How to Position a Sundial for Use at Home
Sundials have been used as timepieces for many centuries, with an unchanged basic functional design. A sundial consists of the flat plane where the time segments are marked, and the gnomon, which is the raised part that casts a shadow to determine the time. A correctly positioned sundial will be a fairly accurate timepiece as well as a decorative addition to your garden. Remember, a sundial tells you sun time and not clock time. On a sundial, noon is always when the sun is directly overhead. The rest of the time segments are divided according to the sun’s path across the sky, which changes seasonally.
Prepare a solid pedestal or support to hold your sundial. Once you have the sundial set correctly, it should remain immovable.
Place the sundial so the length of the blade of the gnomon is aligned with the earth’s axis of rotation, which is true north. True north is not precisely the same as magnetic north, which is the pull of a compass needle toward the North Pole, but a compass is close enough for a garden sundial.
Use a plumb line to determine a true vertical to indicate noon. Hang a plumb object from a string about 5 feet long. When it casts no shadow, rotate your sundial on its pedestal until there is no shadow on the sundial plane.
Adjust the angle of the gnomon to correspond with your latitude. For accuracy, the angle of the gnomon must be the same as your angle of latitude. For example, if a gnomon has a 33 degree angle, and your latitude is 38 degrees, it will not be accurate for your area. Use a protractor to adjust the angle by tilting the sundial plane 5 degrees, and wedge it solidly in place. The gnomon will then be angled at 38 degrees.
Seasonal differences occur due to the earth’s tilt (longer or shorter day length), and these differences will vary according to your latitude. True noon, when the sun is directly overhead, varies one degree of longitude every four minutes. This is because the movement of the sun across the sky is 15 degrees westward per hour.
- Seasonal differences occur due to the earth's tilt (longer or shorter day length), and these differences will vary according to your latitude.
- True noon, when the sun is directly overhead, varies one degree of longitude every four minutes. This is because the movement of the sun across the sky is 15 degrees westward per hour.
- Base for sundial
- Plumb line
- Directional compass
- Protractor (optional)