It sounds like something from a gothic horror novel but a "Blood Moon" is very real, although the only connection it has with blood is its color. Around twice a year, the Moon turns red. The event only happens during a total lunar eclipse and is caused by the same thing that keep us alive: the Earth’s atmosphere.

There are four eclipses happening in 2022. We've had the first partial solar eclipse and now we're about to experience the first total lunar eclipse, on May 15-16. So, what is a Blood Moon?  

Why is the Moon red?

The Moon becomes red for the same reason sunsets and sunrises are red. Sunlight is filtered through the Earth’s atmosphere and scattered. Blue light is scattered less than red, so the sky appears blue during the day but when the Sun is low on the horizon, this tints the heavens in crimson hues.

This has a peculiar effect. Even though the atmosphere is tiny compared to the size of the planet, the Earth’s shadow gets a bit of this diffuse light making it appear a bit red. During a total lunar eclipse, the Moon crosses Earth's shadow and once it is fully obscured it will appear red.

It is certainly a fantastic spectacle, seeing the Moon getting progressively darker as the shadow of our planet stretches across its silvery surface to then turn into a sanguine color at the moment of totality. Unlike solar eclipses that last just a few minutes and are only seen in a small region of the planet, lunar eclipses are visible to anyone anywhere on that specific night-side and can last up to two hours.

Why doesn’t the moon become red every month?

Blood moons are fairly rare. The eclipses follow long cycles and depend on the position of the Sun, Earth, and Moon. Given that the lunar orbit is inclined with respect to Earth’s orbit around the Sun, the eclipses can only happen when the three bodies are perfectly aligned – they are in syzygy. There are around four to seven eclipses in a calendar year, divided between solar and lunar eclipses. Total lunar eclipses tend to happen twice a year for a couple of years before a gap, while there are two to five solar eclipses, and a total solar eclipse takes place around every 18 months.

Many wonder, given that the Earth’s shadow is red if we shouldn’t see that coloring on the Moon regularly as its near side changes phase, but the shadow cast during a waning or waxing Moon doesn’t come from the Earth. The dark region of phases of the Moon is the shadow of the Moon itself as some lunar areas move into daytime and others move into nighttime.  

Why is the moon not red during a partial lunar eclipse?

The reason it's only around totality that the Moon becomes red and not before is that the Moon is bright in its reflection of sunlight, and that drowns out the subtle coloring of the Earth’s shadow.

The moment the Moon turns red is not exactly predictable either because it strongly depends on the atmosphere and its nebulosity at a particular time.

When is the next blood moon?

The next total lunar eclipse is this weekend, the night of May 15 and 16. It will be fully visible from half of North America, most of central, and all of South America. Part of Western Africa and Antarctica will also catch the spectacle before dawn. You can also catch it on the Virtual Telescope's live stream here. The next one will be on November 8, 2022, visible across the Pacific. After that, we'll have to wait for March 14, 2025.