Solar eclipses roll around like clockwork — astronomers can predict them thousands of years in advance. But the clock is like something from the Middle Ages, with lots of complicated moving parts. So it took a long time to figure out how the parts all work together.

A solar eclipse occurs when the Moon passes directly between Earth and the Sun, blocking some of the Sun’s light. If the alignment is just right, the Moon completely covers the Sun, creating a total eclipse. If the alignment is off, though, then the Moon covers only part of the Sun.

Calculating an eclipse requires detailed knowledge of Earth’s orbit around the Sun, and the Moon’s orbit around Earth. Both orbits are slightly lopsided, so the distance between Sun and Earth, and between Earth and Moon, changes from day to day. And orbital speed changes depending on the distance.

The angle of the Moon’s orbit is important, too, because it tells us where on Earth that shadow will appear. So is Earth’s rotation on its axis, because it’s not steady. Thanks to the tides — caused by the Moon — the rate at which Earth spins can vary — one of the most complicated factors in calculating a solar eclipse.

And astronomers have calculated that there will be a partial eclipse early tomorrow. It’ll be visible from the Arctic Ocean down through Greenland, Scandinavia, and parts of Asia. The U.S. misses out on this one, though — skipped over by the clockwork motion of the solar system.

Script by Damond Benningfield