Give Now

A Moment of Science

Why Does The Moon Look Red During A Complete Lunar Eclipse?

Some parts of the world will see a complete lunar eclipse on January 31, 2018.

a super blood moon

Photo: Martin Heigan (Flickr)

An example of a blood moon right before it hits totality.

Have you ever seen a complete lunar eclipse? It’s spectacular.  The moon appears red; it’s sometimes called the blood moon.

There’s a scientific explanation for why the moon appears that way. In this type of eclipse, the sun, the moon, and the Earth must be perfectly lined up. Otherwise, it’s not actually a complete lunar eclipse.

While the sun, moon, and Earth have to be perfectly lined up for solar eclipses, as well, the placement of the bodies is different. In a complete solar eclipse, the moon comes between the Earth and sun. 

All the phases of a blood moon/total lunar eclipse

From NASA: “Total Lunar Eclipse Over NASA’s Johnson Space Center
From open prairie land on the 1625-acre Johnson Space Center site, a JSC photographer took this multi-frame composite image of the so-called “Blood Moon” lunar eclipse in the early hours of April 15.”

Lunar Eclipse

During a complete lunar eclipse, the Earth is between the sun and the moon. So, the Earth’s shadow falls across the moon.

This isn’t a confirmation that the Earth’s umbra, also known as its shadow, is red. The reddish color is actually a result of the Earth’s atmosphere and its influence.

Remember, the Earth’s atmosphere goes up about fifty miles or so, and the sun’s rays always need to pass through this space. This is also true for when the Earth is casting its shadow across the moon.

Color Frequencies

So, that causes the light to be red because the sunlight’s rays include the color frequencies in the light spectrum – you know, like ROY G BIV (Red, Orange, Yellow, Blue, Indigo, Violet).

So, the atmosphere causes the sunlight to be filtered so that the green through violet parts of the light spectrum aren’t visible. So, what’s left over is the reddish part of the light spectrum.

So, we’re then left with a reddish moon, also called, the blood moon. In many parts of the United States, the next complete lunar eclipse that is visible takes place on January 31, 2018.

Sources And Further Reading:

  • Byrd, Deborah. “Why a totally eclipsed moon looks red.” EarthSky. September 25, 2015. Accessed November 1, 2017.
  • Howell, Elizabeth. “Why the Blood Moon Eclipse Turns Red.” Space.com. September 25, 2015. Accessed November 1, 2017.

Stay Connected

What is RSS? RSS makes it possible to subscribe to a website's updates instead of visiting it by delivering new posts to your RSS reader automatically. Choose to receive some or all of the updates from A Moment of Science:

Support for Indiana Public Media Comes From

About A Moment of Science

Search A Moment of Science