How to Set a Sundial

If you have a sundial in your garden, and you want it to tell time accurately, today is one of the best days to set it. Go out at noon (1:00 p.m., if you’re currently observing daylight savings time), and set yours to 12 o’clock. Sundials can be set on four dates each year for an accurate read. Those dates include:

• April 15
• June 15
• September 1
• December 24

Choose the Right Spot for Your Sundial

Place your sundial somewhere it’ll receive full sun throughout the day. You can place your sundial on the ground, or up on a pedestal, just be sure that it’s level and that it isn’t near any tall-growing plants that might eventually cast a shadow over it.

How to Set Your Sundial

Set the gnomon (that’s fancy for the part that sticks up), so that it’s facing true north. If you’ve set your sundial correctly (and it’s noon/1:00 p.m. dst), there won’t be any shadow on the face of the dial.

Check your sundial a few times throughout the day to see if it’s keeping accurate time. If you’re currently observing daylight savings time, your sundial should be one hour behind clock time. Just remember that you need to add one hour to whatever time is shown, if you want to know what time it is locally.

If your sundial isn’t reading accurately on the day you set it, the gnomon may be the wrong size for your latitude. To adjust for this, just lift the bottom of the sundial until the correct time is shown. Then, place something under your sundial to keep it at this angle. The angle of the gnomon must be parallel with the Earth’s axis in order to show the proper time.

How to Find True North

True north is different than magnetic north, which is what a compass shows. To calculate true north for your location, look up your latitude online. Then, plug it in to the National Centers for Environmental Information’s Magnetic Declination Calculator to determine how you need to adjust your compass.

My location has a latitude of 35.7565° N. When I plugged that number into the calculator, I got a 5 degree westward declination. That means to find true north for my location, I need to turn my compass 5 degrees in the opposite direction (so, in this case, 5 degrees east).

How to Make Your Sundial as Accurate as Possible

Solar time and clock time line up at noon on April 15, June 15, September 1 and December 24, which is why those days are recommended for setting a sundial. If you want your sundial to be as accurate as possible, reset your dial on each of those dates. This will help to keep the difference between the time shown on your sundial and your watch to a minimum. Since solar time moves faster during some parts of the year than others, it won’t ever be in perfect agreement with your watch. In fact, depending on where you live, your sundial could be as much as 45 minutes ahead or behind local time.