Age Of Moon
The age of the Moon is the number of days since the last new moon.
The phases of the Moon cycle roughly once every 29 days. The Moon is full when it is around 14 days old, and approaches new moon again when it is around 29 days old.
This cycle is often further subdivided into four week-long intervals. At the end of the first interval, the Moon is exactly half-illuminated at first quarter. At this time, it is visible in the evening sky and sets at around midnight.
Around day 14, it reaches full moon, rising at dusk and setting at dawn. Not only does it appear at its brightest at full moon, but it is also visible for much or all of the night.
Around day 21, it is half-illuminated once again at last quarter, but this time it is visible in the morning sky, rising at around midnight.
Finally, after 29 days, it returns to new moon once again and passes close to the Sun, disappearing from view.
Waxing and waning phases
The Moon is said to be waxing through the first half of this cycle, from new moon through to full moon. During this two-week period, it is visible in the evening sky and sets during the course of the night, a little over an hour later each day.
Conversely, the Moon is said to be waning through the second half of this cycle, from full moon through to new moon. During this two-week period, the Moon rises during the course of the night and is visible in the morning sky.
When to observe the Moon
The Moon is usually best observed at around first or last quarter.
These times, when the Moon shows half phase, are ideal for observing it with a pair of binoculars or a small telescope. The border between the light and dark portions of the Moon's disk is the best place to look for detail on its surface, because along this line, the Moon's surface is illuminated at a very shallow angle. As a result, mountains and crater rims cast long shadows which are very easy to see. An observer on the Moon would see the Sun on the horizon, casting long shadows just like the ones we see on Earth at sunrise and sunset.
At first quarter and last quarter, when the terminator line is down the middle of the Moon, it is best presented for view, without any foreshortening.
At full moon, comparatively few craters are visible, though the imposing crater Tycho, the maria, and many smaller features remain readily visible.