Introduction: Ancient people used shadows to tell the time before the invention of the clock. The device they used to read these shadows was called a sundial. Use this activity to make your own sundial.
Note: the Sun will NEVER be directly overhead (i.e. at the zenith) in the continental United States. AVOID saying "the Sun is overhead". IT IS NOT. Say, "when the Sun reaches its highest point in the sky it is noon".
Local noon: When the Sun is as high as it gets in the sky for the day, it is local noon. Therefore, the shadow cast by the style will be its shortest at local noon. At local noon the shadow will also point due north because the Sun will be due south at this time.
Length of shadows: The shadows are longer in the morning and afternoon and are shortest at local noon, when the Sun is highest in the sky.
In the summer the shadows are shortest (at a given time of the day) and in the winter they are longest. This is due to the fact that the Sun is higher in the sky in the summer than in the winter. Also, after vernal equinox in the spring and before autumnal equinox in the summer the shadows can sweep across more than 180 degrees, since the Sun rises north of east and sets north of west during these times of the year.
If your class makes careful measurements of the angle that the shadow makes at the same time for different weeks you will discover that the angle changes a little. That is because the sun's time, as measured by a sundial, and clock time do not stay in perfect agreement. Sometimes the Sun is a little ahead of the clock, and sometimes it is a little behind the clock. The difference can be as much as +/- 16 minutes.
On some globes, in the middle of the Pacific Ocean, is an elongated figure eight, known as an analemma. Around the perimeter of the analemma are the months of the year. Find today's date and look at the time before or after the vertical center line. This will tell you how many minutes ahead or behind the clocks the Sun is today. Also notice the vertical location of the position on the analemma. This tells you the latitude on earth where the Sun will be directly overhead (i.e. at the zenith) at noon on the date specified. Notice that the largest latitude listed is 23½ degrees;, the Tropic of Cancer, which is well south of the continental US.
The following table should be used to figure the angles between the hour graduations.
|Noon and 1 p.m. also noon and 11 a.m.||12 degrees||Noon and 2 p.m. also noon and 10 a.m.||24.5 degrees|
|Noon and 3 p.m. also noon and 9 a.m.||38 degrees||Noon and 4 p.m. also noon and 8 a.m.||54 degrees|
|Noon and 5 p.m. also noon and 7 a.m.||71 degrees||Noon and 6 p.m. also noon and 6 a.m.||90 degrees|
|Noon and 7 p.m. also noon and 5 a.m.||109 degrees||Noon and 8 p.m. also noon and 4 a.m.||126 degrees|
|Noon and 9 p.m. also noon and 3 a.m.||142 degrees||Noon and 10 p.m. also noon and 2 a.m.||136 degrees|
|Noon and 11 p.m. also noon and 1 a.m.||169 degrees|
©Copyright 1998 Elizabeth
Beckett, Holly Bernitt, and Vishwa Chandra.