Full Moon Calendar 2013 | Next Full Moon

The moon shows its full face to Earth once a month. Well, sort of. In fact, when things are perfectly aligned and the moon would be 100 percent full, there is a lunar eclipse, so in reality the moon is never perfectly full. And sometimes — once in a blue moon— it is full twice in a month. See the calendar of full moons below. First, how it works:

Just a phase

The moon is a sphere that travels once around the Earth every 29.5 days. As it does so, it is illuminated from varying angles by the sun — what we see when we look at the moon is reflected sunlight. On average the moon rises about 50 minutes later each day, which means sometimes it rises during daylight and other times during nighttime hours. Here’s how the phases go:

This image of the full moon was taken by Alamelu Sundaramoorthy from Portland, Ore. on July 3, 2012.
Credit: Alamelu Sundaramoorthy

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At "new moon," the moon is between the Earth and sun, so that the side of the moon facing toward us receives no direct sunlight, and is lit only by dim sunlight reflected from the Earth. As it moves around the Earth, the side we can see gradually becomes more illuminated by direct sunlight.

After a week, the moon is 90 degrees away from the sun in the sky and is half-illuminated from our point of view, what we call "first quarter" because it is about a quarter of the way around the Earth.

A week after this, the moon is 180 degrees away from the sun, so that the sun, Earth and moon form a line. The moon’s disk is as close as it can be to being fully illuminated by the sun, so this is called "full moon." [Related: How 2012's Full Moons Got Their Strange Names]

A week later the moon has moved another quarter of the way around the Earth, to the third quarter position. The sun's light is now shining on the other half of the visible face of the moon.

Finally, a week later, the moon is back to its new moon starting position. Because the moon’s orbit is not exactly in the same plane as Earth’s orbit around the sun, they rarely are perfectly aligned. Usually the moon passes above or below the sun from our vantage point, but occasionally it passes right in front of the sun, and we get an eclipse of the sun.

Credit: Karl Tate, SPACE.com

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Each full moon is calculated to occur at an exact moment, which may or may not be near the time the moon rises where you are. So when a full moon rises, it’s typically doing so some hours before or after the actual time when it’s technically full, but a casual skywatcher won’t notice the difference. In fact, the moon will often look roughly the same on two consecutive nights surrounding the full moon.

Full moons in 2013

— Tim Sharp, Reference Editor