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Moon Phases

Phases of the Moon

It is not very often we can watch the phases of the Moon change in a few minutes, as they do here overhead. In the real world you notice the shape of the Moon changing over the course of a few weeks. Phases are the different shapes the Moon appears to have during the night and day. They are a result of the ever-changing arrangement of the Sun, Moon, and Earth.

The Moon Orbits Earth

Three things affect the way the Moon looks throughout a month: its spin rate, its orbit around our planet, and the illumination of half its surface by the Sun.

The Moon spins one time on its axis as it orbits Earth once every 27.3 days. As a result, we always see the same side of the Moon turned toward Earth.

A Walk Through the Phases

The Moon cycles through its phases every 29.5 days. It starts out looking like a thin crescent in the sky. Over the next two weeks, its appearance slowly changes to a luminous Full Moon that lights up the night. The Moon takes another two weeks to shrink back to a crescent before disappearing briefly during New Moon. A few days later, it shows up again as a crescent. This recurring cycle of phases is the basis for our month.

The Far Side of the Moon

© USGS AstrogeologyWe always see the same side of the Moon. The far side was hidden from our view until astronaut photographs and spacecraft images gave us our first look at this lunar wilderness. Those views revealed a more rugged and cratered surface than the face of the Moon we see. Although the far side is sometimes incorrectly referred to as the "dark side of the Moon," it receives just as much sunlight as the side facing Earth.