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How to use the sundial

Before you can use the sundial properly, you need to do a few simple tasks. First, you need to find a flat surface on which to place the sundial. In order to maintain the accuracy, the surface should be as horizontal as possible. If you wish to be meticulous, you can accomplish this by aligning a level both North-South and East-West on the table and adjusting the legs until both levels are level. But this is not necessary. You can eyeball a level surface, and generally that is good enough. Note: the further the sundial is from the horizontal, the less accurate the sundial is. Secondly, you need to orient the sundial as indicated in the upper right corner of figure 3. One can use a compass to find north, but do note that a compass points to magnetic north, and not to true north. For Bloominton magnetic north is about degrees to the west of true north. For accurate alignment, one must correct for this discrepancy. However, for the sundial in this handout, magnetic north will do. Lastly, you need to acquire a gnomon. The enclosed sundial uses a vertical gnomon that is specifically 0.5 inches in height. Any thing will work. Some ideas are a tooth pick, a twig, or a triangluar sheet of paper. The later has the advantage that it can be taped to the paper and placed into the vertical position whenever you want to use the sundial.

Figure 1: This shows the construction of the triangular gnomon.

To set up the triangular gnomon, you need to obtain a sheet of paper with square corners, a protractor, a ruler, and a pair of scissors. Make sure the sheet of paper you obtain has square corners. If you use a piece of paper that does not have square corners, your sundial will not give the correct time. On your sundial, you will see two x's. If you look at figure 3, you will see that these x's are labeled the Vertical Gnomon Position, and the Polar Gnomon Position. With your protractor, you need to make a mark on one edge of the paper that is exactly one-half an inch from a corner. Then from this point you need to use the protractor to draw a line that makes an angle with the edge of the paper which is equal 90.0 degrees minus your latitude. For example, in Bloomington, Indiana the latitude is about 40 degrees. The angle between the line you draw on the page and the edge of the page with the 0.5 inch mark should be 90.0 - 40.0 = 50.0 degrees. You should have something that looks like figure 1. Now cut along the drawn line. You should be left with a small triangle with one edge that is 0.5 inches in length. This edge is your gnomon. Now all you need to do is attach the triangle to the sundial as shown in figure 2 in a vertical position.

Figure 2: Method used to attach the triangular gnomon to the sundial.

Once you have obtained a suitable gnomon, you need to place it on the sundial in the appropriate position. On the sundial you will see two x's. One is the polar gnomon position, and the other is the vertical gnomon position. The gnomon/stick should be place on the x labeled the vertical gnomon position. Please see attached sheet for a diagram. Once you have done the above, the sundial should be in working order.

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Department of Physics & Astronomy
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