Before we could really track the movement of the moon in relation to the sun, a lunar eclipse, and the blood red color associated with it were seen as a threatening display from the heavens. Ancient Mesopotamians thought it meant the moon was being attacked by demons. Some Chinese cultures believed that the moon’s blood red color meant famine and disease were coming. The Incans would throw spears and stones at the moon during an eclipse.

Ancient Greeks, however, correctly accepted the Earth’s shadow during a lunar eclipse as evidence that the Earth was round.

But what about the color? Why does the moon appear red?

The answer is in the Earth’s atmosphere. If the Earth didn’t have an atmosphere, the moon would just disappear during an eclipse. And there’d be no one here to enjoy it. During a lunar eclipse, the Earth is between the sun and the moon. And while the Earth completely blocks the sun, some of the light gets refracted through the atmosphere, but only the longer wavelengths make it through. And since red is the longest wavelength of visible light, that’s what we see. And it’s the same reason certain sunsets can have a beautiful red hue.

You may have heard that this weekend is also a supermoon—meaning the moon is as close as it can be to the Earth—what’s called perigee. But the difference between the full moon at perigee and the opposite, apogee, is like the difference between a Susan B. Anthony Dollar, and a quarter. Barely noticeable.

The total lunar eclipse will begin happening Sunday, January 20, 2019. You’ll start to see the moon change around 10:30 p.m. At midnight, the moon will be a deep red.