Seeing Shadows

Groundhog Day approaches! Legend has it that if the groundhog, a large rodent related to squirrels and chipmunks, sees its shadow on February 2, we’re in for six more weeks of winter. If, on the other hand, the shadow stays hidden, we’ll have an early spring.

No, dear readers, I won’t try to convince you of the folly of this notion, nor tease out the subtle science behind rodent prognostication. Instead, let’s consider the groundhog’s shadow itself. What is a shadow, anyway?

Well, you might be thinking, it’s the place where the light doesn’t fall, of course! Everybody knows that! But the truth lurking in the shadow is a bit more intriguing than that.

First, consider that whenever an object blocks light, there are actually three shadows produced. The first shadow is found on the object itself. This is a shadow we experience at least once every day – we call it night time. The bulk of our planet blocks the Sun, so that half of our world is perpetually bathed in shadow. It’s always midnight somewhere.

The second shadow is the one cast into space. Imagine placing your hand behind a sunlit groundhog. Your hand passes into the shadow produced by the rodent’s bulk and instantly goes dark. Note that this shadow is always invisible! We only know about it because of the effects on objects that pass into its path. The Earth itself produces a long, thin shadow that travels at the speed of light into the deepest recesses of the universe. The shadow we made on Groundhog Day four years ago is just now reaching our nearest starry neighbors. In fact, one way we detect planets around other stars is by watching for the barely detectable shadows impacting our planet from so far away.

The third shadow is one the groundhog will see (or not see) on the Big Day. This is the shadow that falls on objects lying in the background. It’s the shadow that crawls across the Moon during a lunar eclipse, the shadow that makes your hand look like bunny ears when projected on the wall, and it’s the shadow that darkens the ground during the rare and spectacular solar eclipse.

Next, consider that these shadows are not all black and white. Shine a flashlight in a darkened room and you’ll see what I mean. The shadow made by any object in the path of that flashlight (maybe even your very own stuffed groundhog Billy M.) will have two parts. Only the center part, called the umbra, is truly black. This is the part of the shadow from which all light is blocked. Around the edge of the umbra is the penumbra, where only part of the light is blocked, resulting in a gray, partially lit shadow. The larger your light source, the fuzzier the shadow edges become. So what if the groundhog only sees its penumbra? Does that mean only three more weeks of cold?

Last, imagine you and your groundhog on a faraway world orbiting a pair of colorful stars. Our Sun is yellowish, but the light we receive from it is just about white. Other stars can be very different colors, including orange, red, and even blue. What if you lived on a world of two stars, one blue and the other red? All objects would then have two shadows, one red shadow (with the blue star blocked) and another blue shadow (where the red star is covered). Best of all, where the two star colors mix, everything would be bathed in a purple glow! What groundhog (or human) wouldn’t want to see that, even if it did mean six more weeks of winter?

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