What's the Difference Between a Blood Moon and a Lunar Eclipse?

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. Spoiler alert: It doesn't.

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're seeing 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 the 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.

Given that shadows on Earth aren't red, why does Earth's shadow cast a red hue on the moon? The answer takes into account another aspect of life on Earth: our atmosphere. During a total eclipse, the the only sunlight reaching the moon is passing through Earth's atmosphere. As rays of the sun's light pass through the atmosphere and bend around Earth, they pick up a reddish color. The more dust or clouds in Earth's atmosphere during an eclipse, the redder the full moon will look.

This is the same reason that sunrises and sunsets appear red too. Here's how NASA puts it: "Blue light has a shorter wavelength and is scattered more easily by particles in Earth’s atmosphere than red light, which has a longer wavelength. Red light, on the other hand, travels more directly through the atmosphere. When the sun is overhead, we see blue light throughout the sky. But when the sun is setting, sunlight must pass through more atmosphere and travel farther before reaching our eyes. The blue light from the sun scatters away, and longer-wavelength red, orange, and yellow light pass through." As we said earlier, all the sunlight reaching the moon during a lunar eclipse is passing through Earth's atmosphere and thus will be red.

If you want to see the next blood moon – that is to say, the next total lunar eclipse, mark your calendars: it will occur on Nov. 8, 2022 (or 9th, depending on your longitude). During that eclipse, the blood moon will be visible across all of North America, South America, Asia, and Oceania if the skies are clear. Only those in Europe and Africa will be unable to see the lunar eclipse or blood moon during this event.

You don't need any special equipment to see this phenomenon, though binoculars or a telescope will enhance the view. Totality of the eclipse will begin at 5:17 a.m. ET (2:17 a.m. PT) on Tuesday, Nov. 8 and end at 6:42 a.m. ET (3:42 PT), according to NASA.

November's full moon will be the last blood moon for three years. The next blood moon occurs on March 14, 2025, according to NASA.