There are always headlines when a blood moon is coming, but they don't always explain what a blood moon is — and how it differs from a lunar eclipse. A blood moon is the same thing as a total lunar eclipse (one of three types of lunar eclipses that can be seen from Earth). Whenever you read a news story about an upcoming blood moon — or see beautiful pictures of one that has just happened — the scientifically accurate term for what you see is a total lunar eclipse.

To help make more sense of the blood moon phenomena, it helps to understand how eclipses work. An eclipse occurs any time one celestial body (such as Earth or the moon) passes in between the sun and another celestial body and casts a shadow on that other object. In the case of a lunar eclipse, Earth passes between the sun and the moon and our shadow is cast onto the moon.

However, it's even more complex: There are two different parts to a shadow. There's the penumbra, the partially shaded outer region of a shadow, and the umbra, the fully shaded inner region of a shadow. The penumbra is the lighter area that causes your shadow to appear fuzzy on the ground on a sunny day; the umbra is the darker area of your shadow.

Given all that, we can now make sense of the kind of lunar eclipse that creates a blood moon or a total lunar eclipse. When the moon passes completely into the Earth's umbra, it is in our shadow and appears with a reddish tint.