Moon Phase Calendar for Mac

Download a detailed Moon Phase Calendar for 2008 - 2015, giving you exact dates and times of all major moon phases.

You may have personally observed that the moon goes through a complete moon phases cycle in about one month. That's true, but it's not exactly one month. The synodic period or lunation is exactly 29.5305882 days. It's the time required for the moon to move to the same position as seen by an observer on earth. If you were to view the moon cycling the earth from outside our solar system (the viewpoint of the stars), the time required is 27.3217 days, roughly two days less. This figure is called the sidereal period or orbital period.

Why is the synodic period different from the sidereal period? The short answer is because we see the sunlit moon from a slowly moving position: the earth! During the moon cycle, the earth has moved approximately one month along its year-long orbit around the sun, altering our angle of viewpoint, and thus, the phase. The earth's orbital direction is such that it lengthens the period for earthbound observers.