What Is A Blood Moon?
QUICK ANSWER
A blood moon is the reddish color the Moon takes on during a total lunar eclipse. The color comes from sunlight refracted through Earth's atmosphere, which filters out blue light and lets red and orange light reach the Moon. The effect is essentially every sunset on Earth illuminating the Moon at once.
A blood moon is the dramatic reddish color the Moon turns during a total lunar eclipse. The term has caught on in recent years as social media has amplified interest in eclipses. Despite the ominous-sounding name, a blood moon is just a beautiful optical effect produced by sunlight bending through Earth's atmosphere. The same physics that makes sunsets red gives the eclipsed Moon its red glow.
Why does the Moon turn red?
Earth's atmosphere refracts sunlight. According to NASA, during a total lunar eclipse, the Moon enters Earth's umbra (the dark inner shadow) where direct sunlight is completely blocked. But the Moon doesn't go entirely dark. Earth's atmosphere bends some sunlight around the planet and onto the Moon's surface. Blue wavelengths scatter away in the atmosphere, while red wavelengths pass through and reach the Moon. The effect is similar to a sunset, in which the Sun looks red because its light is passing through a longer column of atmosphere.
Why is it called a blood moon?
Because of the deep red color it can take on. The exact shade varies from a coppery orange to a deep crimson, depending on atmospheric conditions. Particles in Earth's atmosphere (like volcanic ash or pollution) can make the color deeper. Some cultures throughout history have viewed blood moons as omens, while others have simply seen them as beautiful natural events. The term blood moon is informal and not an official astronomical term, though it's now widely used in news coverage of total lunar eclipses.
How often do they happen?
About one to two times per year on average. Blood moons happen during every total lunar eclipse, which occur roughly once or twice per year somewhere on Earth. Most years have at least one total lunar eclipse, though some years have none. Total lunar eclipses are visible from the entire night side of Earth (unlike solar eclipses with their narrow path of totality), so individual observers will likely see several blood moons in their lifetime without traveling. Each lasts up to about 100 minutes.
What about other meanings?
Some religious traditions use the term differently. In some interpretations of biblical prophecy, blood moon refers to specific lunar tetrads (four consecutive total lunar eclipses) that have been claimed as prophetic signs. However, lunar tetrads are normal astronomical events. They've happened many times throughout history. The most recent tetrad was in 2014-2015, with no extraordinary events corresponding to the eclipses. Scientifically, blood moon refers only to the visual phenomenon of a total lunar eclipse.
A blood moon is the reddish color the Moon takes on during a total lunar eclipse. The color comes from sunlight refracted through Earth's atmosphere, with blue wavelengths scattered away and red wavelengths reaching the lunar surface. The effect is similar to a sunset, only on a global scale. Blood moons happen during every total lunar eclipse, roughly once or twice per year. They're beautiful but normal astronomical events with no special significance.
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