Boston Children's Museum
308 Congress Street, Boston, MA 02210
When we look up at the sky, it seems as though the sun moves across it throughout the day. This apparent movement of the sun across the sky is predictable, and can make observations about the sun by noting the relationship between its position in the sky and the shadows it creates on Earth. You even can tell approximately what time of day it is by observing the sun’s position in the sky. But that apparent motion can also lead to some confusion – is the sun moving across the sky, or is the Earth moving? Simple investigations like this one can provide early concrete connections to the sun, and a beginning understanding of the relationship between the sun and the Earth.
Scout your location and hope for some sun!
Ask your children, if they have ever noticed how their shadow might be short or long, wide or skinny, or pointing different directions? Do their shadows point in the same direction all day, or do they point different ways at different times of the day? Why might their shadows point in different directions at different times of the day? If they think that shadows move throughout the day, how could we prove it?
If they don’t come up with the idea themselves, you can suggest to children that you could all go outside, trace a shadow, and then go back to the same spot a little later and trace the same shadow again. In this activity, we’ll trace our shadows, then make predictions about what they’ll look like later on in the day. All we need is sunlight, sidewalk chalk, and a bit of patience!
Trace your shadows and see how they change throughout the day!
When everyone has had their shadow traced, discuss the results. Are the shadows long, short, or in between? Was it hard to trace them? Were the shadow edges fuzzy or distinct?
Tell your students how long it will be before you go back to check on the shadows they drew, and ask them to make predictions as to whether the shadows’ sizes and shapes will change or not, and if so, how. If you will be hanging out in the general vicinity of your tracings for the rest of the day, it may be fun to check them regularly and do repeat tracings to see how they move. Remember to have kids stand where their feet were traced for their first shadow. If you are planning to leave the area and come back later at a specified time, you can make predictions as to what direction the shadows will have moved and how their shapes will have changed. Why might the shadows change size and shape over time? Where have students observed this phenomenon before? What causes these changes?
If you are returning inside in between shadow drawings, while you are waiting in between those drawings, try this activity with your students:
Some time later…
Return to the shadow tracings and have students repeat the procedure of tracing each other’s shadows, taking care to stand in exactly the same place as before. What do they notice about the new tracings? Are they different from the previous ones? Where is the sun in the sky? Were any of their predictions correct? Why did the shadows move and change? Discuss the movement of the sun. Did the sun actually move or was it us (Earth) that moved? What predictions can students now make about what the shadows would look like if they could come back again, later? Or first thing the next morning?
Ask your students, if they had the chance to do this activity again, what other shadows would they trace? And what time of day would be best for trying this activity? Do they think the outcome would be different depending on the month or the season? The important point to get children to understand is that the length and position of a shadow is related to the height and position of the sun in the sky. As this height and position changes throughout the day, the shadows do as well. You can approximate some of this inside with flashlights and objects (or kids).
Why did the shadows change? It might seem that the Sun is moving across the sky, but we are really the ones who moved! Most of us have seen a globe or a diagram of the Earth spinning on its axis, but we don’t always connect the Earth’s motion with the Sun “moving” across the sky or the changes in our shadows. One simple way to demonstrate this is with a flashlight and a globe (or any round object to represent Earth, with different locations marked on it as reference points.) Shine the flashlight on the globe and notice how a certain location will change from night to day as the Earth spins. If you attach a small figurine or stick on the globe, you will also see its shadow change as you spin rotate the globe. Another way to demonstrate why the Sun looks to be “moving” across the sky can be done with a stationary reference point, either a sphere held in place to represent the Sun or simply spot on the wall. You or your participants represent “Earth” and stand so that the “Sun” is to your left. Point to where the “Sun” is located. Now slowly rotate counterclockwise (to your left) as the Earth would rotate, finger pointed toward the “Sun” the whole time. Notice how the “Sun” never moved, but to your perspective it went from being on your left (sunrise) to directly in front of you (noon), to on your right (sunset), and to behind you (night). A fun way to combine these two demonstrations is to print a world map so that it spans the length of a few sheets of paper, enough to wrap a person around the waist. You might want to mark your city or other interesting locations on the map. As the person rotates like the Earth, with the “Sun” flashlight pointing toward the map, others can see your city changing from night to day and back again. At the same time, you can do the pointing activity in the second demonstration to notice how the stationary “Sun” seems to be at different places relative to the map-person. See the link below for a “wrap around” map:
Earth and Space science activities were developed with the support of NASA. This material is based upon work supported by NASA under grant award number NNX14AQ83G. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Aeronautics and Space Administration (NASA).