Why Sundials Tells Time

A Sundial will serve you faithfully if it is used. Many sundials are not used because the basic principle of their construction and the reason why they tell time are not understood. You can not use your sundial to the fullest extent if you know nothing about it other than the fact that it is supposed to tell time but why? There is no deep mystery connected with the hour lines on your sundial plate or the motion of the shadow cast on it. You will recall from the previous section that at the time of Berosus, the picture of our universe was that of a hollow sphere, with the earth at its center. About 150 years later, 225 B.C., Eratosthenes, an astronomer and mathematician of Alexandria, devised an instrument which consisted of many rings put together in the form of a hollow sphere, with a globe in the center. It was made to show the heavens encircling the earth, and it resembled one of those gyroscopes we have all wondered at as children and delighted in playing with as adults. This instrument was called an armillary, where only three rings have been used one to represent the horizon, another the celestial equator, and the third a true north and south line or meridian circle, with an axis pointing to the celestial pole. To be sure, the sphere looks complicated, nay almost understandable.

However, this jumble of rings is not entirely unfamiliar, for from childhood we have seen them on globe atlases of the world; they correspond to those imaginary lines or circles which divide the earth into the tropic, temperate, and frigid zones. These circles, beginning at the north or uppermost part of the sphere are called Arctic Circle, tropic of Cancer, equator, tropic of Capricorn, and the Antarctic Circle. In addition a sloping ring which cuts across the equator and extends north as far as the tropic of Cancer and south as far as the tropic of Capricorn, shows the path in which the sun and planets appear to move, commonly called the ecliptic. Such was the armillary. It was used by ancient astronomers for observational purposes; but as time went on it was found to be useful in solving many problems of the sphere. No mention is made of it as a sundial, in early Latin, Greek, and Arabian manuscripts. Vitruvius does not mention it as such, nor does Albategni. This is peculiar for it is the simplest form of a sundial just a circle. Even as late as the 17th and 18th century it was referred to and used as an instrument to "solve problems of the sphere and to lay out sundials". When it was first used to tell time is not known.

The Principles of Sundials

The principles of the sundial are more easily understood if the form of the armillary is fixed in the mind in lieu of a working model. The earthly circles have their counterparts in the sky where they bear the same names, for the heavenly circles are just the prolongation of the planes of the earthly ones. This is easily seen if we mark and name the various circles on the outside of a small rubber balloon representing the earth; then if the balloon could be blown up so large that we could step in to the center, the circles would be seen from the inside of the balloon as they would appear in the sky, if the balloon expanded the same amount in all directions. You will no longer have to wonder what the rings mean when you next see this type of instrument, which is often used today as a sundial. Its application will be obvious as we proceed. Our present day method of timekeeping, used in everyday life, is wholly man made. We have seen fit to make each day of equal length and divide the day into twenty-four equal parts, beginning at midnight, with hours running consecutively either from one to twelve or from one to twenty four. In the United States we use the former division of the day, by counting twelve hours from midnight to midday, and twelve hours from midday to midnight; whereas in some countries the hours are counted from midnight and continue consecutively throughout the day to midnight. It is this division of the day into equal hours that enables us to tell time by the sundial. more on why the sundial keeps time...

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