What is a blood moon?

A blood moon is another name for a total lunar eclipse, when Earth’s shadow completely covers the moon and turns it a deep red or reddish-brown color. This happens only during a full moon, when the sun, Earth, and moon line up perfectly.

Most full moons don't create a blood moon because the moon's orbit is slightly tilted compared to Earth's path around the sun. But when the alignment is just right, Earth blocks direct sunlight from reaching the moon. The only light that makes it through is filtered through Earth's atmosphere, scattering blue light and letting red and orange hues reach the moon's surface, creating the dramatic "blood moon" effect.

Related: Lunar eclipses 2025: When, where & how to see them

What happens during a blood moon?

Not all lunar eclipses create a blood moon. If Earth only partially blocks the sun, the darkest part of its shadow covers part of the moon's surface, creating a partial lunar eclipse; it looks as though a dark bite has been taken out of the moon. Sometimes the moon passes through the lighter outer shadow, known as the penumbra, causing a penumbral eclipse that is so subtle that only experienced skywatchers notice the faint dimming.

During a total lunar eclipse, the moon is fully inside Earth's shadow, and that's when the striking blood moon effect appears. Sunlight passing through Earth's atmosphere gets filtered and scattered, blue light is removed, leaving only the longer red and orange wavelengths to reach the moon. This is the same effect that makes sunrises and sunsets appear red, and it bathes the moon in a coppery glow.

The exact shade of a blood moon can vary depending on atmospheric conditions. Volcanic ash, wildfire smoke, dust, or pollution can make it appear darker or more intense. According to NASA, two to four lunar eclipses occur each year, and each can be seen from roughly half the planet. While planets and moons across the solar system cast shadows, only Earth produces lunar eclipses that completely cover its moon — a cosmic alignment that won’t last forever, as the moon is slowly drifting away by about 1.6 inches (4 centimeters) each year.

While there are planets and moons all over the solar system, only Earth is lucky enough to experience lunar eclipses because its shadow is just large enough to cover the moon completely. The moon is slowly drifting away from our planet (at roughly 1.6 inches or 4 centimeters a year) and this situation won't persist forever. There are roughly two to four lunar eclipses every year, according to NASA, and each one is visible over about half the Earth.

How a blood moon saved Christopher Columbus

Some ancient cultures didn't understand why the moon turned red, causing fear. At least one explorer — Christopher Columbus — used this to his advantage in 1504.

According to a Space.com Skywatching columnist Joe Rao, Columbus and his crew were stranded on an island now known as Jamaica. At first the Arawak people who lived there were welcoming, but over time, Columbus' crew grew restless and murdered or robbed some of the indigenous people. Understandably, the indigenous people weren't eager to help the crew search for food, and Columbus realized famine was drawing near.

Columbus had an almanac with him foretelling when the next lunar eclipse would take place. Armed with this information, he told the Arawak that the Christian god was unhappy that Columbus and his crew received no food. God would turn the moon red as a symbol of his anger, Columbus said. As the event took place, frightened people "with great howling and lamentation came running from every direction to the ships laden with provisions, praying to the Admiral to intercede with his god on their behalf," according to an account by Columbus' son Ferdinand.

Additional resources

If you have a budding astronomer at home who wants to learn more about eclipses check out Solar and Lunar Eclipses (Explore Outer Space) by Ruth Owen or take a look at NASA's Space Place webpage on the different types of moons.

Bibliography

K. G. Strassmeier et al "High-resolution spectroscopy and spectropolarimetry of the total lunar eclipse January 2019," Astronomy & Astrophysics, Volume 653, March 2020, https://doi.org/10.1051/0004-6361/201936091

Natural History Museum, "Lunar eclipse guide: what they are, when to see them and where," May 27, 2021.

Royal Museums Greenwich, "How to see a lunar eclipse," January, 2022.

Jessica Marshallsay, "Blood moon rising: How lunar eclipses might help us find life on other planets," The University of Queensland, January, 2022.