The armies battled beneath the hot sun. Neither side seemed to be winning. It was May 28, 585 BCE, the sixth year of war between the Lydians and the Medes in what is now Turkey. Suddenly, the light grew thin and the air turned cold. A hole appeared in the sky where the sun should be, surrounded by a halo of light. Surely this was a sign from the gods that they should make peace.

A total eclipse of the sun, as those ancient armies witnessed, could be frightening if you didn’t know it was coming. That fear could even stop a war! Nowadays, we know what causes eclipses. We can predict when and where they’ll happen, down to the minute and mile. Instead of frightening people, an eclipse can bring millions of us together in a wondrous moment of awe.

What Causes an Eclipse?

Earth orbits the sun, and the moon orbits Earth. The moon passing between the sun and Earth causes a solar eclipse. Although the moon is 400 times smaller than the sun, it’s 400 times closer to Earth. That means they look the same size when viewed from Earth.

During an eclipse, the moon casts a fuzzy shadow on the Earth. The center part of the shadow is called the umbra; people in the umbra see a total eclipse. For these viewers, darkness falls as the moon entirely covers up the sun. The gray, fuzzy part of the shadow is the penumbra; people here see a partial eclipse. The farther you are from the center, the less sun is covered up.

The moon takes about 27 days to orbit the Earth, but we don’t have eclipses every month. That’s because the moon’s orbit is tilted five degrees to our orbit around the sun. So the moon is often too high or too low when it’s between us and the sun. An eclipse happens only when the sun, the moon, and Earth line up perfectly.