History Online - Time
[an error occurred while processing this directive]Time measurement
A brief history of the clock
Celestial bodies, the sun, moon, planets, and stars have provided us a reference for measuring the passage of time throughout our existence. Ancient civilizations relied upon the apparent motion of these bodies through the sky to determine seasons, months, and years.
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.
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:
In 1656, Christiaan Huygens, a Dutch scientist, made the first pendulum clock, regulated by a mechanism with a "natural" period of oscillation. Although Galileo Galilei, sometimes credited with inventing the pendulum, studied its motion as early as 1582, Galileo's design for a clock was not built before his death. Huygens' pendulum clock had an error of less than 1 minute a day, the first time such accuracy had been achieved. His later refinements reduced his clock's errors to less than 10 seconds a day.
The Shortt clock was replaced as the standard by quartz crystal clocks in the 1930s and 1940s, improving timekeeping performance far beyond that of pendulum and balance-wheel escapements.
Quartz clock operation is based on the piezoelectric property of quartz crystals. If you apply an electric field to the crystal, it changes its shape, and if you squeeze it or bend it, it generates an electric field. When put in a suitable electronic circuit, this interaction between mechanical stress and electric field causes the crystal to vibrate and generate a constant frequency electric signal that can be used to operate an electronic clock display.
Little is known about the details of timekeeping in prehistoric eras, however, records and artifacts that are discovered, show that in every culture, people were preoccupied with measuring and recording the passage of time. Ice-age hunters in Europe over 20,000 years ago scratched lines and gouged holes in sticks and bones, possibly counting the days between phases of the moon. Five thousand years ago, Sumerians in the Tigris-Euphrates valley in today's Iraq had a calendar that divided the year into 30-day months, divided the day into 12 periods (each corresponding to 2 of our hours), and divided these periods into 30 parts (each like 4 of our minutes). There are no written records of the creating of Stonehenge, built over 4000 years ago in England, but its alignments show its purposes apparently included the determination of seasonal or celestial events, such as lunar eclipses, solstices and so on.
The earliest Egyptian calendar was based on the moon's cycles, but later the Egyptians realized that the "Dog Star" in Canis Major, which is now called Sirius, rose next to the sun every 365 days, about when the annual inundation of the Nile began. Based on this knowledge, they devised a 365-day calendar that seems to have begun in 4236 B.C., the earliest recorded year in history.
In Babylonia, again in Iraq, a year of 12 alternating 29-day and 30-day lunar months was observed before 2000 B.C., giving a 354-day year. In contrast, the Mayans of Central America relied on not only the sun and moon, but also the planet Venus, to establish 260-day and 365-day calendars. This culture flourished from around 2000 B.C. until about 1500 A.D. They left celestial-cycle records indicating their belief that the creation of the world occurred in 3113 B.C. Their calendars later became portions of the great Aztec calendar stones. Other civilizations, including the modern West, have adopted a 365-day solar calendar with a leap year occurring every fourth year.
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
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. Two 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 forms that were more elaborate. 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
- A regular, constant or repetitive process or action to mark off equal increments of time. Early examples of such processes included movement of the sun across the sky, candles marked in increments, oil lamps with marked reservoirs, sand glasses ("hourglasses"), and in the Orient, small stone or metal mazes filled with incense that would burn at a certain pace.
- A means of keeping track of the increments of time and displaying the result. Our means of keeping track of time passage include the position of clock hands and a digital time display.
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 did not 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 manikins 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 could never achieve excellent accuracy. People were naturally led to other approaches.
In Europe during most of the Middle Ages (roughly 500 to 1500 A.D.), technological advancement was at a virtual standstill. Sundial styles evolved, but didn't move far from ancient Egyptian principles.
During these times, simple sundials placed above doorways were used to identify midday and four "tides" of the sunlit day. By the 10th Century, several types of pocket sundials were used. One English model identified tides and even compensated for seasonal changes of the sun's altitude.
Then, in the early-to-mid-14th century, large mechanical clocks began to appear in the towers of several large Italian cities. There is no evidence or record of the working models preceding these public clocks that were weight-driven and regulated by a verge-and-foliot escapement.
Verge-and-foliot mechanisms reigned for more than 300 years with variations in the shape of the foliot. All had the same basic problem: the period of oscillation of this escapement depended heavily on the amount of driving force and the amount of friction in the drive. Like water flow, the rate was difficult to regulate.
Another advance was the invention of spring-powered clocks between 1500 and 1510 by Peter Henlein, a German locksmith from Nuremberg. Replacing the heavy drive weights permitted smaller (and portable) clocks and watches. Henlein nicknamed his clocks "Nuremberg Eggs". Although they slowed down as the mainspring unwound, they were popular among wealthy individuals due to their size and the fact that they could be put on a shelf or table instead of hanging from the wall. They were the first portable timepieces. However, they only had an hour hand, minute hands did not appear until 1670, and there was no glass protection. Glass over the face of the watch did not come about until the 17th century. Still, Henlein's advances in design were precursors to truly accurate timekeeping.
Accurate Mechanical Clocks
Around 1675, Huygens developed the balance wheel and spring assembly, still found in some of today's wrist watches. This improvement allowed 17th century watches to keep time to 10 minutes a day. And in London in 1671 William Clement began building clocks with the new "anchor" or "recoil" escapement, a substantial improvement over the verge because it interferes less with the motion of the pendulum.
In 1721, George Graham improved the pendulum clock's accuracy to 1 second a day by compensating for changes in the pendulum's length due to temperature variations. John Harrison, a carpenter and self-taught clock-maker, refined Graham's temperature compensation techniques and added new methods of reducing friction. By 1761, he had built a marine chronometer with a spring and balance wheel escapement that won the British government's 1714 prize (of over $2,000,000 in today's currency) offered for a means of determining longitude to within one-half degree after a voyage to the West Indies. It kept time on board a rolling ship to about one-fifth of a second a day, nearly as well as a pendulum clock could do on land, and 10 times better than required.
Over the next century refinements led in 1889 to Siegmund Riefler's clock with a nearly free pendulum, which attained an accuracy of a hundredth of a second a day and became the standard in many astronomical observatories. A true free-pendulum principle was introduced by R. J. Rudd about 1898, stimulating development of several free-pendulum clocks. One of the most famous, the W. H. Shortt clock, was demonstrated in 1921. The Shortt clock almost immediately replaced Riefler's clock as a supreme timekeeper in many observatories. This clock consists of two pendulums, one a slave and the other a master. The slave pendulum gives the master pendulum the gentle pushes needed to maintain its motion, and also drives the clock's hands. This allows the master pendulum to remain free from mechanical tasks that would disturb its regularity.
Quartz Clocks
Quartz crystal clocks were better because they had no gears or escapements to disturb their regular frequency. Even so, they still relied on a mechanical vibration whose frequency depended critically on the crystal's size and shape. Thus, no two crystals can be precisely alike, with exactly the same frequency. Such quartz clocks continue to dominate the market in numbers because their performance is excellent and they are inexpensive. But the timekeeping performance of quartz clocks has been substantially surpassed by atomic clocks.
