A Human Sundial

Creating a “Human Sundial”

You have probably noticed that on sunny days the length of your shadow changes. It might start off in the morning long, get shortest some time around lunchtime, and appear longest right before the sun goes down. You may have also noticed that shadows cast at different times of day also seem to change direction. For example, if you use an umbrella for shade at the beach, you’ll probably need to move or change the position of your umbrella over the course of the day to stay in the shade.

We wanted to learn a little more about these shadow phenomena. For four days, we asked Science Friday fans to go outside, face north, and tweet a photo of their feet and their shadow with #HumanSunDial. We used the time stamp in the tweet to figure out approximately what time of day the photo was taken. When we arranged them chronologically, here’s what we found:

We assumed that for the most part, people were tweeting their photos promptly and that they were mostly in the continental U.S. We also adjusted the time of each tweet to the local time of the tweeter.

Shadow Basics:

The position of our planet relative to the sun is what determines the length and direction of shadows on the earth’s surface. The planets in our solar system revolve around the sun at different speeds and distances. We define a year to be the amount of time it takes Earth to complete its revolution around the sun. As it travels around the sun, it also rotates around an axis, almost like a spinning top. It takes about 24 hours for Earth to complete one full rotation on its axis; we call that complete rotation a day. Earth’s rotation exposes all of its sides to sunlight, with the most direct light hitting the part of earth that is nearest the sun at any given moment.

When an object sticks out from the part of earth’s surface that is facing the sun, it will cast a shadow, much like our #HumanSunDial participants noticed. However, because Earth rotates, the angle of an object (or a person) relative to the sun changes, and with it the size and direction of the object’s shadow. That’s why your shadow moves throughout the day!

If you need to convince yourself that Earth’s rotation around its axis causes your shadow to move, we recommend that you try making a model Earth out of clay and stab a toothpick in the side. Rotate it on its axis next to an illuminated light bulb (“sun”) to see how the toothpick’s shadow changes.

Try one of the following simple tools for using the relative positions of Earth and the sun to calculate and observe interesting phenomena:

Tell the time using a sundial by printing out and making a sundial

Predict the length of your shadow using a shadow calculator

Figure out where the sun will be with this sun calculator

Find the angle of the sun relative to the ground in this Khan Academy tutorial

Create your own Stonehenge and mimic ancient sun observations