What is a Moon Phase?

The phases of the moon are determined by the moon's position in relation to the Sun and Earth. Half of the moon is always illuminated by sunlight. How much of the lighted portion of the moon that we see, when observed from Earth, gives the moon a distinct appearance, the Moon Phase, and that appearance changes and also repeats itself over a period of time. 

Phases of the Moon

The location of the moon, relative to the sun and earth, determines the different moon phases that can be observed. The first quarter moon and last quarter moon occur when both the moon and the Earth are the same distance away from the sun, illuminating half of the moon. These moon phases are referred to as “quarter” moons because when the moon is in this position, it has either completed one-quarter of its orbit or has one-quarter of the orbit left in its path to be completed. A full moon appears when the moon is the farthest away from the sun, exposing the fully illuminated surface of the moon to people on earth. The new moon phase occurs when the moon’s orbit takes it closest to the sun. At this point, the sun is illuminating the “far” side of the moon, leaving the dark or unilluminated side facing the earth.

The Cycle

A complete moon cycle takes approximately 29-and-a-half days to progress through the eight distinct moon phases. Since this cycle is shorter than the average calendar month, a single moon phase may occur twice in one month. You will be able to see the same phase at the very beginning and very end of the calendar month.

Many AcuRite products display the moon phase to help you track the current phase of the lunar cycle. The following image is one example of the moon phase icons included on AcuRite products: