A Trick Of The Light Turns The Moon Red During The Lunar Eclipse

Supermoons are great and all, but blood moons are way cooler, right? We're pretty sure that scientifically speaking, blood moons occur every time a vampire attacks a gloomy eastern European village. But then again, we're not scientists. Let's find out what blood moons really are.

Blood Moon On The Rise

Let's get one thing straight. A blood moon isn't the result of any kind of dark prophecy, and it doesn't signify the appearance of a sexy undead blood-drinker (dang!). Actually, it happens between two and four times a year — every time there's a lunar eclipse. When the moon passes into the Earth's shadow, it turns a rusty shade of red. But if it's cast in shadow, then why can we see it at all?

The answer is Earth's atmosphere. The air you breathe extends about 50 miles above the ground, forming a kind of atmospheric halo. Normally when sunlight shines directly through the atmosphere, air molecules scatter the short-wave blue light and make the sky appear blue. The sky turns red at sunset because the sun being lower on the horizon forces its light to travel through more of our atmosphere, scattering and rescattering the blue light until mostly long-wave red light comes through.

A blood moon, then, is sort of like a moon watching a sunset. When the Earth fully blocks the sunlight on its path to the moon, the little light that does get through travels through a whole lot of atmosphere. The atmosphere scatters the blue light and sends only the red light the moon's way. A blood moon can also happen when there's a lot of smoke or other haze in the air since that can scatter blue light in the same way.

The Cosmic Dance

When you consider the physical orientation of the Earth, moon, and sun during a lunar eclipse, you start to see how eclipses can only come when the moon is full. When the Earth is between the moon and the sun, people on Earth see the bright side of the moon in its entirety. But why isn't every full moon a lunar eclipse? Because the moon orbits at about a 5-degree angle, meaning it only cuts across the Earth's shadow two to four times a year.