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How Sundials Work

Sundials rely on the fact that, as the earth turns, the sun seems to travel across the sky. Any object standing in the sun casts a shadow. On a stationary object, this shadow will move "clockwise" (in the northern hemisphere). This means that the relative times of the day can be marked around the stationary object, and during daylight hours an approximation of the actual time can be made.

How a Sundial Works image.
The only point at which true sun time can easily be determined is at 12 noon; when, in the northern hemisphere, the shadow cast by the gnomon (the vertical part of the sundial) will be at its most northerly. If the Earth were not tilted as it orbits, then the sundial would be regular, and it would be quite easy to tell time by a sundial once the position of the hours around the sundial were established. However, the fact that the number of daylight hours waxes and wanes over the course of the year, and the fact that the angle any particular part of the planet will be, relative to the sun, changes on a daily basis means that the shadow cast by a sundial perpendicular to a tangent of the Earth will not be as regular as a clock. (This also stems from the fact that the Earth's orbit is not exactly circular, but is slightly elliptical.)

how a tilted sundial works image.
The curvature of the Earth creates another problem for the sundial watcher. The angle relative to the sun of a stick perpendicular to the surface will be different for various latitudes. This means that every hour's passing will move a shadow more or less depending on its geographic position.

Yet these factors can be compensated for by setting up the sundial so that the style (the gnomon on a tilted sundial is called a "style") is perpendicular to the equator and the base is made parallel with the equator. This means the base of the sundial should be raised on the north side to make it equal to the degree of latitude.

Even with this adjustment, two sundials will read a different time of day as long as they are on different lines of longitude. This is also due to the curvature of the Earth coupled with its spinning motion. This difference from actual clock time can also be compensated for by tilting the sundial. The direction and degree of tilt depend on the actual location within a time zone on which the time of that region is based and whether or not Daylight Saving Time is in effect. (Yes daylight savings time does affect a sundial.) The sundial will have to be tilted slightly toward the preferred point as if in obeisance to its prominence.

Again, because of the tilt of the Earth this type of sundial will not be able to tell time on the equinoxes (the first day of spring and the first day of autumn). The sun will be directly above the style and the course of the sun, if marked on a sundial, would etch a straight line from west to east. Also, the time will be shown on the north side of the sundial in winter and on the south side in summer.

Many types of sundials have been designed including equinoctial sundials, vertical sundials, and dyptichs. Heliochronometers are highly accurate sundials which, at one time, played a prominent role in scientific and astronomic studies. Analemmatic sundials correct for the irregular elliptical orbit of the Earth.

Because the sundial requires significant initial adjustment to tell time accurately, coupled with the fact that it does not tell time at night, the sundial has lost much of its popularity as a time-piece in the modern world. However, sundials do make for beautiful and utilitarian art and architecture, and are often found as a decoration in gardens. They also make an excellent learning tool for people learning geography, science, and navigation.

Quite common in ancient and medieval times, the sundial was largely supplanted by the mechanical clock as soon as it became practical and affordable.

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