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How to Build a Simple Sundial

Copyright May 20, 2011 by Robert Wayne Atkins, P.E.
All Rights Reserved.



Sundial When the sun is shining a sundial may be used to estimate the approximate time of day. However, because the earth travels around the sun in an elliptical orbit, the position of the sun in the sky will be different during the different seasons. This difference is most noticeable in the middle of the winter when the sun will appear lower in the sky relative to the horizon, and in the middle of the summer when the sun will appear higher in the sky relative to the horizon.

If you push a straight stick into the ground then the shadow cast by that stick will point west at sunrise, north at midday, and east at sunset in the northern hemisphere. (Note: In the southern hemisphere the shadow will point south at midday.) In the northern hemisphere the shadow will move in a clockwise direction around the stick. This is the same direction that the hands of a clock move around the face of the clock.

To make a simple sundial you must first find a level spot on the ground that receives full sun for most of the day. Then use a magnetic compass to determine the directions west, north, and east. (Although magnetic north and true north are slightly different, this difference will not have a significant impact on a simple sundial.) Then draw a straight line in the dirt on some level ground from due west to due east. In the middle of that east-west line draw another straight line due north. Write the number 6 (or 6:00 am) at the west end of the line, the number 12 (or 12:00 noon) at the tip of the north line, and the number 6 (or 6:00 pm) at the east end of the line.

Face of Sundial Now draw a line that approximately divides the left side of the sundial in half and write the number 8 (or 8:00 am) at the tip of that line. Draw a line that approximately divides the right side of the sundial in half and write the number 4 (or 4:00 pm) at the tip of that line.

Now draw another line for 7 am on the left side but draw the line a little closer to the number 8 than the number 6 am. Draw a line for 5 pm on the right side but draw it a little closer to the number 4 than then number 6 pm.

Now separate the space from 8 am until 12 noon using three lines, with the spacing between the lines becoming a little less as they get closer to the number 12 noon. Separate the space from 12 noon to 4 pm with three lines with the spacing between the lines being a little less as they get closer to the number 12 noon.

Push a long thin straight stick (at least 12 inches long) into the ground at the point where the east-west line and the north line intersect (see the illustration) and point the opposite end of the stick towards Polaris, or the North Star.

Your exact location on the earth is determined by the intersection of your longitude and your latitude. Your latitude is the angle from level ground at your location to the North Star. For example, if you were at the North Pole then the North Star would be directly above your head and the angle would be 90 straight up, and this would also be your latitude. If you were at the Equator then the angle from the ground to the North Star would be 0, and this would also be your latitude. If you moved from the Equator towards the North Pole then the angle to the North Star (your latitude) would gradually increase to 10, then 30, then 50, then 70, until you finally reached the North Pole and your latitude would be 90. You can find your exact latitude on a map that shows latitudes and longitudes. However, for the purpose of constructing a simple compass an estimate will be okay.

Angles to the North Star In the United States of America if you live in a state that touches the Canadian border then your average latitude is approximately 45. If you live in a state about halfway between Canada and Mexico then your latitude is approximately 38.5. If you live in a state that touches the Mexican border or the Gulf of Mexico, then your latitude is approximately 32. This will be the angle between your straight stick that points to the North Star and the level ground below the stick. Push your long thin straight stick into the ground at the correct angle so it points North towards the North Star. Then use another shorter stick, with a fork at its top, between the ground and your North Star stick to support your North Star stick so it does not begin to gradually lean over closer to the ground.

This completes your simple sundial. Your simple sundial will allow you to determine the approximate time of day. However, during daylight saving time you will need to add one hour to your sundial time to arrive at the current daylight savings time.

Throughout the entire world we now live in designated time zones that are several hundred miles wide. This means the shadow of the stick will be directly below the stick at 12 noon if you live in the center of a time zone. But if you live near the west or east borders of a time zone then the shadow of the stick will be a little to the right or left of the stick at 12 noon and not directly below the stick.

If you live outside the continental United States of America and you wish to construct a sundial, or if you simply wish to improve the accuracy of your sundial, and you have access to a clock and you can determine your exact latitude, then you could easily adjust the angle of the stick to match your latitude, and you could adjust the positions of the numbers around the outside edge of your sundial to match the exact hour of the day the stick's shadow is pointing in a specific direction. Once you have constructed a simple sundial and you have determined the approximate distances between the individual lines for the hours of the day, then you could easily duplicate that sundial whenever you wanted to for the rest of your life.

After you have gained a little experience constructing temporary simple sundials in the dirt on the ground you may decide that you want to build a better sundial using a large round block of flat wood. You could then mount your wooden sundial on top of a wooden post that you anchor or bury in the ground. This would put your sundial a little higher than waist level and this would make it easier for everyone to read.



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