How to Use a Trigon in Sundials

Using a sundial as a clock dates back to prehistoric times, reports Iowa State University. A horizontal dial is one type of sundial. In the center of this sundial is the primary object used to tell time, the gnomon supported by a trigon or triangle-shaped dial. The gnomon is a long, straight bar that runs along the top of the triangle and extends past the triangle. As the Earth rotates, the sun casts a predictable shadow depending on the time of day. You use the gnomon's shadow on the sundial to tell time. You have to set up your sundial and use the triangle properly to have the sundial give you accurate information.

Instructions

- 1
  Use your compass to find which direction is true north, which is different from magnetic north. If your compass does not adjust for magnetic declination, the difference between magnetic and true north, then you will need to make the adjustments yourself. The National Geophysical Data Center's Estimated Value of Magnetic Declination website calculator can give you the declination for your location.
- 2
  Put your sundial in a sunny location. To have it work all day, the sundial needs full sunlight all day, so avoid any locations that receive shade even part of the day.
- 3
  Position your sundial with the shorter side of the triangle facing north. This will make the part of the gnomon that extends beyond the triangle also point to the north.
- 4
  Look at your sundial from above and view where the shadow of the triangle is on the surface of the sundial. The shadow should fall on a number that corresponds with the current time.

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