Keeping A Sundial In Time With The Clock Requires Careful Calibration: Are You Prepared?

You could return to the most basic of timekeeping methods: the sundial. But be forewarned -- a temporally accurate sundial requires more than just putting a stick in the ground. (Note: All the following advice applies to the basic, iconic horizontal sundial; there are many other variations to consider as well!)

Although lots of gardens have sundials, many of these are more decorative than useful. Sundials have to be carefully calibrated to show the correct time. One of the first things to do is to figure out your position’s latitude and longitude -- your north-south and east-west coordinates, respectively.

Correcting for latitude avoids the problem of the shadow of your sundial's pointer, or gnomon, pointing in a different direction depending on seasonal variations of the sun’s apparent path in the sky. To compensate for your local latitude, your gnomon should point toward the celestial pole (the north celestial pole if you’re in the Northern Hemisphere, the south celestial pole if you’re in the Southern Hemisphere). This makes the angle of the gnomon equal to the geographic latitude of your position.

The celestial poles are those points in the sky that the stars appear to rotate around, and extend in a line from the Earth’s axis of rotation. The north celestial pole is very close to Polaris, the North Star. Finding the south celestial pole is a bit trickier, requiring you to use one of three methods involving southern constellations.

Correcting for your local longitude is important as well, because your local time zone is fixed to a specific east-west coordinate. Adjusting for the difference between your longitude and your local time zone meridian, converted into minutes, will give you a more accurate time.

Take this example from the North American Sundial Society: A dial located at 78.15 degrees West longitude (near Buffalo, N.Y.) is in a time zone fixed to the Eastern Time Zone meridian of 75 West longitude. That difference of 3 degrees, 15 minutes is equivalent to 3.25 degrees, which when multiplied by four (because time varies by four minutes for each degree of longitude) equals a 14-minute offset. Adjust your hour marks accordingly, or just add the appropriate time offset to your shadow reading when you look at your sundial.

Even if you adjust your sundial for your latitude and longitude, you still have to correct for the difference between “sun time” and “clock time.” The sun’s apparent path around the sky is not actually uniform, thanks to both the fact that Earth’s orbit is not perfectly circular, and because the Earth is tilted. Due to both of these factors, the sun’s apparent speed through the sky varies throughout the year, causing sun time to go faster than or fall behind clock time.

Thankfully, people have already calculated this variation, and given us the Equation of Time, which tells you how many minutes you need to add or subtract on a certain day of the year. As seen in this graph:

Photo: Wikimedia Commons/Drini At points in the line above the x-axis a sundial will appear faster than local clock time, and at points in the line below the x-axis sundial will appear slower. You may need to subtract 16 minutes from your sundial reading in early November or add 14 minutes in mid-February.

Finally, if you’re on Daylight Savings Time, just add one hour and you’re good to go!