Description of a MechanismThe Sundial INTRODUCTIONDifferent devices have been used for centuries to mark time. Before clocks and watches were invented in the 18th century, the sundial was the most common device used to gauge the time of day. The sundial relies on the sun and its angle in the sky to indicate the time. Though a sundial’s total dependence on the sun makes them less useful on cloudy or rainy days, sundials have long been an important time teller and calibrator for more sophisticated time telling devices. Even at the beginning of the 20th century, sundials were used to regulate watches and clocks. Sundials tell “sun time.” Clocks and watches tell “clock time.” Neither kind of time is intrinsically better than the other; they are both useful for their separate purposes. Sun time is anchored around the idea that when the sun reaches its highest point in the sky, it is noon. The next day when the sun is at its highest point in the sky, the time will be noon again. The time that has elapsed between successive “noons” is sometimes more and sometimes less than 24 hours of clock time, depending on the time of the year. In the middle months of the year, the length of the day is close to 24 hours, but around mid-September the days are slightly shorter than 24 hours and around Christmas the days are slightly longer than 24 hours. Clock time is anchored around the idea that each day is exactly 24 hours long. This is not actually true, but it is much more convenient to have a consistent measure of time that is the same each day. As interaction among cities increased, it became increasingly important to have the same measure of time around the world. Components of a SundialSundials consist of two parts: a gnomon and a dial plane. The gnomon (a Greek word that means pointer) is the shadow-producing device, usually a metal plate set parallel to earth’s axis and pointing toward the celestial pole. The gnomon is a triangular-shaped plane that sits perpendicular to the dial plane. The inclined edge of the gnomon, called the “style,” produces the working edge of the shadow that is used to tell the time. The dial plane is the flat surface marked with the times of day. The dial plane is the surface where the shadow reflects which is cast by the gnomon. Although the dial plane can be any shape, the numbers used to read time rely on mathematical formulae for their precise placement and spacing. A sundial is specifically designed for its location on the earth, and tables are used to calculate standard time from solar time, because solar time changes daily with the position of the sun. Virtually anything casting a shadow can be made into a sundial; the difficult part is to calculate the proper placement of the time marks. The color or material used to construct a sundial is limited only by the imagination. An element of the sundial construction that is important is the position of the gnomon relative to the specific location of the sundial.
How a Sundial WorksA sundial measures time by the location of a shadow on a plane. The gnomon, which is exposed to sunlight, casts the shadow. The shadow is cast onto a plane that is inscribed with numbers representing the hours of the day.Time represented by a correctly designed and constructed sundial is referred to as Local Apparent Time (LAT). Local Apparent Time reads noon when the sun is viewed directly over the observer’s meridian. No two locations would share the same LAT noon, or any other hour, unless they were on the same line of longitude. In addition, due to the orbit of the earth around the sun, a day defined by the length of time it takes the sun to reappear over the same earth meridian can vary depending upon the time of year. To avoid the problems presented by variations in the length of solar days, and to establish a uniform system of time reference, modern civilizations have adopted Standard Time Zones (STZ). Though somewhat arbitrary, STZs enable consistent reference to time in any particular location from day to day.A sundial can be designed to indicate Standard Time by offsetting the hour lines on the dial by the arc to time difference between the STZ reference longitude and the dial’s longitude. To account for the variation in the length of the solar day, a correction to the dial’s reading is also required. This correction, known as the “Equation of Time,” can be obtained from published sources in either tabulated or graphical form.1 Types of SundialsHorizontal Sundial The horizontal sundial is the type commonly found on pedestals in gardens. The dial plane is horizontal to the earth’s surface. Numbers, representing the hour of the day, are inscribed on the dial plane. Figure 1 illustrates a horizontal sundial.Vertical SundialAs the name indicates, the vertical sundial hangs perpendicularly to the ground surface. The vertical sundial has traditionally been found on the walls of churches. Today, vertical sundials are still used though their function is more decorative than utilitarian. In the northern hemisphere, the vertical sundial is most accurate when it faces due south. Any variation of angle will impact the precision of the sundial’s ability to tell time. Figure 2 illustrates the Vertical Sundial. Figure 1 – Horizontal Sundial Figure 2 – Vertical Sundial 1 World Wide Web. Welcome to Bob’s Sundial Page. www.se.mediaone.net