Using Sun and Shade to Tell Time: The Making of a Sun Dial
The sundial is the most basic device for telling time. It reveals time according to the position of shadow that is cast by the sun. In the past, human beings did not have clocks or watches, and they used a shadow stick to keep track of time. When sunlight fell on the stick, it would create a shadow, and ancient people could tell time by observing the position and length of the shadow. Even though modern people have advanced devices for telling time, there are circumstances that may require a person to use a sundial. For instance, people may get lost in the woods, and they may not have watches or other time-telling devices with them. In such a situation, they can just create a sundial and get a rough estimate of the time. It is very easy to make a sundial, and here are instructions on how to do it.
Making an Equatorial Sundial
The materials that are needed for making an equatorial sundial include:
- protractor
- piece of card about 25 cm long and slightly wider than the protractor
- knitting needle or drinking straw
1) Draw three lines across the card, so that the length will be divided into four sections that measure 1 cm, 10 cm, 14 cm, and 1 cm. The 14 cm length is suitable for latitudes of 50° or more, but those who are living at lower latitudes will need a longer length. The three lines should be called A, B, and C. Mark a center point on line A and name the point O. Then, mark another point at the center of line C and call it P. Draw a line to connect points O and P.
2) Place the protractor on line A in such a way that the center of the protractor is right on the point O. Draw around the protractor and make a pin-sized hole through the point O. Turn over the card, and place the center point of the protractor on the hole. After that, draw around the protractor again to create two back-to-back semicircles.
3) Mark intervals of 15° on the perimeter of the semicircle, and write hour numbers at the marks, from 6.00 am to 6.00 pm. Repeat the process on the other semicircle.
4) Fold the card along lines A, B, and C.
5) Make the hole at point O larger and put the straw through the hole. The straw should be perfectly perpendicular to the card.
6) Place the lower end of the straw on line OP, at a point where the angle between the straw and the horizontal is equal to the latitude of the place where the sundial will be used. Examples of latitudes are 30° for New Orleans and Perth, 45° for Minneapolis and Bordeaux, and 51.5° for London. The correct length of the straw can be calculated with the formula 10 x co-tangent of latitude. For the above mentioned examples, the lengths should be 17.3 cm for New Orleans and Perth, 10 cm for Minneapolis and Bordeaux, and 8 cm for London.
7) After the position of the lower end of the straw is determined, cut a tab from the edge of the paper to end of the straw. Then, curl the tab and insert it into the straw.
8) The straw acts as the gnomon. Its shadow will fall on the topside of the sundial during summer time and the underside during winter. The shadow will show the time of the day.
9) The sundial should be positioned in such a way that the dial plate is parallel to the equator and the straw is parallel to the axis of the Earth.
There are many different types of sundials, and most of them are easy to construct. Other than the equatorial sundial, you can also make the horizontal sundial, the analemmatic sundial, the diptych sundial, the equiangular sundial, and the Capuchin sundial. All these sundials can tell time effectively on a sunny day. Those who are interested in sundials can join societies around the world to share their passions and learn more about sundials.
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