It’s a flower of a different sort. Favored by gardeners for centuries, this perennial thrives in bright sunlight, it tolerates cloudy days and cold weather, and it can be found in all latitudes. It is at once attractive, ornamental and utilitarian, and the Greeks named its pistil “gnomon,” meaning “one who knows,” because it registered the sun’s movement.
Now mostly a garden ornament, the sundial waxed, waned and waxed again as man’s understanding of mathematics flourished, its gnomon casting a shadow across graceful Roman numerals, keeping time simply and accurately. Just as the pistil, its stem called a style, is the flower’s central organ, making possible the continuation of the species, so the gnomon, its shadow-casting edge also called a style, makes possible the telling of time.
The sun and moon were man’s first time indicators, and sundials started as sticks and obelisks. The sun was the principal timekeeper for 4,000 years, from ancient Egypt to the dominion of the clock. Sundials were neglected during the Dark Ages and became popular again in the Renaissance, their advances paralleling the rediscovery of and advances in astronomy and mathematics from 1500 to 1800.
Timekeepers could be cubes, hollowed-out globes, flights of numbered steps, the flat dials that we know, or portable compass sundials. Dials used in Egypt ca. 1500 B.C. still exist, the earliest known being L-shaped, the short leg of the L serving as the vertical indicator that casts the shadow. Horizontal dials are the types found in gardens; sun clocks were vertical, as seen on the walls of English churches; and equinoctial are at right angles to Earth’s axis. The armillary sphere is a set of rings, like a globe missing its skin, showing the important circles of the celestial sphere, which are the major great circles of Earth extended into space. A great circle has its plane intersecting the center of the Earth — the equator and a circle drawn from pole to pole are two — and an armillary sphere includes the equatorial and polar circles, with an arrow, representing the Earth’s axis, casting a shadow on a circle bearing numbers representing the hours.
To register accurate time, the dial plate on a horizontal sundial must be level, the noon line must point north, and the style must also point north and be parallel to Earth’s axis. To make the style parallel to Earth’s axis, its angle from the sundial’s base must equal the dial’s degree of latitude. In my area, the style would be angled at 41 degrees above the base. Hour lines are laid out using geometry, and the hours close to noon are closer together because the sun’s shadow moves more slowly near midday. Hour lines connect like hours: 6 a.m. and 6 p.m., for example, oppose each other on the dial face.
The compass sundial, developed in the 15th century, was accurate and portable and could be carried in the pocket. The compass was used to find north, and the gnomon was a string or folding triangle, lowered or raised to suit the latitude. Drake’s portable dial, built in 1569, opens like a pocket watch. The dial tilts up on an arc marked with degrees to set the sundial for latitude, and the compass aligns the dial with the meridian. It includes a tide table, a nocturnal to find time at night, and a diagram of planetary aspects showing positions of the planets.
During the 17th century, gnomonics, or dialing, the art of using or making sundials, was a special branch of education. The Bavarian astronomer and mathematician Christoph Clavius, who developed the proposal that was adopted as the Gregorian calendar in 1582, published an 800-page quarto volume in 1612 containing all that was known of gnomonics at the time.
Sundials are quite reliable when adjusted properly, reading to within a minute, and were used to correct mechanical clocks until the latter were improved, gradually replacing sundials in the 18th century except in gardens and rural areas, but they give sun time, which differs from clock time by as much as 16 minutes. “The Old Farmer’s Almanac” includes a column giving sun fast in minutes.
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