Archive for December 18th, 2009

Blue shadows around the Front Range

by Jeff Bowell, guest blogger

Sometimes, shadows aren’t black.

I was reminded of this during my career as a musician with many, many hours spent on stage. Some stages would have overhead banks of lights, and these lights could produce shadows of the “opposite” color. White lights, of course, produced black shadows, but if the overhead lights were an intense red, your hand held over a white page of music would produce a dark green shadow.

Image from ehow.com

The opposite applied as well. If the overhead lights were green, a faint but still noticeable red shadow would appear on a white surface. Strong purple lights would yield a faint yellow shadow, and an orange light left a shadow that was distinctly blue. And an orange light left a shadow that was distinctly blue.

Actually, blue shadows are something that residents of the Front Range can see for themselves these wintry days, if you know where to look and when.

We think of our sun as just that – the “Sun,” but it’s also a star. Different stars shine at different frequencies of visible color, as well as producing frequencies of color invisible to the human eye, like the ultraviolet that can give you sunburn and fade paint.

If you look up into a clear night sky during the winter, you’ll see stars which, at first, all appear to be the same shade of white. But, if you keep looking for a few minutes you’ll notice that the stars are many different colors: some silvery white, some almost brick red, and some a mix of orange-and-yellow.

Star field photographed by the Hubble telescope, courtesy of the Space Telescope Science Institute

Our sun is one of the latter stars: not so young and hot that it shines brilliant white, but not so old and cool that it shines with a reddish color. No, “Sol” (as it’s known to astronomers) shines a yellowish-white color, and the shadows it gives are an intriguing shade of blue-purple.

This time of year is the best to see the blue shadows because there’s snow on the ground. If you look carefully on a day when the sun is shining, you’ll see that shadows cast onto snow will look just s-l-i-g-h-t-l-y blue. If the same shadow falls onto something not white, like the grey of a street, you’ll see a distinct difference in the shadow’s color.

Image courtesy of Gary Czerwinski

The best time to see this subtle phenomenon is when the sun is low and casting long shadows. Early morning or in the late afternoon before sunset when the skies are clear is best. For example, if you’re driving along a side road, try this: first be sure that it’s safe to briefly glance away from the road. Then, look at the fields on either side of the road. Snow tends to collect along slopes and in hollows, and if there’s fence between the sun and the snow, you’ll see a bluish shadow cast by the fence. Or, if you’re outside shoveling snow on a sunny day, you can see for yourself that your own shadow isn’t black.

Many stars — indeed, more than you might first expect — are “double stars”; that is two stars orbiting around a central point, and they make double shadows. If our yellow-white Sol was half of a double star, say with another star that was red, everything outside would have two shadows: the bluish shadow that Sol produces, and a green shadow from the red star.

Compared to certain more impressive astronomical events, things like eclipses and meteor showers, a colored shadow might seem insignificant. However, there’s the idea in astronomy of how “everything is its own opposite.” When the moon is full, its position relative to the earth is the opposite from where it was when it was new. The full moon is in the part of the sky where the sun will be in six months (and twelve hours!). And, the “color” that makes up a sun-cast shadow is the opposite color of the light given by the sun.

It’s a small and subtle thing, but treat yourself to the blue shadows against the snow. Once you recognize them, you’ll find that you see them more often, and it’s very likely that you’ll find that you enjoy the sight!

Speaking of shadows, the planet Venus sometimes can produce a shadow, although conditions have to be exactly right for this to happen. Venus has to be high and bright, the sun must either have set or not yet risen, the skies must be close to perfectly clear, and a white background (like snow) is essential. Some people have seen a Venus-produced shadow. I haven’t, but when Venus re-appears in the evening skies this winter and spring, and if there’s snow on the ground in April, we might just see it.

As always, “Good seeing.”


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