What is a Blood Moon?

The January 2019 Blood Moon during totality.
Photo: Dimitar Dilkoff/AFP/Getty

“Blood Moon” is another name for the Moon when it is totally eclipsed. For the Moon to eclipse, it must be at its full moon phase and pass through the Earth’s shadow. There are different types of lunar eclipses, but the Moon is only considered a Blood Moon when the Moon is fully eclipsed by the Earth.

An illustration of the Earth blocking the Moon from the Sun creating a total lunar eclipse.
Photo: NASA

Why is the Moon red?

A total lunar eclipse is sometimes called a Blood Moon because of the reddish hue that the Moon takes on during totality. The Moon appears this way because of the type of light it is able to reflect from the Sun. 

Usually, the Moon is lit by the Sun and appears white. It looks this way because the Sun emits all wavelengths of the color spectrum which combine to create white light. When the wavelengths reach the Moon, it is able to reflect a percentage of this light which creates its bright white appearance. 

During a total lunar eclipse, however, the Earth completely blocks the Moon from receiving the Sun’s light directly. At this point, the Moon is only able to reflect the light that travels through the Earth’s atmosphere first. When white light travels through the Earth’s atmosphere, it goes through a process called Rayleigh scattering. Through this process, some of the light particles pushed through the atmosphere are filtered out depending on their wavelength. For example, blue light, which has a lower wavelength, is scattered more easily than red light.

The visible color spectrum including the wavelengths of four colors in nanometers.
Photo: Ken Jeffery

This is why colors like red and orange are not filtered out when they go through Earth’s atmosphere and are able to reach the Moon to be reflected.

Types of Blood Moons

During January 2019, a spectacular and rare lunar event occurred called the Super Blood Wolf Moon. We now know what a Blood Moon is, but what do these other terms mean?

The term “Super Moon” is not exclusive to Blood Moons. A Super Moon is simply a full moon that appears slightly larger in size. This happens when the Moon gets a little closer to Earth due to its elliptical orbit. Super Moons happen fairly regularly, usually a couple times a year.

A “Wolf Moon” is a full moon in the month of January. There are special names for the full moons that occur every month depending on the month. These names are usually historical or seasonal in origin and are not named based on appearance. A Wolf Moon by itself looks just like any other full Moon.

The combination of these terms is used to describe what the Moon looks like and when it is occurring. The term Super Blood Wolf Moon means that the full moon in January was slightly closer to Earth and eclipsing.

Another term in the same world as Blood Moon is the “Half-Blood Moon”. A Half-Blood Moon is another name for a partial lunar eclipse. Half-Blood Moons, as the name suggests, look like half of a Blood Moon. During a partial lunar eclipse, the Moon is illuminated half by the Sun and half by the Sun’s light that travels through Earth’s atmosphere. Partial lunar eclipses occur sporadically and can occur from zero to a few times a year.

A Half-Blood Moon.
Photo: GettyImages

Other descriptions of Blood Moons

The term “Blood Moon” is sometimes used to refer to the full moon that occurs in the month of October. Typically, this moon is called a Hunter’s Moon. The full moon during this month is no different in appearance than a full moon in any other month. Its namesake originates from historic North America where hunters would hunt into the night to collect meat for the coming winter. Calling an October moon a Blood Moon refers to the gore of hunted animals, not the color of the moon.

When can we view Blood Moons?

A montage of images of the Moon as it eclipses.
Photo: Mona Sorayaei

Blood Moons are visible to the part of the Earth that is facing away from the Sun (the “night” side of Earth) at the time of the eclipse. This type of lunar event occurs, on average, 0-3 times a year. The total duration of a lunar eclipse can last a couple hours. As shown above, the Moon moves into Earth’s shadow and slowly turns red. The Moon can be in totality (completely behind the Earth) for minutes to around an hour. The Moon will then slowly make its way out of Earth’s shadow and turn its familiar shade of white again. 

  • Asia, Australia, and the Pacific will be able to view the next blood moon eclipse on May 26, 2021
  • North America, South America, Europe and Africa will be able to see a blood moon next on May 16, 2022
  • In between, there will be plenty of partial lunar eclipses viewable during this time period

Viewing Blood Moons from Earth

Blood Moons and Half-Blood Moons are safe to view with the naked eye, but the experience can be enhanced with astronomy gear. Cameras with a long exposure can capture the vibrant sight in detail but smartphone cameras also get the idea. Telescopes and binoculars can increase the clarity in which the backyard astronomer can see the event. Of course, a clear night is required to witness the eclipse so check your weather report.

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