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The Shadow of the Dog

The dogs I know like to be walked at least twice a day -- once in the morning, and once in the evening. But even though I'm the same person and it's the same dog in the morning and evening, our shadows change...

In the morning, our shadows point in one direction. This is true regardless of which way we are facing. The shadows of trees, buildings, cars, fire hydrants, and blades of grass all point in the same direction as ours. In the evening, our shadows all point in approximately the opposite direction.

And if we happen to take our walk closer to sunset, our shadows are longer than if we go out in mid-afternoon.

What makes shadows act this way?

Grade Level: Upper Elementary (3-5)
Curriculum Topic Benchmarks: S1.2.6, S1.3.5, S11.1.1, S11.3.3, S12.1.2, S12.3.3
Subject Keywords: Shadow, Dog, Sun, Season, Day, Fire Hydrant

Author(s): Ralph Kahn
PUMAS ID: 06_19_96_1
Date Received: 1996-06-19
Date Revised: 1996-12-26
Date Accepted: 1996-12-26

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Comment by Elizabeth Roettger on March 8, 1999
"Weird shadows --
I've been noticing a brown shadow from my brown chair on my blue rug. (At first I thought it was a stain on the rug.) Yesterday, I noticed my cat had a yellow-brown shadow on a black&white chair. I finally tracked down the diffuse light source that was filling in these otherwise dark shadows (turns out to be light through yellow slats, reflected several times and thus made more diffuse and difficult to locate.)

So, when you're walking the dog on a white surface (especially snow), do you notice that the shadows are sometimes a bit blue? The diffuse light from the sky can fill in the shadows. (It's not bright enough to notice where the sunlight hits directly.)

Are there any other colored shadows that are easy to notice in daily life? (I know the dramatic ones can be produced intentionally.)

Author's Response: On snow, if the day is clear and the sky blue, the dog's shadow will have a bluish cast. You can often see bluish skylight on other types of ground as well, such as in the shadows of trees on a clear summer day.

In fact, for this reason, shades of blue are referred to as "cool" colors, whereas shades of yellow and red are called "warm" colors. Spectrally, you'd expect the shorter-wavelength, higher-energy blue photons to be thought of as "hotter." But stand in the yellow sunlight and then in the bluish shadowlight on a hot summer day and you'll know why it's the other way around."

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