Q:

What does "moon phases" mean?

A:

Quick Answer

Moon phases refers to the appearance of the moon in the night sky and how much of its visible surface is illuminated. As the moon travels through its orbit, the percentage of its illuminated surface facing Earth waxes and wanes between full illumination and full darkness. This creates several distinct phases: full, gibbous, quarter, crescent and new.

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Full Answer

When the moon is directly opposite the sun as seen from Earth, its entire surface is illuminated, creating a full moon. As it orbits the Earth, its relative position changes, and when only 75 percent of the surface is lit, it is called a gibbous moon. The quarter moon is the phase when the moon appears half-lit in the sky, and the crescent moon occurs when only 25 percent of its surface is lit. Finally, the new moon occurs when the moon is directly between the Earth and sun, and its illuminated surface is pointing directly away from the planet. The moon then completes its journey, showing more of its surface over the latter half of the lunar month until it reaches a full moon once again. The phases that occur as the moon travels from full to new are waning phases, while those that occur as the moon fills out again are waxing phases.

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