A sundial is an instrument that tells time by measuring the apparent passage of the sun through its daily course in the sky. The vast majority of sundials consist of a pointer, called a style or gnomon, and a calibrated plate upon which the pointer casts a shadow.
The calibration of the dial can be understood by imagining a sundial placed so that the style lies on the earth's north-south axis and the plate is secured to a horizontal surface on the north pole. Thus the plate would be perpendicular to the style and parallel to the earth's equator. As the earth rotates about its axis, the shadow cast by the style would sweep out equal periods of time, since the earth rotates at an approximately constant velocity. Therefore the plate of the sundial could be calibrated by dividing it into 24 equal arcs. If the dial were moved from the north pole to any other place on earth and the plate kept parallel to the equator, these calibrations would not need to be changed. This type of sundial is called an equatorial or equinoctial sundial.
If the sundial is to have a plate in any position other than one parallel to the equatorial plane, such as horizontal, vertical, or in another plane, equal arcs on the plate will not correspond to equal periods of time. However, the correct calibrations can be determined by means of either geometry or trigonometry.
Sundial time, or solar time, does not correspond to clock time, or mean solar time. Clock time corresponds to a period measured by the sun's apparent movement from zenith to zenith, assuming fictitiously a sun rotating at constant velocity around the earth in the equatorial plane. Actually, the earth revolves around the sun in an elliptical orbit, and the plane of the orbit is inclined about 23° from the plane of the earth's equator. As a result of these two factors the sun's apparent velocity fluctuates. Solar time, therefore, varies from clock time according to the day of the year, except at the equinoxes and solstices. The difference between solar time and clock time (mean solar time) is called the equation of time.
Types of Sundials
There are three major types of sundials. The dials discussed above depend basically only upon the rotation of the earth about its axis. These dials are often given names corresponding to the placement of the calibrated surface—horizontal, vertical, inclined, or equatorial. All sundials of this class are called directional sundials because their styles must always be aligned with the north-south axis of the earth.
Another type of sundial depends not only upon the rotation of the earth about its axis but also upon the changes in position of the sun above and below the equator in the course of a year. The advantage of this type of dial, called an altitude sundial, is that it does not require a compass for orientation. Typical of altitude sundials is the pillar, or shepherd's sundial. The calibrations of the instrument reflect the change in the altitude of the sun in the sky with the succeeding months. The months are marked off by the vertical lines and the hours by the diagonal lines. When the dial is suspended from its rings, with the pointer, or gnomon, erected over the line of the proper month, the instrument is rotated until the shadow falls directly beneath the gnomon, indicating the time.
A third group of sundials measures the sun's azimuth (horizontal angle between the sun and the south point of the horizon), rather than its altitude. Some dials incorporate the principles of two types. For example, the universal ring dial is an equatorial dial with a time-indicating device that must be changed from month to month.
History of the Sundial
The most ancient of scientific instruments is probably the sundial. A stick placed in the ground gave early man a simple way to tell time. The most important early developments in the construction of sundials occurred in Egypt, Babylonia, and Greece. Most sundials from these civilizations divide daylight into equal hours, called temporary hours, taking no account of the variation in the period of daylight during the year.
By the beginning of the Christian era, the principles of construction of most of the sundials discussed above were known. Before the invention of the compass, portable sundials were mainly of the self-orienting altitude type because of the difficulty in aligning a sundial of the directional class.
The introduction of trigonometry into mathematics by the Greeks by about 150 A.D. supplied the tool for plotting the hour lines with simple arithmetical calculations instead of the more cumbersome geometric constructions. The method was exploited by the Arabs and subsequently by European Renaissance sundial makers.
The great age of the European sundial lasted from the 16th to the 19th century. The development of the mechanical clock spurred the search for more accurate sundials for their regulation. With the increasing accuracy of the watch and the wide adoption of standard time in the 19th century, the sundial lost its practical value. Nevertheless, sundials have not gone completely out of use.