A longitude dial sits in my backyard atop a granite pedestal centered on a patio Andrewes designed in the style of a compass rose. Andrewes typically visits the site of any proposed garden or monument dial, paces around to pick the most Sun-favored spot and fixes its position with a hand-held GPS. But he downloaded my latitude and longitude from a computer database of ordinance survey maps. The coordinates became the raw data for the gnomonic projection centered on the house where I've lived for the past 20 years—and now may never leave, since its location is set in stone on my dial, with the latitude and longitude expressed in degrees, minutes and seconds of arc. It was plotted to work just here and nowhere else. In this sense it is a far more personal possession than the wristwatch I wear every day. And lovelier too. Early in the morning, I find the dew has collected on the continents, clouds floating overhead are reflected in the dial, and I can read the wind's direction by their passage. The gold-plated bead on the gnomon wire throws a small round shadow on the part of the map where the Sun is precisely overhead. The bead's shadow will cross the map along the straight line of the Equator each year on the days of the vernal and autumnal equinox, and on June 21, the summer solstice, it will trace out the curved Tropic of Cancer. Because I consult my dial most in warm-weather months, when daylight saving time is usually in effect, I chose to have it constructed to that system.
"With each dial I discover some new technique that makes me want to redo the earlier ones," Andrewes says. "But of course I can't do that." He can, however, incorporate innovations in the next dials, such as the monument-size one that has just been commissioned for an English country house. "The joy for me—and one of the most exciting things about a good sundial—is that once it's leveled and oriented correctly, it will never fail you when the Sun is shining. If anything goes wrong with the Earth, this dial will show it. You could be among the first to know. But if that suddenly happens, don't call me. Pray."
Dava Sobel, author of Longitude and The Planets, co-authored The Illustrated Longitude with William Andrewes.