The Winged Chariot of Time

Earliest Clocks

        Not until somewhat recently (that is, in terms of human history) did people find a need for knowing the time of day. As best we know, 5000 to 6000 years ago great civilizations in the Middle East and North Africa initiated clock-making as opposed to calendar-making. With their attendant bureaucracies and formal religions, these cultures found a need to organize their time more efficiently.

Sun Clocks

     After the Sumerian culture was lost without passing on its knowledge, the Egyptians were the next to formally divide their day into parts something like our hours. Obelisks (slender, tapering, four-sided monuments) were built as early as 3500 B.C. Their moving shadows formed a kind of sundial, enabling citizens to partition the day into two parts by indicating noon. They also showed the year's longest and shortest days when the shadow at noon was the shortest or longest of the year. Later, markers added around the base of the monument would indicate further time subdivisions.

        Another Egyptian shadow clock or sundial, possibly the first portable timepiece, came into use around 1500 B.C. to measure the passage of "hours." This device divided a sunlit day into 10 parts plus two "twilight hours" in the morning and evening. When the long stem with 5 variably spaced marks was oriented east and west in the morning, an elevated crossbar on the east end cast a moving shadow over the marks. At noon, the device was turned in the opposite direction to measure the afternoon "hours."

        The merkhet, the oldest known astronomical tool, was an Egyptian development of around 600 B.C. A pair of merkhets were used to establish a north-south line by lining them up with the Pole Star. They could then be used to mark off nighttime hours by determining when certain other stars crossed the meridian.

        In the quest for more year-round accuracy, sundials evolved from flat horizontal or vertical plates to more elaborate forms. One version was the hemispherical dial, a bowl-shaped depression cut into a block of stone, carrying a central vertical gnomon (pointer) and scribed with sets of hour lines for different seasons. The hemicycle, said to have been invented about 300 B.C., removed the useless half of the hemisphere to give an appearance of a half-bowl cut into the edge of a squared block. By 30 B.C., Vitruvius could describe 13 different sundial styles in use in Greece, Asia Minor, and Italy.

   Elements of a Clock

     Having described a variety of ways devised over the past few millennia to mark the passage of time, it is instructive to define in broad terms what constitutes a clock. All clocks must have two basic components:

        The history of timekeeping is the story of the search for ever more consistent actions or processes to regulate the rate of a clock.

   Water Clocks

        Water clocks were among the earliest timekeepers that didn't depend on the observation of celestial bodies. One of the oldest was found in the tomb of Amenhotep I, buried around 1500 B.C. Later named clepsydras ("water thief") by the Greeks, who began using them about 325 B.C., these were stone vessels with sloping sides that allowed water to drip at a nearly constant rate from a small hole near the bottom. Other clepsydras were cylindrical or bowl-shaped containers designed to slowly fill with water coming in at a constant rate. Markings on the inside surfaces measured the passage of "hours" as the water level reached them. These clocks were used to determine hours at night, but may have been used in daylight as well. Another version consisted of a metal bowl with a hole in the bottom; when placed in a container of water the bowl would fill and sink in a certain time. These were still in use in North Africa this century.

     More elaborate and impressive mechanized water clocks were developed between 100 B.C. and 500 A.D. by Greek and Roman horologists and astronomers. The added complexity was aimed at making the flow more constant by regulating the pressure, and at providing fancier displays of the passage of time. Some water clocks rang bells and gongs, others opened doors and windows to show little figures of people, or moved pointers, dials, and astrological models of the universe.

        A Greek astronomer, Andronikos, supervised the construction of the Tower of the Winds in Athens in the 1st century B.C. This octagonal structure showed scholars and marketplace shoppers both sundials and mechanical hour indicators. It featured a 24-hour mechanized clepsydra and indicators for the eight winds from which the tower got its name, and it displayed the seasons of the year and astrological dates and periods. The Romans also developed mechanized clepsydras, though their complexity accomplished little improvement over simpler methods for determining the passage of time.

        In the Far East, mechanized astronomical/astrological clock-making developed from 200 to 1300 A.D. Third-century Chinese clepsydras drove various mechanisms that illustrated astronomical phenomena. One of the most elaborate clock towers was built by Su Sung and his associates in 1088 A.D. Su Sung's mechanism incorporated a water-driven escapement invented about 725 A.D. The Su Sung clock tower, over 30 feet tall, possessed a bronze power-driven armillary sphere for observations, an automatically rotating celestial globe, and five front panels with doors that permitted the viewing of changing mannikins which rang bells or gongs, and held tablets indicating the hour or other special times of the day.

        Since the rate of flow of water is very difficult to control accurately, a clock based on that flow can never achieve excellent accuracy. People were naturally led to other approaches.

    Perfecting the clock

        Accuracy in timekeeping has improved steadily, as shown, since the first mechanical clocks were made in the 14^th century.  Early clocks, like the Dover Castle, were regulated by weighted bar called the foliot, which pivoted back and forth to move a single hand.  Although foliot mechanism became increasingly precise over the next three centuries, clocks still varied by several minutes a day until the pendulum clocks came into general use in 1656.  Then, for the first time clocks became accurate enough to record minutes as well as hours.

        Over the next 256 years, clockmakers developed better escapements for regulating the pendulum, and then began to improve the pendulum itself.  In 1721 George Graham was the first t compensate for the fact that temperature changes cause steel pendulums to vary in speed.  His clock had a temperature-independent mercury-vial pendulum, which varied only by one second a day.  Accuracy continued to improve as pendulums were superseded by quartz crystals that vibrated so precisely that they made possible a clock accurate to a few thousandths of a second a day.

        Even more precise clocks came into being in the 1950s, when atomic oscillations were used to regulate the vibrations of a quartz crystal.  Atomic clocks paced by vibrating cesium atoms were perfected until they were precise to several billionths of a second per day, or about one second every 350,000 years.  For time periods shorter than a day, as the graph shows, hydrogen master cocks were even more exact than other types.  However, for measuring long periods of time the cesium clock remains most accurate timepiece that man has devised.

©1999 ThinkQuest Team 28906