First, download one of the PDF files listed below and print it out. (You'll need Adobe Reader to do this.)
Sundials need to be adjusted according to your approximate latitude — how far you are north or south of the equator. If you don’t know your latitude, you can find it on most maps, or at one of these websites:
Cut and fold the printout according to directions printed on it. You might want two copies so that you can work on one while reading the directions on the other. The only hard part is pushing the pencil point through the center of the small circle. It helps to twirl the pencil as you push.
When you’re done, the pencil should be perpendicular to the sundial’s face (not to the base). If the pencil wants to topple over, try taping the whole thing to another piece of paper or (better) to a sheet of cardboard. Mounting the sundial on cardboard also makes it easier to carry around. Cereal boxes are a great source of free cardboard.
Point the pencil due north — or south if you live in the Southern Hemisphere. You can use a compass or a map to determine the proper direction, or you can just orient the sundial so that it agrees with your watch. (Subtract one hour from the watch if you’re on daylight-saving time.) Then watch the shadow on the sundial change as the Sun moves from east to west over the course of the day.
This design is called an equatorial sundial because the face with the numbers is parallel to Earth’s equator. It’s easiest to use at midwinter and midsummer, and hardest near the equinoxes around March 20th and September 23rd.
How Equatorial Sundials Work
The horizontal sundial, with a readout face parallel to the ground, is the most common design — the one that’s often found in garden stores. Horizontal sundials are easy to use but tricky to design and build.
The model you’ve just built is called an equatorial sundial because its readout face is parallel to Earth’s equator. Conceptually, it’s the simplest design, but it suffers from one practical disadvantage. Depending on the season, the pencil’s shadow may fall either on the top or the bottom of the readout face. Because the face is made of translucent paper, it’s easy to see the shadow even when it’s on the underside. This would be extremely awkward if the face were made of metal.
To see how this sundial works, imagine first that you lived at the North or South Pole. In that case, a stick planted upright in the ground would form both a horizontal and an equatorial sundial — the two designs being identical at those locations. During the spring and summer, the Sun would rotate a full 360° around you every day, staying the same height above the horizon all the time. The stick’s shadow would behave exactly like the hour hand on a 24-hour clock, staying the same length all day long while rotating at a constant rate. During autumn and winter, the Sun would never rise, and your sundial would be useless. But you would be too cold to care.The pencil in the sundial you’ve just built is parallel to that stick at the Pole, and the readout face is parallel to the ground there. So the pencil’s shadow behaves exactly like the shadow of the
stick at the Pole, except that the shadow is visible even in the winter. That’s because unlike our planet, the sundial is hollow, allowing the Sun to shine up from below.
Do you find that your sundial isn’t as accurate as you would like? That may be partly due to construction problems. Paper isn’t very rigid, and no matter how carefully you do the folds, the sundial won’t hold its shape perfectly. But there are also some more fundamental reasons why sundials and clocks tell different time. For more information, see our article Time and the Amateur Astronomer.