Based on ancient designs, armillary sundials--when properly sited and positioned--are surprisingly accurate. In use since Grecian times, armillaries are composed of complex, interlocking spheres supporting an arrow-shaped gnomon. As the sun travels across the sky, the gnomon casts a shadow onto a surface that indicates the hour. Armillary sundials are usually made of cast brass, bronze or aluminum. Sizes, materials and designs vary, but placement instructions are the same for any style.
Choose your location. The longer your sundial is in direct sunlight, the longer it will give accurate readings.
Position the base of the sundial and level it carefully. Take horizontal and vertical readings and adjust the base as needed. Be sure the base is securely attached to the ground. If the base becomes tilted, your sundial will read inaccurately. Periodically check that the base is level, particularly after frosts and freezes.
Place the sundial on the base but do not secure it. Use the compass to locate true north and point the gnomen in that direction. For the greatest accuracy, the gnomon must point to magnetic north, not true north but to get a "ballpark" directional reading, a standard compass reading is fine.
Obtain your longitude and latitude to calculate your declination and locate magnetic north.
Before you throw up your hands and give up, visit NOAA's Geophysical Data Center (see reference 4). The site will compute these numbers for you. It will tell you the number of degrees and the direction to which you should turn your compass for a magnetic north reading.
According to Rick Curtis of the Outdoor Action Program at Princeton University, "the angle of declination varies from about 20 degrees west in Maine to about 21 degrees east in [the state of] Washington."
Fine tune your direction more easily by using a clock. At exactly noon, adjust your sundial so that the gnomon casts no shadow. At "high noon," the sun will be directly overhead and true vertical objects will not cast shadows.
Secure the sundial to the base and enjoy.
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- Longitude and latitude
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