Marking time with a sundial not for everyone

The Working House

Making and marking time with a sundial is a very personal matter. Barbara, Leonard and Winthrop, who have each made their own sundials, would find it annoying to wear a beepin’ digital watch.

“Count only the sunny hours.” That is the inscription Barbara chiseled into the face of the homemade sundial which graces the center of her rose and herb garden.

When the shadow is two marks past Roman numeral III, she’ll leave her gardening knowing that there is just enough time to chill the champagne before the dinner guests arrive.

Leonard Rhoades, who taught math, astronomy and navigation at Exeter Academy, set up his sundial the hard way — with trigonometric calculations. “Do you realize that there is a nine-second difference in time between the east and west sides of Manhattan,” he asked. Some people are late and they don’t even know it.

Leonard chose a vertical dial which he attached to the front of his house next to the entrance. It is a handsome and eye-catching adornment to his Newcastle, N.H., home that he reads coming and going.

In 1725, Winthrop Randolf, an English gentleman who had trouble rising before noon after a night of debauchery, devised a new use for sundials. His adjustable sundial had a magnifying lens that focused light on the fuse of a miniature cannon, firing it at eight a.m. Reload the cannon at bedtime. Sleep in on cloudy days.

You might not choose to build the cannon alarm clock, but Leonard’s vertical dial and Barbara’s horizontal dial are useful instruments that are easy to build (we’ll do the mathematical calculations for your specific location). If you buy a sundial from a store or catalogue, you’ll get the same results that Fred Flintstone got from his wrist sundial — cartoon time. The accuracy of a sundial depends upon its calibration to a specific site such as your backyard.

If you make your own dial, you can calibrate it by this very simple method (purists will cringe): make a gnomon (the triangular pointer on a typical dial) as described below, set it into your unmarked dial face, orient the pointer to true north and make the dial face plumb and level. On a sunny day, mark the hours on the face with the help of an (ugh!) digital watch.

This method uses “mean time” (a clock) to measure “apparent time” (the sun) which varies by as much as 15 minutes a year due to the earth’s elliptical orbit. In other words, your clock will really only be accurate one day each year. Our average day is actually 23 hours 57 minutes and 55.91 seconds, which explains why you can never finish everything you want to in a 24-hour “clock” day.

Cut the gnomon described above from an 1/8-inch piece of sheet metal with a hacksaw. As a rule of thumb, the dial should be twice as big as the gnomon. Include a half-inch “tongue” on the bottom that will be glued with epoxy into a groove cut into the face of the dial. For Bangor, the angle of the gnomon’s pointer should measure 43 degrees.

Teak, which weathers beautifully, makes an excellent choice for the garden sundial’s face. From 1-inch stock, cut a circle or square twice as wide as the length of the gnomon.

After the hours have been marked according to the above described method, draw in the rays with a ruler, and then carve them with a veiner or V-tool. Make at least two passes with the carving tool so that the second pass straightens any wrinkles left during the first.

Finish the teak with an oil such as Watco Oil Finish, which will provide protection for the wood and give a sheen, but will not flake off like a varnish. Glue and screw the sundial onto a sturdy, level base (a 4-by-4 pressure-treated post will do) that won’t shift in frost or be knocked out of place.

Leonard’s vertical dial can be placed on a garage peak or sunny exterior wall. The fewer obstructions (like trees) and the more directly south-facing the wall is, the better. The gnomon for this dial is simply a half-inch dowel about a foot long that is glued into a hole drilled into the face of the wall at a specific angle — the angle for Bangor is 47 degrees, which was found by subtracting our latitude (43 degrees) from 90 degrees.

Once the gnomon is in place, establish the hour rays as their shadows are projected by the sun. For the most accurate results, do this on April 16, June 16, September 1, or December 25, when the difference between mean time and apparent time is minimal.

A second method of scribing lines is to lay a protractor down on the face of the dial and mark off the following set of degrees that we have calculated for Bangor: 0, 5, 10.5, 16, 21.5, 27.5, 34.5, 41.5, 50, 58.5, 68.5, 79. The marks indicate half hours from noon (0 degrees) to 6 p.m. (79 degrees).

Return to zero and mark the same degrees in the other direction for the hours, 6 a.m. to noon. You’ll notice that there are not 24 hours on these clocks; the sun only shines for half of the day. You could use a flashlight…

Doug and Cynthia Edmunds are renovators from Kittery.