What Is a Blood Moon?
A "Super Blue Blood Moon" will light the skies over Alaska, Hawaii and western North America Jan. 31, 2018.
Credit: NASA

A "blood moon" happens when Earth's moon is in full eclipse. While it has no special astronomical significance, the view in the sky is striking as the usually whiteish moon becomes red or ruddy-brown. The next "blood moon" will happen during the total lunar eclipse of Jan. 31, which will be visible from parts of North America, Australia, the Pacific and Asia.

The last lunar eclipse (a partial eclipse) was on Aug. 7, 2017. NASA has a list of all the lunar eclipses until 2100, and here are a few coming up:

  • Jan. 31, 2018: Total eclipse. Visible from Asia, Australia, Pacific Ocean, western North America.
  • July 27, 2018: Total eclipse. Visible from South America, Europe, Africa, Asia, Australia.
  • Jan. 19, 2019: Total eclipse. Visible from North and South America, Europe, Africa.
  • July 16, 2019: Partial eclipse. Visible from South America, Europe, Africa, Asia, Australia.

The moon orbits around Earth, while Earth orbits around the sun. The moon takes about 27 days to orbit Earth and goes through regular phases in a 29.5-day cycle. The difference in these two cycles has to do with the relative positions of the sun, Earth and moon, which change during the moon's orbit.

Lunar eclipses can only happen during a full moon, when the sun fully illuminates the surface. Usually a full moon has no eclipse because the moon orbits in a slightly different plane than the Earth and the sun do. However, at times the planes coincide. Earth passes in between the moon and the sun and cuts off the sunlight, causing an eclipse.

If Earth partially blocks the sun, and the darkest part of its shadow falls across the moon's surface, it is called a partial eclipse. You will see a black shadow taking a bite out of the moon. Sometimes, the moon passes through the lighter part of Earth's shadow, causing a penumbral eclipse. Only seasoned skywatchers will be able to tell the difference, because the moon only darkens very slightly.

This global map shows the areas of visibility (weather permitting) for the "super blue blood moon" eclipse of Jan. 31, 2018.
This global map shows the areas of visibility (weather permitting) for the "super blue blood moon" eclipse of Jan. 31, 2018.
Credit: NASA

During a full eclipse, however, something spectacular happens. The moon is fully in Earth's shadow. At the same time, a little bit of light from Earth's sunrises and sunsets (on the disk of the planet) falls on the surface of the moon. Because the light waves are stretched out, they look red. When this red light strikes the moon's surface, it also appears red. 

How red the moon appears can depend on how much pollution, cloud cover or debris there is in the atmosphere. For example, if an eclipse takes place shortly after a volcanic eruption, the particles in the atmosphere will make the moon look darker than usual.

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.

Ancient cultures often 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 Jamaica. At first the people there were welcoming, but over time, Columbus' crew grew restless and murdered or robbed some of the natives. Understandably, the natives 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 Jamaicans 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, the frightened Jamaicans "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.