What Time Is It?
ES0405  Sundials

Early methods of keeping time arose from tracking the position of the sun across the sky. People discovered that shadows cast by objects in sunlight could be used to track the sun's position, because they always point directly away from the sun. A sundial is an example of a simple timekeeping device that uses shadows to tell time.

Marshall Space Flight Center and Science@NASA
A tall pillar serves as a simple sundial.

The pillar in this image is directly south of the viewer. The left side of the image represents the eastern horizon, and the right side represents the western horizon.

  • Can you tell what time it is from the shadow?
  • Do you think it shows morning, noon, or afternoon?

  !   Click the image to see an animation of the shadow's changes through a day.

Note: Shadow motion is shown for the Northern Hemisphere.

  • How would you start if you wanted to design a sundial around a flagpole near your home?
  • What would it take to "set" your sundial to the right time?

9. Sketch a map view of a flagpole area and show the times that the flagpole's shadow would indicate when it pointed west, north, and east.

Sundials can be useful locally, but if you could view a large number of them from different locations, all at the same time, you would see that their shadows point in different directions. Thus, if we relied on sundials for timekeeping, every location on Earth would have its own unique time.

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