Total solar eclipses are rare, but our position in relation to the sun and moon suggests it should be pretty common. Every month, the moon passes between the Earth and the sun, giving the moon an easy opportunity to blotch out our glowing star. Alas, this happens infrequently, and the moon’s “wobble” is to blame.

The moon doesn’t orbit in a flat path, instead moving up and down as it circles the Earth. Scientists call this movement a wobble.

As the moon makes a complete lap around Earth, it certainly does pass between the Earth and sun, but because of its wobble, sometimes it passes too high or too low to actually eclipse the sun. When the moon’s wobbling transit brings it too high, the moon’s shadow “overshoots” the Earth. The opposite happens when its orbit is too low.

The moon’s wobble, however, is not random. Instead, it’s on a strict schedule, repeating itself approximately every 18 years, 11 days and, eight hours — a periodicity astronomers call the Saros cycle. Its consistency is one reason why scientists can make such precise predictions about when the next total solar eclipse will arrive.

NASA illustrates the point well in the 45-second video below.

Because of the moon’s scheduled wobble, the coming August 21 total solar eclipse is rare to begin with, but it’s extraordinarily rare for another reason. As the moon completely obstructs the sun, it will cast its shadow across the entirety of the United States — a thin, 70-mile wide path from Oregon to South Carolina. This cross-country route hasn’t occurred since 1776, when the United States declared its independence.

The last total solar eclipse to graze the U.S. occurred nearly 40 years ago, in 1979, but not nearly as many Americans lived close enough to see it. The moon’s shadow just passed over the Northwest, from Washington to North Dakota.

For guidance on how to watch the wobbling moon pass in front of the sun on August 21, Inverse provides insight on solar viewing glasses here and guidance on when to take them put them on (and take them off) here.

Photos via NASA (1, 2)