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. So why does our moon turn red and not black when bathed in 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. When the moon starts to pass into the central portion of Earth’s shadow, called the umbra, that’s when the fiery glow stands out.
“When the moon is within the umbra, it will turn a reddish hue. Lunar eclipses are sometimes called ‘Blood Moons’ because of this phenomenon,” NASA said.
Related: How to watch the total lunar eclipse of May 2022 online
As for why the moon looks red, it has to do with the way that light scatters. A phenomenon called Rayleigh scattering causes some wavelengths of light to 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. Even though Earth is way bigger than the sun, the light rays are able to bend around the edges of our planet before being reflected onto the moon. Even so, the sun’s light first passes through Earth’s atmosphere; and during that trek, particles in the atmosphere preferentially scatter the shorter-wavelength blue light. In that way, the longer-wavelength orange and red light bathe the moon’s surface.
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). But the shorter wavelengths — such as violets and blues — get absorbed and then scattered every which way, giving them more chances to hit 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. Atmospheric conditions can also affect the brightness of the colors. For instance, extra particles in the atmosphere, such as ash from a large wildfire or a recent volcanic eruption, may cause the moon to appear a darker shade of red, according to NASA.
The moon doesn’t always hide completely behind Earth’s shadow. During partial lunar eclipses, the sun, Earth and moon are slightly off in their alignment, and so our planet’s shadow engulfs just part 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.