Posts Tagged 'shadows'

The history and science of Groundhog’s Day

by Katie Bowell, Curator of Cultural Interpretation

Groundhog’s Day is upon us and it seems the rodent community has been pulling the wool…er…fur over our eyes. Here’s why: Tradition holds that, on February 2nd, if a groundhog emerges from its burrow and sees its shadow, we’ll have six more weeks of winter. If there’s no shadow, spring is on its way. Now, groundhogs are good at many things – climbing trees, whistling, and inspiring tongue twisters (how much wood could a woodchuck chuck?), but they probably need to take weather prediction off their resumes.


The tradition of the groundhog and its shadow began as a Pennsylvania Dutch custom, and has similarities to the medieval Catholic holiday Candelmas and the Pagan festival of Imbolc. Other animals have also been used to predict spring, including the badger, bear, and hedgehog. It’s easy to understand why those hibernating animals would be hallmarks of spring, but why do we check to see what a groundhog is doing on February 2nd?

First, we need to know a little background on the solstices and equinoxes. On June 21st of this year, the North Pole will be as far tilted towards the sun as it gets, and here in the Northern Hemisphere we’ll have the longest amount of daylight/day for the year. That’s our summer solstice. Six months later, on December 21st, the South Pole will be tilted as close to the sun as it gets and the Northern Hemisphere will have the shortest amount of daylight/day for the year – our winter solstice. On March 20th and September 22, halfway between each of those dates, are the spring and fall equinoxes. On those dates, everyone in the world has the same amount of daylight and night. Since none of those dates are near February 2nd, where am I going with this? Well, while we pay attention to the solstices and equinoxes to mark the beginnings of our seasons, early Northern Europeans also paid attention to the dates between those events – called cross-quarter days. Their seasons began on those days, and the beginning of February is pretty close to being between the winter solstice and the spring equinox, and would have marked the beginning of the next season for pagan Europeans.

So what does this have to do with shadows? February also marks the beginning of being able to see the sun taking a higher path across the sky. This has been happening since the winter solstice, but it’s difficult to see when it’s just beginning. Changes in the sun’s path across the sky also mean changes in the shadows we see: the higher the sun, the shorter the shadow. And, finally, winter days in which you can see your shadow tend to be colder because there are no clouds present to insulate the earth.

Put all the pieces together and you can see how the tradition of checking on a hibernating animal on the cross-quarter day between the winter solstice and spring equinox and its ability to produce a shadow could have developed. However, whether or not anyone sees their shadow on February 2nd, spring still won’t start until March 20th. And while it’s a fun tradition, if the groundhogs are anything like me I bet they’d like the chance to sleep in a bit longer…

Update: Per reader Robin’s request, I did a little digging into the accuracy of groundhogs on Groundhog’s Day. According to StormFax Weather Almanac, Punxsutawney Phil has been accurate 39% of the time since 1887.


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

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.”

July 2018
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