No matter where you are in the universe, the speed of light is 186,000 miles per second (around the world seven times with time to spare). The speed of light is a constant … it doesn’t change …
… until light hits something and has to go through it. When light traverses glass, water, even air, it slows down.
Moving in water, light slows to three-fourths its vacuum speed. Changing speed when it moves from water to air causes light to bend, which is why a spoon in a half-glass of water looks broken.
Bending light is useful. We make lenses of glass to slow light down, to bend it this way and that. Even air puts the brakes on light, just a bit, and bends it, making stars twinkle, making the air above hot pavement shimmer.
Because air bends light, a lens can be made out of air: A lens the size of the Earth.
The body of the Earth is opaque. It blocks light entirely and casts a shadow out into space. But surrounding the body of the Earth is a layer of transparent air. As light from the sun enters that layer of air, it slows down and bends. Earth’s atmosphere, acting as a lens, sends sunlight into Earth’s shadow.
Earth’s shadow is usually empty — there’s nothing in the shadow to catch the light sent into it. But Monday, a few moments before 11 p.m., the moon will cross into Earth’s shadow.
The full moon shines by reflecting sunlight. Earth blocks that sunlight. The moon in our shadow would be invisible but for the light bent into the shadow by the atmosphere.
Sunlight is a mix of all the colors of the rainbow, but not all those colors make it into Earth’s shadow. Stir a few drops of homogenized milk into a glass of water. Turn out the lights and shine a flashlight into the side of the glass.
Homogenized milk contains tiny globules of butterfat. These globules scatter light every which way … but not all colors equally. Blue is scattered a lot, red, hardly at all. From the side, scattered blue light gives the milky water a bluish tinge. Seen straight-on, and missing some blue, the flashlight takes on a slightly reddish cast.
Like globules of fat, molecules of air also scatter light every which way, blue more than red … thus, the sky is blue.
When the sun is on the horizon, rising or setting, its light travels through more atmosphere than usual and scatters more blue than usual. With less blue remaining, the rising or setting sun looks orangey-red.
A few minutes after midnight, Monday night, the moon will fall completely within Earth’s shadow. The only light striking the moon will be light bent into the shadow from opposite sides of the Earth — light from the sun rising over Europe, from the sun setting over Asia: reddish light. How much dust and smoke and cloud hangs in the skies of Europe and Asia will determine how red will be the light that strikes the moon, and how bright.
Every eclipse is different, depending on what’s happening in space. Monday night, planet Mars — bright yellow-orange — will lie close to the moon in the sky and also, that night, closest to Earth on this orbit. More personally, eclipses differ by time of day or night, weather local and distant, people sharing it, close and far away.
Al Stahler’s science programs can be heard on alternate Tuesdays at noon on KVMR (89.5 FM). He teaches classes to students of all ages, visits classrooms, and may be reached at firstname.lastname@example.org.