On the recent winter solstice (Dec 20, 2020) I asked a friend to help me capture my noon shadow.
“How would I do that?” she asked.
I unrolled a length of black felt and gave her a piece of tailor’s chalk. “Follow me outside. But be quick,” I added. “Solar noon is about to happen.”
Shadows are so much fun to have in your drawer. I used to have an entire collection of them, which I would take to elementary schools. Those have been lost during my several home moves, so I decided to start a new collection. Now I asked my friend, “How long do you think my shadow will be?” “Well, it will be short since it’s noon. Maybe you won’t even have one.” She eyed my yardage. Then we went outside. I looked at my watch: 11:59 a.m., exactly solar noon.
I unrolled the fabric and stood so that my shadow fell on it. My friend began tracing it with the chalk. She had to move quickly because the shadow began shifting away from her tracing. Luckily, it was noon, when the shadow moves slowest. If we’d been working at sunset, she wouldn’t have been able to keep up. When I had caught sunset shadows for my previous collection, two of my children would trace frantically to beat the sun as it advanced and my shadow slid out from under me. “Oh, Mom,” they would complain. “Why do we have to do this?” I had interrupted their computer game.
On winter solstice, the sun carves its lowest arc across the sky. A low angle meant shadows would be longer than on any other day of the year. I’d purchased 3 yards of felt at the fabric store the previous night and even that wouldn’t be long enough to capture my shadow. I had to cut off a piece of fabric from the excess folded part and use it to extend the fabric. Even I was surprised at the length of my shadow: it was approximately 10 feet!
Now my friend’s husband Mike joined us. “What is the angle of the sun now?” he wondered. “Can you measure it from your shadow?” Yes, you could. My vertical body and the length of the shadow created a right triangle. The angle of the solar height could be computed from this. The angle cast by a stick and its shadow on a summer solstice several thousand years ago is the means used by Eratosthenes to calculate earth’s circumference. That will eventually be another lesson plan!
Mike took out his cellphone and used an app to find true north and we marked its line directly on the shadow for reference. I was surprised, but shouldn’t have been, to find that my shadow was pointing due north, directly toward the north pole. At noon in the northern hemisphere, shadows will always point true north, which is why taking noon sightings were so important for mariners in the days before geo-positioning satellites. (You can have fun confounding people by asking them if they can make their shadow reverse direction, pointing in the opposite direction. Often, they will try repositioning their body before they realize it’s impossible and then they will ask why it’s impossible. We all know that shadows move as the day goes on. Why can’t you get it to point south? North of the tropic of Cancer the sun is always to our south, meaning that shadows will always point northward)
Think of all the ways shadows can be used! Really, the list is endless. Sundials, moondials, Marsdials. Shadows give you an idea of the quality of light in pictures and are used to create realism in cartoons and paintings. As above, shadows can give direction and help determine latitude. Shadows have helped astronomers identify craters on the moon. Over 2000 years ago, Eratosthenes used a noon shadow at summer solstice to determine the circumference of the earth.
What is “solar noon?” That’s when the sun is at its highest point in the sky. It would be easy to say, “when the sun is directly overhead” but that’s not true on most parts of the earth. Where I live at latitude 38 degrees north, the sun will never be directly overhead. And, solar noon is rarely at 12:00 on our watches. Time zones have put our watches out of sync with the sun. To find the “watch time” that corresponds to solar noon, you can look at the NOAA website or merely google solar noon on the date in which you are interested. It was a coincidence of factors that made solar noon nearly the same as clock noon on this latest solstice 11:59 p.m.
What is “true north”? That is the line that runs directly to the north pole, or to the north star at night. It is not the same as magnetic north. because the magnetic north pole is to the east of the true north pole. True north is the one used for navigation. The difference between true north and magnetic north varies from place to place and is computed as an angle. At latitude 38 north, the difference is a whopping 15 degrees. Before cellphone apps, you could use a compass to find magnetic north, look up the angular difference on a table; then use a protractor to redraw the line. Of course, you can still do this. Pay attention to the direction of the angle! Since magnetic north is to the east, the true north line will be rotated to your left.
The best time to take standardized shadows is on the solstices and equinoxes, but any time is fun. Best to take each shadow at solar noon for accurate comparison among them. So much fun! Use felt because it doesn’t ravel when cut. I like to decorate my shadows with embroidery.
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