Science For Everyone - The Red Ring in the Sky

Copyright © Karl Dahlke, 2022

The moon orbits the earth every 29.5 days, presenting the same face to us at all times. The plane of this orbit is not perfectly aligned with the sun, thus, when the moon swings around behind the earth, it usually travels below the shadow of the earth, or above it. However, once or twice a year, our moon passes directly through the earth's shadow. This is a lunar eclipse.

You may have seen this, if you're lucky. You are on the night side of the earth, and a full moon shines overhead. It is big and white, bright enough to read the headlines of a newspaper. Slowly, the earth's shadow creeps across the face of the moon, and cloaks it in darkness. And yet, the moon does not disappear; it does not fade to black. Instead, the moon turns a dark red. You need a clear sky, and an absence of city lights, to see it. If you have some advanced notice, it's well worth a trip to the country.

After a few hours, the moon leaves the shadow of the earth, and is bright and full once again. But why does it turn red? Why doesn't it disappear altogether?

Every culture was amazed by this recurring phenomenon. It was often referred to as a blood moon, since the red color was similar to blood. The Inca thought a jaguar was attacking and eating the moon. Might the jaguar come for the earth next? They shouted and shook their spears, hoping to turn the jaguar away.

In a similar vein, the Chinese believed a legendary celestial dragon consumed the moon. They too would bang drums and shout, to frighten the dragon away. Many other myths have been recorded.

Isaac Newton, scientist extraordinaire, wasn't interested in myths; he wanted to know. More than any other man in history, Newton unlocked the secrets of the universe. Calculus, physics, newtonian mechanics, gravitation, orbital mechanics, the tides, the motions of the planets, optics, and the rainbow, all brought to light by his unimaginable genius. ancient questions, which evaded the greatest minds in history, were answered for the first time. The blood moon is just one example.

Newton figured out that the air in our atmosphere bends light. But it doesn't bend all colors equally. Red light bends more than green, which bends more than blue. The red sky that heralds the sunrise, which had been a mystery since the down of time, was finally explained. Just before the sun pokes up above the horizon, its red light is bent slightly, following the curve of the earth. Reds and oranges and a splash of yellow streak across the sky, then the sun climbs above the horizon and the day begins. At night, the process is reversed. The sun dips below the horizon, while its red light bends around the curve of the earth and forms the glow of sunset.

Picture the sun far off to the right, the earth in front of you, and the moon to the left. The moon is about to enter the shadow of the earth. The right half of the earth is lit by the sun, in other words, the day side. The left half of the earth is dark. An imaginary circle runs around the earth, separating day and night. This is called the terminator. Red and orange sunlight bends around the earth as it passes through our atmosphere, from right to left, across the terminator. If you stood on the earth, just on the dark side of the terminator, you would experience the glow of sunrise or sunset on the horizon. trace the red light rays past the earth and out into space. They bend inward, just a hair, and enter the shadow of the earth. A quarter million miles away, this red light bathes the moon in a gentle glow. All the sunrises, and all the sunsets, along the terminator, shine their red-orange light onto the moon, and give it the soft red hue that is the hallmark of a lunar eclipse. If our planet had no air, all the sunlight would travel in straight lines, and the moon would disappear behind the shadow of the earth. But then again, without an atmosphere, we wouldn't be here to see it.

What if you lived on the moon? That was unthinkable when Newton published his book, A Treatise of the Reflexions, Refractions, Inflexions and Colours of Light, in 1704, but today, a moon base is technically feasible. You are sitting outside, in a space suit of course, looking up at the heavens. Don't look directly at the sun; with no air, it's even brighter than on earth. There is no blue sky, everything is black and white. The land around you is stark white as far as the eye can see, “Magnificent Desolation”, as Buzz Aldrin famously stated when he first set foot on the moon during Apollo 11. Suddenly the glaring light seems to dim. If you could look at the sun, you would see the earth taking a bite out of it. Over the next hour, the earth completely covers the sun. More accurately, the moon, which you are sitting on, moves into the shadow of the earth. Now it is safe to look up, and the sight is spectacular. A bright red ring shines down from above. this is the red light that bends past the terminator of the earth. Inside the ring is pure black. Outside the ring lies a field of stars, as if it were night. The stars don't twinkle, since there is no air on the moon. They stand perfectly still in their constellations, as if they will shine for all time. Look down, and the ground is a red-orange color, illuminated by the red ring above. The rocks are red in every direction; even your moon base, 100 meters away, is red. this soft magic persists for over an hour, then the moon moves out of shadow, and the sun's white glare returns.

With all the rovers and satellites we have sent to the moon, somebody must have a picture of this red ring, right? Well I couldn't find one on the internet. Maybe we never pointed our cameras toward the sun at just the right time. If you find such a picture, let me know and I'll included here. One thing is sure; when we finally establish a moon base, the crew will be sending back pictures of the red ring in the sky, and the rust-colored landscape below.