Rotation and Phases of the Moon

Lunar Orbit

• Scientists recognize two types of lunar cycles. The first is called the synodic period, or lunation, and is the time it takes for the moon to complete its visible cycle and return to its initial phase. For example, the time between one new moon and the next is one synodic period. This period takes 29.5305882 days, or approximately one month. The time it takes for the moon to complete its actual orbit and return to its initial orientation in regard to the earth and the sun, however, takes slightly less time and is called its sidereal or orbital period. This period lasts 27.3217 days. It is shorter than the synodic period because the earth is also moving, altering the angle at which we view the moon.

New Moon

• When the moon is between the sun and the earth, it is said to be new. A new moon is a dark moon -- that is, the side that faces the earth is not the illuminated side. A new moon begins the lunar cycle and rises and sets within minutes of the sun. Because it is visible during the day instead of at night, people sometimes say that there is "no moon" during this phase.

Waxing Phases

• The new moon only lasts for one day. After this, more of the illuminated side is visible on earth each night. This makes the moon appear to grow and creates the waxing phases. The first of these phases, which begins the day after the new moon, is the waxing crescent. During this phase, less than half of the visible side of the moon is illuminated. This phase lasts until the first quarter moon, which occurs exactly one-fourth of the way through the synodic period and appears as half of a full moon. Like the new moon, this phase lasts only one night and then gives way to the waxing gibbous, which grows until the night of the full moon.

Full Moon

• On the night of the full moon, the entire illuminated side of the moon faces the earth. During this phase, the moon is on the opposite side of the earth from the sun and is halfway through its synodic period. During the waxing phases, the moon rises and sets later each day until the full moon, which is at its peak at midnight. Occasionally, the moon, earth and sun are perfectly aligned during this phase, which causes the earth to cast a shadow over the moon, creating a lunar eclipse.

Waning Phases

• After the full moon, the moon appears to decrease in size, but its phases are mirror images of the corresponding waxing phases. For instance, the waning gibbous shrinks toward the left instead of growing toward the right, and the left half of the moon is illuminated during the third quarter. After this, the waning crescent shrinks each night until the cycle begins again with a new moon.