Telling Time With Shadow Sticks
A Schoolyard Investigation

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Have you ever noticed what happens to shadows during the day? If you've already made outdoor observations, you probably discovered what ancient humans learned.

Long ago, people discovered that shadows point north when the sun is highest. A shadow is long in the morning as the sun appears in the east. It is shortest around noon. Then it gets longer again until the sun sets in the west — but the shadow was on the other side of the object. (It had moved "clockwise"!) How do you think they used this information to keep track of time? Jot your ideas. Then read on!

They created simple sundials (clocks) called shadow sticks. 2,000 years ago, scientists discovered that if the shadow stick was slanted, it kept more accurate time than a straight object. They learned that if the stick (called a gnomon) was slanted to the same degree as the latitude where it is placed, it kept accurate time all year long. (This is because the earth's axis is also tilted.)

Try this! Set up your own shadow stick clock like these fourth grade students in Minnesota did. Here's how:

1. Use an atlas or other resource to look up the latitude of your town.

2. Use a "protractor" to insert a post (stick or yardstick) into the ground at the same angle as your latitude. Use a compass to make sure the shadow stick points North. That aligns it with earth's axis. What next?

3. Go outside in the morning. Use small rocks to mark your stick's shadow. Put the first rock at the top of the shadow and work your way down to the base of the stick. Every hour through the school day, do the same thing.

What patterns did you notice? Did the shortest shadow (when the sun was most directly overhead) appear when your watches pointed to noon? Did the times on your shadow stick clock match the times on your watches? Probably not! How would you explain the differences? Discuss this with your class. Then see what scientists say.

A clock and a shadow stick (sundial) measure two different things.

What Do Clocks Measure? A clock measures time passing in minutes in standard time zones that humans created.

What Do Shadow Sticks and Sundials Measure?
These measure the Earth's actual movement as it rotates on its axis each day (called "local apparent time"). The length of shadows and actual daylength differ from place to place and from day to day as the earth revolves around the sun.

How Do Time Zones Fit In?
Each time zone is an hour earlier than the one east of it and an hour later than the one west of it. But Earth rotates to the east on its axis at a rate of one degree every 4 minutes (15 degrees in an hour). So the sun appears overhead in eastern parts of the time zone before it does in western locations. Try This! Find Houston, TX, on this time zone map. Then find San Antonio, TX. Watches in both cities would say 9:00 because they are in the same time zone. In which city would a shadow stick clock (sundial) show 9:00 first? (Answer below map.)

Answer: A shadow stick clock (sundial) in Houston would show 9:00 first because it is east of San Antonio. (The sun would pass over it first.)

More! Houston is at about 95 degrees W longitude. San Antonio is at about 98 degrees W longitude. The distance between them is 3 degrees longitude. A sundial in Houston would show 9:00 (Local Apparent Time) about 12 minutes before the sundial in San Antonio. (3 degrees longitude X 4 minutes per degree = the time difference.)

 

 

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