Overview of Lesson

The crosscutting concept to convey in this lesson is how our perception of time is tied to the rotation of the Earth on it axis, one rotation giving us one day-night cycle (counted as one day). We have no control over the speed of the Earth’s rotation, the duration of one day. However, there is a practical need to specify times within a day. (Allow kids to consider and discuss what it would be like if we did not specify times within a day–animals don’t.

Therefore, early in history humans decided to break the day into 24 equal segments we call hours, divide each hour into 60 segments we call minutes, and each minute into 60 segments we call seconds. (Allow kids to consider other ways days might have been divided to give specific times.) However, deciding to divide a day into hours is one thing, measuring those hours is something else. Have kids consider and discuss how people might have been able to tell time within a day before the development of technologies to make clocks and watches.

The observation of how shadows move in a regular pattern in response to the sun’s movement across the sky was key. (Guide kids to do the exercise described in the text to gain first hand experience of this.) The only additional step is to designate particular positions of the shadow as times of day. The highest point of the sun in the sky, the shortest shadow, is taken as 12:00 noon. (If you are in the northern hemisphere, it also points due north.)

There are countless examples of sundials throughout the world, some ancient, some modern. Type into your browser: sundial images

Note that sundials work the same regardless of size. Include: world’s hugest sundials

The obvious difficulty with sundials is that they don’t work at night or when the sun is behind clouds. Thus other means were developed for keeping time over night. Some of these are noted in the following video.

Kids will have fun making and calibrating their own sundials. Follow instructions in the text. In the course of this lesson, you may come across the datum that the time for the Earth’s rotation on its axis is 11 hours, 56 minutes. How does this square with our day being exactly 24 hours. See the explanation in the text.

The time duration of one day (day-night cycle) is determined by the rotation of the Earth. Have kids consider and discuss what our perception of time might be like if we lived on another planet with a different rotation. Ask: what other time duration is determined by the Earth’s movement? Answer: one year is the duration of time that it takes for the Earth to make one revolution around the sun as will be addressed in Lesson D-6.

How did your kids do with this lesson? Please post comments or questions.