Time, Measurement of

The history of time measurement is the story of the search for more consistent and accurate ways to measure time. Early human groups recorded the phases of the Moon some 30,000 years ago, but the first minutes were counted accurately only 400 years ago. The atomic clocks that allowed mankind to track the approach of the third millennium (in the year 2001) by a billionth of a second are less than 50 years old. Thus, the practice and accuracy of time measurement has progressed greatly through humanity's history.

The study and science of time measurement is called horology. Time is measured with instruments such as a clock or calendar. These instruments can be anything that exhibits two basic components: (1) a regular, constant, or repetitive action to mark off equal increments of time, and (2) a means of keeping track of the increments of time and of displaying the result.

Imagine your daily life—getting ready in the morning, driving your car, working at your job, going to school, buying groceries, and other events that make up your day. Now imagine all the people in your neighborhood, in your city, and in your country doing these same things. The social interaction that our existence involves would be practically impossible without a means to measure time. Time can even be considered a common language between people, and one that allows everybody to proceed in an orderly fashion in our complicated and fast-paced world. Because of this, the measurement of time is extremely important to our lives.

The Origins of Modern Time Measurement

The oldest clock was most likely Earth as it related to the Sun, Moon, and stars. As Earth rotates, the side facing the Sun changes, leading to its apparent movement from one side of Earth, rising across the sky, reaching a peak, falling across the rest of the sky, and eventually disappearing below Earth on the opposite side to where it earlier appeared. Then, after a period of darkness, the Sun reappears at its beginning point and makes its journey again. This cyclical phenomenon of a period of brightness followed by a period of darkness led to the intervals of time now known as a day. Little is known about the details of timekeeping in prehistoric eras, but wherever records and artifacts are discovered, it is found that these early people were preoccupied with measuring and recording the passage of time.

A second cyclical observation was likely the repeated occurrence of these days, followed by the realization that it took about 30 of these days (actually, a fraction over 27 days) for the Moon to cycle through a complete set of its shape changes: namely, its main phases of new, first quarter, full, and last quarter. European ice-age hunters over 20,000 years ago scratched lines and gouged holes in sticks and bones, possibly counting the days between phases of the Moon. This timespan was eventually given the name of "month." Similarly, the sum of the changing seasons that occur as Earth orbits around the Sun gave rise to the term "year."

For primitive peoples it was satisfactory to divide the day into such things as early morning, mid-day, late afternoon, and night. However, as societies became more complex, the need developed to more precisely divide the day. The modern convention is to divide it into 24 hours, an hour into 60 minutes, and a minute into 60 seconds.

The division into 60 originated from the ancient Babylonians (1900 b.c.e.–1650 b.c.e.), who attributed mystical significance to multiples of 12, and especially to the multiple of 12 times 5, which equals 60. The Babylonians divided the portion that was lit by the Sun into 12 parts, and the dark interval into 12 more, yielding 24 divisions now called hours.

Ancient Arabic navigators measured the height of the Sun and stars in the sky by holding their hand outstretched in front of their faces, marking off the number of spans. An outstretched hand subtends an angle of about 15 degrees at eye level. With 360 degrees in a full circle, 360° divided by 15° equals 24 units, or 24 hours. Babylonian mathematicians also divided a complete circle into 360 divisions, and each of these divisions into 60 parts. Babylonian astronomers also chose the number 60 to subdivide each of the 24 divisions of a day to create minutes, and each of these minutes were divided into 60 smaller parts called seconds.

Early Time-Measuring Instruments

The first instrument to measure time could have been something as simple as a stick in the sand, a pine tree, or a mountain peak. The steady shortening of its shadow would lead to the noon point when the Sun is at its highest position in the sky, and would then be followed by shadows that lengthen as darkness approaches. This stick eventually evolved into an obelisk, or shadow clock, which dates as far back as 3500 b.c.e. The Egyptians were able to divide their day into parts comparable to hours with these slender, four-sided monuments (which look similar to the Washington Monument). The moving shadows formed a type of sundial, enabling citizens to divide the day into two parts.

Sundials evolved from flat horizontal or vertical plates to more elaborate forms. One version from around the third century b.c.e.) was the hemispherical dial, or hemicycle, a half-bowl-shaped depression cut into a block of stone, carrying a central vertical pointer and marked with sets of hour lines for different seasons.

Apparently 5,000 to 6,000 years ago, civilizations in the Middle East and North Africa initiated clock-making techniques to organize time more efficiently. Ancient methods of measuring hours in the absence of sunlight included fire clocks, such as the notched candle and the Chinese practice of burning a knotted rope. All fire clocks were of a measured size to approximate the passage of time, noting the length of time required for fire to travel from one knot to the next. Devices almost as old as the sundial include the hourglass, in which the flow of sand is used to measure time intervals, and the water clock (or "clepsydra"), in which the water flow indicates the passage of time.

