How to Build a Sundial That Keeps Time

Sundials are limited to telling time during sunny hours.

To see form and function combine effortlessly, build a working sundial to let the direction of the sun's rays tell you what time it is. Sundials have been used throughout history to gauge the passage of time by the movement of the sun. As the suns position changes in the sky, the shadows it casts move as well. A reliable time-telling instrument can be constructed by carefully marking where the shadows are located at regular intervals throughout the day. You can build a working sundial from common household items in a couple of days.

Things You'll Need

• Pencil compass
• Poster board or plywood
• Permanent marker
• Drill
• Modeling clay
• Disposable pen
• Clock with alarm

Instructions

1. Position a sheet of poster board or plywood in a clear and sunny area where long shadows will not cover the sundial. Draw a circle with an eight-inch diameter in the middle of the poster board using a pencil compass. Trace the circle outline with permanent marker.

2. Press a 1-inch ball of modeling clay down in the center of the circle. Twist slowly as you stick the pointed end of a pen straight down into the ball of clay.

3. Set an alarm to go off on the clock every 15 to 30 minutes. Use the same time interval each time to be consistent. Each time the alarm goes off, mark the where the shadow falls on the circle. Draw a dash mark on the shadow position and write the corresponding time next to the mark.

4. Continue adding position times to the sundial until you have shadow marks from sunrise to sunset. It may take a couple days to complete the shadow positions, depending on what time of day you start.

5. Read the homemade sundial by checking the position of the shadow cast by the center pen against the times that were previously recorded. The sundial must stay in the same position without moving for the readings to be accurate.

Tips & Warnings

• Stake down a sundial or mark the position where it was originally calibrated so that it can be return to the exact spot to be used.

• Overcast and stormy weather make it nearly impossible to get a defined show from the sundial for a time reading.

References

• Science Kids at Home: Sundial
• Eye on the Sky: Sundials: Observing and Using Shadows

Resources

• Read this Article in Spanish