In the early hours of May 26, 2021 the western U.S. will see a full “Flower Moon” turn a reddish color for precisely 14 minutes and 30 seconds as our satellite briefly glides through the edge of Earth’s dark shadow in space.
It’s called a “Blood Moon,” but that colloquial name for a total lunar eclipse is both of questionable quasi-religious origin and, more importantly, betrays the actual colors you should expect to see during this relatively rare and beautiful sky event.
For the first time in over two years the full Moon—also a bigger-than-average “supermoon”—will turn a reddish-orangey-pinkish-copper color, with the brevity of the event expected to keep the northern limb of the Moon rather bright.
Semantics aside, the Moon will receive red light at night on May 26. So why will the Moon turn red? The incredible answer involves Earth’s atmosphere, sunsets, and our blue daytime sky.
“The red is the projection of all the sunrises and sunsets onto the lunar surface,” said Dr. Noah Petro, Project Scientist for NASA’s Lunar Reconnaissance Orbiter mission that's currently orbiting the Moon. “We see it turn red not because of some mythical fire-breathing dragon, but because of the properties of the Earth’s atmosphere scattering light.”
The physics at play is something called Rayleigh scattering.
What is Rayleigh scattering?
It’s the deflection of light off molecules of nitrogen and oxygen in the Earth’s atmosphere. Visible light is a form of electromagnetic radiation, which is made-up of photons—particles of light—that travel in different wavelengths.
It’s named after Lord Rayleigh (John William Strutt), a British physicist, who wrote a paper about it in 1871.
Why is the sky blue?
It’s not! It just looks blue during the day. Blue and purple have the shortest wavelengths of light, which more easily strike molecules and so more easily scatter.
The result is a blue sky because the Sun doesn’t give off much purple light, and besides, human eyes aren’t particularly sensitive to it.
Why are sunsets and sunrises orange?
Again, they’re not! The yellowy (and normally way to bright to look at) Sun isn’t doing anything differently when it’s close to the horizon, but to reach the viewer’s eyes the light from it is traveling through a lot more atmosphere.
The radiation is therefore striking more molecules. Red and orange have the longest wavelengths of light, so they more easily travel through Earth’s atmosphere to reach your eyes.
The intensity of light is less, too, having been filtered by the atmosphere, so you can glance at a setting Sun with your naked eyes.
The exact same physics is at play for a a sunrise, a moonrise and a moonset, but what about during a total lunar eclipse?
Why is the ‘Blood Moon’ red?
The science for a lunar eclipse is identical. Just as when you watch a setting Sun it bathes your surroundings in a reddish, orangey light, so Sun does the same to the Moon during lunar totality.
Since the full Moon is in Earth’s shadow no direct sunlight can reach it. The only light that can is sunlight filtered by Earth’s molecule-filled atmosphere.
Why you might see a ‘Blue Blood Moon’
However, the keen-eyed may see traces of blue at the beginning and end of the eclipse. That’s light that’s gone through Earth’s ozone layer, which absorbs red light and allows blue light to pass through it.
The effect of Earth’s atmosphere on lunar totality
Although the science doesn’t change, exactly what shade of reddish-orangey-pinkish-copper color light we’ll see on the full Moon on May 26, 2021 will depend on airborne particles.
For example, a lot of forest fires or a recent volcanic blast—even one many thousands of miles away—can change how the light interacts with particles in the atmosphere, and so can (drastically) effect the colours on the Moon.
Disclaimed: I am the editor of WhenIsTheNextEclipse.com
Wishing you clear skies and wide eyes.
