solar time

Solar time, time defined by the position of the sun. The solar day is the time it takes for the sun to return to the same meridian in the sky. Local solar time is measured by a sundial. When the center of the sun is on an observer's meridian, the observer's local solar time is zero hours (noon). Because the earth moves with varying speed in its orbit at different times of the year and because the plane of the earth's equator is inclined to its orbital plane, the length of the solar day is different depending on the time of year. It is more convenient to define time in terms of the average of local solar time. Such time, called mean solar time, may be thought of as being measured relative to an imaginary sun (the mean sun) that lies in the earth's equatorial plane and about which the earth orbits with constant speed. Every mean solar day is of the same length. The difference between the local solar time and the mean solar time at a given location is known as the equation of time. Tables used by navigators list the equation of time for different times of year so that an observer can calculate his mean solar time from his local solar time (found by determining the sun's hour angle). Mean solar time is the basis for civil time and standard time.

watch

Watch, small, portable timepiece usually designed to be worn on the person. Other kinds of timepieces are generally referred to as clocks. At one time it was generally believed that the first watches were made in Nuremburg, Germany, c.1500. However, there is now evidence that watches may have appeared at an earlier date in Italy. Early watches were ornate, very heavy, and made in a variety of shapes, e.g., pears, skulls, and crosses; the faces were protected by metal latticework. Watch parts were made by hand until c.1850, when machine methods were introduced by watch manufacturers in the United States. The introduction of machine-made parts not only cut manufacturing costs but increased precision and facilitated repairs. To insure the accuracy of a watch over a long period, bearings made of jewels (usually synthetic sapphires or rubies) are utilized at points subject to heavy wear. The mechanical watch contains a mainspring to drive the watch's mechanism. Part of the mechanism includes a hairspring and an oscillating balance wheel to control the rate at which the mechanism moves. The mainspring is wound by the wearer when he turns a knob outside the watch's casing. The automatic, or self-winding, watch has a mainspring that is wound by an oscillating weight, contained in the watch, that is set into motion by the movements of the wearer. The stopwatch can be stopped or started at will by pressing a tiny button on its edge and is used for timing such events as races. The electric watch, which was introduced by the Hamilton Watch Company in 1957, also uses a hairspring and a balance wheel to regulate the rate at which its mechanism moves, but it has no mainspring. In recent years sophisticated electronic watches have been developed. One type uses the vibrations of an electrically driven tuning fork to determine the rate at which a small motor drives the hands. In another type a crystal oscillator provides a signal that regulates this motion. In the most common type a quartz crystal oscillator is joined to digital counting and digital display circuits, thus eliminating all moving parts. Quartz watches with digital displays now account for nearly half of all watch production, since they are inexpensive to produce but are accurate to within several seconds per month. Electric and electronic watches are powered by tiny long-lasting batteries. See chronometer.

clepsydra

Clepsydra (klĕp`sĭdrə) or water clock, ancient device for measuring time by means of the flow of water from a container. A simple form of clepsydra was an earthenware vessel with a small opening through which the water dripped; as the water level dropped, it exposed marks on the walls of the vessel that indicated the time that had elapsed since the vessel was full. More elaborate clepsydras were later developed. Some were double vessels, the larger one below containing a float that rose with the water and marked the hours on a scale. A form more closely foreshadowing the clock had a cord fastened to the float so that it turned a wheel, whose movement indicated the time. A further step was the use of gear wheels and a turning pointer. It is believed that clepsydras were used in Egypt c.2000 B.C.; from Egypt they were introduced into Greece and later from there into Rome.

hourglass

Hourglass, glass instrument for measuring time, usually consisting of two bulbs united by a narrow neck. One bulb is filled with fine sand that runs through the neck into the other bulb in an hour's time. The date of its invention is unknown, but it was in use in ancient times. Similar devices for marking shorter periods of time, e.g., three-minute sandglasses for timing the cooking of eggs, are still used occasionally.

sundial

Sundial, instrument that indicates the time of day by the shadow, cast on a surface marked to show hours or fractions of hours, of an object on which the sun's rays fall. Although any object whose shadow is used to determine time is called a gnomon, the term is usually applied to a style, pin, metal plate, or other shadow-casting object that is an integral part of a sundial. Forerunners of the sundial include poles or upright stones used as gnomons; pyramids and obelisks were so used in Egypt. Both stationary and portable sundials were probably developed in Egypt or in Mesopotamia. The earliest extant sundial, an Egyptian instrument of c.1500 B.C., is a flat stone on which is fixed an L-shaped bar whose short vertical limb casts a shadow measured by markings on the longer horizontal limb. The sundial was greatly improved (c.1st cent. A.D.) by setting the gnomon parallel to the earth's axis of rotation so that the apparent east-to-west motion of the sun governs the swing of the shadow. The development of trigonometry permitted precise calculations for the marking of dials and stimulated the advance of gnomonics (dial marking). Although watches and clocks came into popular use in the 18th cent., sundials were long employed for setting and checking them. The heliochronometer, a highly accurate instrument in which the shadow is cast by a fine wire, was used until c.1900 to set the watches of French railwaymen. Solar (or apparent) time indicated by sundials and clock (or mean) time are different and must be correlated by the use of tables showing daily variations in sun time. A correction must also be made for the difference in longitude between the position of a sundial and the standard time meridian of a given locality. Although sundials are still used in many areas, including Japan and China, they are regarded today chiefly as adornments. The largest sundial in the world, constructed c.1724 in Jaipur, India, covers almost one acre (.4 hectare) and has a gnomon over 100 ft (30 m) high surmounted by an observatory. Notable collections of sundials are at the Adler Planetarium, the Metropolitan Museum of Art, and the Harvard College Observatory.