When a lunar eclipse occurs and our lone satellite moves inches into Earth’s shadow, the moon’s face is painted red.
While this red hue is most noticeable during a total lunar eclipse, the moon is 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 example, 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 in the edge of Earth’s shadow for a penumbral lunar eclipse. When the moon begins to pass into the central part of the Earth’s shadow, called the umbra, the fiery glow is noticeable.
“When the moon is inside the umbra, it takes on a reddish hue. Lunar eclipses are sometimes called ‘blood moons’ because of this phenomenon,” NASA said.
Related: How to watch the May 2022 total lunar eclipse online
The reason the moon looks red has to do with the way light is scattered. A phenomenon called Rayleigh scattering causes some wavelengths of light to scatter more than others. Specifically, wavelengths of light scatter most of the small particles that are about a tenth of the wavelength of light or smaller.
During a total lunar eclipse, the sun, Soil and the moon are perfectly aligned so that our Blue Planet blocks the sun’s rays from hitting the moon. Although the Earth is much larger than the Sun, the rays of light can bend around the edges of our planet before being reflected off the Moon. Yet the light of the sun passes through the earth’s atmosphere first; and during that journey, particles in the atmosphere preferentially scatter the shorter wavelength blue light. That way, the longer wavelength orange and red light bathes on 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 an array of colors that match their individual wavelengths — are filtered through our atmosphere, where the tiny nitrogen and oxygen gas molecules let through the longer wavelengths, such as red, orange and yellow. go straight to the ground (missing our line of sight). But the shorter wavelengths — like violets and blues — are absorbed and then scattered in every possible way, making them more likely to hit our eyes.
The moon will change different hues during different stages of a total lunar eclipse, changing from an initial grayish to orange and amber. Atmospheric conditions can also affect the brightness of the colors. For example, extra particles in the atmosphere, such as ash from a major wildfire or a recent volcanic eruption, can cause the moon to appear a darker shade of red, according to NASA.
The moon doesn’t always hide completely behind the Earth’s shadow. During partial lunar eclipses, the sun, Earth, and moon are slightly apart, causing our planet’s shadow to engulf only part of the moon.
A novice skywatcher may not even notice the third type of lunar eclipse, the penumbral kind, where the moon is in Earth’s penumbra, or its dim 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 November 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 2018, 2021, and 2022 Super Blue Blood Moon lunar eclipses.
Original article on Live Science.