Prepare yourself for a sight tonight — not to mention some wild behavior, if the legends are true. The biggest full moon in 15 years is set to grace the Northern Hemisphere tonight.

Because the moon orbits along an egg-shaped ellipse, not a circle, its distance from us changes. Today, the moon is approaching its nearest point to Earth, so it should look about 14 percent bigger and 30 percent brighter than this year's other full moons, according to NASA.

Since the moon takes about 28 days to orbit Earth, it reaches its point of closest approach, called perigee, about once a month. But since the moon's orbit isn't a perfect oval — rather, it wobbles — some perigees are closer than others. Tomorrow's approach will be the closest the moon has come to Earth since 1993.

On top of that, tonight's moon will become full just four hours after perigee. The next time these two events will coincide will be in 2016.

The full moon isn't the only boost we get from this special alignment: Tides should be especially big, too. Lunar gravity at perigee pulls tide waters about an inch higher than usual.

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Image: NASA