When a lunar eclipse occurs and our lone satellite approaches the shadow of the Earth, the face of the moon turns red.
Although this red hue is most striking during a total lunar eclipse, the moon is cast in scarlet light even during partial lunar eclipses. So why does our moon turn red and not black when it is immersed in the shadow of the Earth?
For example, the only one lunar eclipse visible in North America this year it happens on May 15th or May 16th, depending on your location. For some viewers, they will see a total lunar eclipse on May 15, while others will see how the moon only moves towards the edge of the 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 shadow, that’s when the fiery glow stands out.
“When the moon is within the shadow, it takes on 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 how the light scatters. A phenomenon called Rayleigh scattering causes some wavelengths of light to scatter more than others. In particular, the wavelengths of light scatter most of the tiny particles which are about one-tenth of the wavelength of light or smaller.
During a total lunar eclipse, the sun, Land and the moon are perfectly aligned so that our blue planet prevents the sun’s rays from hitting the moon. Even though the Earth is much larger than the sun, rays of light are able to bend around the edges of our planet before being reflected on the moon. Even so, sunlight first passes through the earth’s atmosphere; and during that journey, particles in the atmosphere preferentially scatter shorter wavelength blue light. In this way, longer wavelength orange and red light floods the surface of the moon.
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 streak of colors corresponding to their individual wavelengths – are filtered through our atmosphere, where tiny molecules of nitrogen and oxygen let through longer wavelengths. long as red, orange and yellow, go directly to the ground (missing our line of sight). But shorter wavelengths, such as purples and blues, are absorbed and then scattered in every direction, giving them more of a chance to hit our eyes.
The moon will change various shades during the different phases of a total lunar eclipse, transforming from a grayish initial to orange and amber. Weather conditions can also affect the brightness of colors. For example, extra particles in the atmosphere, such as ash from a large fire or a recent volcanic eruption, can make the moon appear a darker shade of red, according to NASA.
The moon does not always hide completely behind the shadow of the Earth. During partial lunar eclipses, the sun, Earth and moon are slightly out of alignment, so our planet’s shadow only envelops part of the moon.
A novice sky observer may not even notice the third type of lunar eclipse, the penumbra type, in which the moon is in the 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 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 Eclipse.
Original article in Live Science.