Discuss: What happens to the shadow of the tree as time passes?
Discuss: Why does it do this?
Discuss: How could you use the shadows to measure the Sun's position?
Predict: How do you think the very first clock got set, when there was no other clock to look at?
Reveal answer
Watch the next video for another clue.
Discuss: Long ago, people did not have the concept of hours or minutes. Someone had to come up with these ways to divide a day into parts.
How would this week have gone wrong for you if you could only measure time in days?
Try It: Count your finger segments using your thumb on that same hand. How many finger segments do you have on one hand?
Reveal answer
The next slide shows the answer.
Discuss: Why are there 24 hours in a day?
Reveal answer
The Egyptians counted by 12 so they divided day into twelve parts and night into twelve parts. 12 + 12 = 24.
Discuss: What happens to the shadow of the tree as time passes?
Reveal answer
It moves across the ground.
Discuss: Why does it do this?
Reveal answer
It does this because the Sun appears to move across the sky. (You know that actually the Earth is spinning, which is why the Sun looks like it moves.)
Discuss: How could you use the shadows to measure the Sun's position?
Reveal answer
The answer is in the next video!
Your students have begun making the connection between the Sun and the time on the clock. They know that the spinning of the Earth causes the Sun to rise and set. You can reinforce students' understanding of the connection between the Sun's position and clock time by introducing time zones.
Each student will need:
Teacher Note: The video mentions that when the students go outside they’ll need to line up their shadow clock’s compass directions correctly. We recommend that in advance of the lesson, you have several chalk compass roses sketched on the ground, to serve as workstations. Then when the students are ready to go outside, break them up into a few small groups.
What’s the easiest way to find north?
The most reliable way to find north is to simply take one of these shadow clocks outside, and using your watch, turn the shadow clock to match the current time. Now the compass rose on the shadow clock will be properly oriented.
A magnetic compass, whether an old-fashioned kind, or the modern one available on many smartphones (such as iPhone’s compass app) actually points the way to the Earth’s magnetic North Pole, which is slightly off from the geographic North Pole depending on where you are. It may cause some error, depending on your location.
If you made skydomes in Mystery 2...
If you made sky domes with your students in Lesson 2, you can use them here. Put the sky dome over the shadow clock (with the directions aligned) to guide the movement of the flashlight. Just keep in mind that the shadow might not be that distinct if the sky dome is small or if your flashlight has multiple bulbs.
Regarding the flashlight, you need one that is bright with a direct beam. If your batteries are low you may need to replace them to get the balloon globe well-lit with a clear shadow. The one we linked to above works well and is inexpensive.
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Why is the sky blue?
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4:41
Why do we call them doughnuts?
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Could a turtle live outside its shell?