The Mechanization of Clocks

By about 2000 b.c.e., humans had begun to measure time mechanically. Eventually, a weight falling under the force of gravity was substituted for the flow of water in time devices, a precursor to the mechanical clock. The first recorded examples of such mechanical clocks are found in the fourteenth century. A device, called an "escapement," slowed down the speed of the falling weight so that a cogwheel would move at the rate of one tooth per second.

The scientific study of time began in the sixteenth century with Italian astronomer Galileo Galilei's work on pendulums. He was the first to confirm the constant period of the swing of a pendulum, and later adapted the pendulum to control a clock. The use of the pendulum clock became popular in the 1600s when Dutch astronomer Christiaan Huygens applied the pendulum and balance wheel to regulate the movement of clocks. By virtue of the natural period of oscillation from the pendulum, clocks became accurate enough to record minutes as well as hours.

Although British physicist Sir Isaac Newton continued the scientific study of time in the seventeenth century, a comprehensive explanation of time did not exist until the early twentieth century, when Albert Einstein proposed his theories of relativity. Einstein defined time as the fourth dimension of a four-dimensional world consisting of space (length, height, depth) and time.

Quartz-crystal clocks were invented in the 1930s, improving timekeeping performance far beyond that of pendulums. When a quartz crystal is placed in a suitable electronic circuit, the interaction between mechanical stress and electric field causes the crystal to vibrate and generate a constant frequency that can be used to operate an electronic clock display.

But the timekeeping performance of quartz clocks has been substantially surpassed by atomic clocks. An atomic clock measures the frequency of electromagnetic radiation emitted by an atom or molecule. Because the atom or molecule can only emit or absorb a specific amount of energy, the radiation emitted or absorbed has a regular frequency. This allowed the National Institute of Standards and Technology to establish the second as the amount of time radiation would take to go through 9,192,631,770 cycles at the frequency emitted by cesium atoms making the transition from one state to another. Cesium clocks are so accurate that they will be off by only one second after running for 300 million years.

Time Zones

In the 1840s, the Greenwich time standard was established with the center of the first time zone set at the Royal Greenwich Observatory in England, located on the 0-degree longitude meridian. In total, twenty-four time zones—each fifteen degrees wide—were established equidistant from each other east and west of Greenwich's prime meridian. Today, when the time is 12:00 noon in Greenwich, it is 11:00 a.m. inside the next adjoining time zone to the west, and 1:00 p.m. inside the next adjoining time zone to the east.

The term "a.m." means ante meridiem ("before noon"), while "p.m." means post meridiem ("after noon"). But military time is measured differently. The military clock begins its day with midnight, known as either or 0000 hours ("zero hundred hours") or 2400 hours ("twenty-four hundred hours"). An early-morning hour such as 1:00 a.m. is known in military time as 0100 hours (pronounced "oh one hundred hours"). In military time, 12:00 noon is 1200 ("twelve-hundred hours"). An afternoon or evening hour is derived by adding 12; hence, 1:00 p.m. is known as 1300 ("thirteen-hundred hours"), and 10:00 p.m. is known as 2200 ("twenty-two hundred hours").

see also Calendar, Numbers in the.

William Arthur Atkins (with

Philip Edward Koth)

Bibliography

Gibbs, Sharon L. Greek and Roman Sundials. New Haven, CT: Yale University Press, 1976.

Landes, David S. Revolution in Time: Clocks and the Making of the Modern World. Cambridge, MT: The Belknap Press of Harvard University Press, 1983.

Tannenbaum, Beulah, and Myra Stillman. Understanding Time: The Science of Clocks and Calendars. New York: Whittlesey House, McGraw-Hill Book Company, Inc.,1958.

Internet Resources

ABC News Internet Ventures. "What is Time: Is It Man-Made Concept, Or Real Thing?" <http://www.abcnews.go.com/ABC2000/abc2000world/Time2000_feature.html>.

MARKING CELESTIAL TIME

Some archeologists believe the complex patterns of lines on the desert floor in Nazca, Peru were used to mark celestial time. Medicine Wheels—stone circles found in North America dating from over 1,000 years ago—also may have been used to follow heavenly bodies as they moved across the sky during the year.

ATOMIC TIME?

Variations and vagaries in the Earth's rotation eventually made astronomical measurements of time inadequate for scientific and military needs that required highly accurate timekeeping. Today's standard of time is based on atomic clocks that operate on the frequency of internal vibrations of atoms within molecules. These frequencies are independent of the Earth's rotation, and are consistent from day to day within one part in 1,000 billion.