When a lunar eclipse occurs and our lone satellite inches into Earth’s shadow, the moon’s face becomes painted red.
Though this red hue is most striking during a total lunar eclipse, the moon gets cast in a scarlet light even during partial lunar eclipses. Why is the moon turning red when it is under Earth’s shadow?
For instance, the only lunar eclipse visible in North America this year happens on May 15 or 16, depending on your location. For some viewers, they will see a total lunar eclipse on May 15, while others will watch as the moon moves into just the edge of Earth’s shadow for a penumbral lunar eclipse. The fiery glow is visible when the moon passes into the umbra (central portion of Earth’s shadow).
As for why the moon looks red, it has to do with the way that light scatters. Rayleigh scattering is a phenomenon that causes certain wavelengths of light scatter more than others. Specifically, wavelengths of light scatter the most off teensy particles that are about one-tenth the wavelength of the light or smaller.
During a total lunar eclipse, the sun, Earth and moon are perfectly lined up so that our Blue Planet blocks the sun’s rays from hitting the moon. Although Earth is much larger than the sun’s, light rays can bend around the planet’s edges before reaching the moon. Despite this, the sun’s rays first pass through Earth’s atmosphere. During that journey, the shorter-wavelength blue light is preferentially scattered by particles in the atmosphere. The longer-wavelength red and orange light that bathes the moon’s surface are reflected back to Earth by the longer-wavelength blue and orange wavelengths.
Perhaps counterintuitively, this phenomenon also explains why the sky is blue. During the day, the sun‘s light waves — which are made up of a swath of colors corresponding to their individual wavelengths — get filtered through our atmosphere, where the tiny nitrogen and oxygen gas molecules let the longer wavelengths such as reds, oranges and yellows, pass through straight to the ground (missing our line of sight). The shorter wavelengths, such as violets or blues, are absorbed and scattered in every direction. This gives them more chances of hitting our eyes.
The moon will change various shades during different stages of a total lunar eclipse, morphing from an initial grayish to orange and amber. The brightness of colors can also be affected by atmospheric conditions. NASA says that extra particles from the atmosphere such as volcanic eruptions or large wildfires can cause the moon’s color to change to a darker hue.
The moon doesn’t always hide completely behind Earth’s shadow. Partial lunar eclipses are when the alignment of the sun, Earth, and moon is slightly off. This causes our planet’s shadow to engulf only a small portion of the moon.
A novice skywatcher might not even notice the third type of lunar eclipse, the penumbral kind, in which the moon sits in Earth’s penumbra, or its faint outer shadow.
The next two total lunar eclipses will occur on May 16, 2022 (visible in the Americas, Europe and Africa), followed by one on Nov. 8, 2022 (visible in Asia, Australia, the Pacific and the Americas), according to NASA.
Editor’s Note: This article was first published in 2016 and updated for the Super Blue Blood Moon lunar eclipse of 2018, 2021 and 2022.
Original article on Live Science.
Jeanna is the managing editor for LiveScience, a sister site to SPACE.com. Jeanna was a reporter at LiveScience and SPACE.com for three years before becoming the managing editor. She was previously an assistant editor at Science World magazine. Jeanna holds a Bachelor’s degree in English from Salisbury University and a Master’s Degree in biogeochemistry, environmental sciences, and science journalism from New York University. Follow Jeanna on Google+ to find out about her latest projects.