimate the time of year or the season simply by counting Moon phases.