Equatorial Sundial
One of astronomy's first tools to measure the flow of time, a sundial is simply a stick that casts a shadow on a face marked with units of time. As Earth spins, the stick's shadow sweeps across the face. The face of the sundial represents the plane of Earth's equator, and the stick represents Earth's spin axis.
 Download the dial face template and latitude strip (29K pdf).
Preparation
First, find your latitude and longitude and an outdoor observing site in a clear area (away from the shadows of buildings or trees). Determine north from a map, or by finding the north star at night and marking its location. Assemble the equipment as described below. Use a flashlight to demonstrate how to position and read the sundial indoors before going out.
Materials and Construction
• Download and print the template and latitude strip.
• Mark a drinking straw using the latitude strip as a guide. First mark the bottom of the scale at one end, then mark each of the latitudes.
• Fold and glue the template. Make sure the dial faces are lined up.
• Cut out the center hole.
• Place the straw in the hole with the top face lined up with your latitude. The straw should fit snugly. Make sure the stick and face are perpendicular. Tape it in place if necessary. The bottom end goes on the ground.
Experiment
On a sunny day, take the sundial outside and aim the pointed end north. Record the time on the sundial at least four times in one day. Each time, also record the "clock" time for your date and location. Try this experiment during different months.
Analysis
1. Why does this sundial have front and back dial faces?
Answers
1. The north face is for use from about March 21 to September 21, and the south face is for use from about September 21 to March 21. The Sun is north of the celestial equator during the first period (spring and summer) and south of the celestial equator during the second (fall and winter).
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