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.
We know little about the details of timekeeping in prehistoric eras, but wherever
we turn up records and artifacts, we usually discover that in every culture,
some 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). We have no written records 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 we call
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 BC, which thus seems to be one of the earliest
years recorded in history.
2000 BC, the Babylonians (in today's Iraq) used a year of 12 alternating 29
day and 30 day lunar months, giving a 354 day year. In contrast, the Mayans
of Central America relied not only on the Sun and Moon, but also the planet
Venus, to establish 260 day and 365 day calendars. This culture and its related
predecessors spread across Central America between 2600 BC and 1500 AD, reaching
their apex between 250 and 900 AD. They left celestial-cycle records indicating
their belief that the creation of the world occurred in 3114 BC. Their calendars
later became portions of the great Aztec calendar stones. Our present civilization
has adopted a 365 day solar calendar with a leap year occurring every fourth
year (except century years not evenly divisible by 400).
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 began to make clocks
to augment their calendars. With their attendant bureaucracies, formal religions,
and other burgeoning societal activities, these cultures apparently found a
need to organize their time more efficiently.
Sumerian culture was lost without passing on its knowledge, but the Egyptians
were apparently 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 BC. Their moving shadows formed a kind of sundial, enabling people
to partition the day into morning and afternoon. Obelisks also showed the year's
longest and shortest days when the shadow at noon was the shortest or longest
of the year. Later, additional markers around the base of the monument would
indicate further subdivisions of time.
Another Egyptian shadow clock or sundial, possibly the first portable timepiece,
came into use around 1500 BC. 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
The merkhet, the oldest known astronomical tool, was an Egyptian development
of around 600 BC. A pair of merkhets was used to establish a north-south line
(or meridian) by aligning them with the Pole Star. They could then be used to
mark off nighttime hours by determining when certain other stars crossed the
In the quest for better 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 BC, 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 BC, Vitruvius could describe 13 different
sundial styles in use in Greece, Asia Minor, and Italy.
of a Clock
Before we continue describing the evolution of
ways to mark the passage of time, perhaps we should broadly define what constitutes
a clock. All clocks must have two basic components:
- a regular, constant or repetitive process or action to mark off equal
increments of time. Early examples of such processes included the movement
of the sun across the sky, candles marked in increments, oil lamps with marked
reservoirs, sand glasses (hourglasses), and in the Orient, knotted cords and
small stone or metal mazes filled with incense that would burn at a certain
pace. Modern clocks use a balance wheel, pendulum, vibrating crystal, or electromagnetic
waves associated with the internal workings of atoms as their regulators.
- a means of keeping track of the increments of time and displaying the
result. Our ways of keeping track of the passage of time include the position
of clock hands and digital time displays.
The history of timekeeping is the story of the search for ever more consistent
actions or processes to regulate the rate of a clock.
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 the Egyptian
pharaoh Amenhotep I, buried around 1500 BC. Later named clepsydras ("water
thieves") by the Greeks, who began using them about 325 BC, 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 in the 20th century.
More elaborate and impressive mechanized water clocks were developed between
100 BC and 500 AD 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 Macedonian astronomer, Andronikos, supervised the construction of his Horologion,
known today as the Tower of the Winds, in the Athens marketplace in the first
half of the first century BC. This octagonal structure showed scholars and 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 AD. 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 AD. Su Sung's mechanism incorporated
a water-driven escapement invented about 725 AD. 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.
Revolution in Timekeeping
Europe during most of the Middle Ages (roughly 500 AD to 1500 AD), technological
advancement virtually ceased. 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" (important times or periods) of the sunlit
day. By the 10th century, several types of pocket sundials were used. One English
model even compensated for seasonal changes of the Sun's altitude.
Then, in the first half of the 14th century, large mechanical clocks began to
appear in the towers of several large Italian cities. We have no evidence or
record of the working models preceding these public clocks, which were weight-driven
and regulated by a verge-and-foliot escapement. Variations of the verge-and-foliot
mechanism reigned for more than 300 years, but all had the same basic problem:
the period of oscillation of the 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 of Nuremberg. Replacing the heavy drive weights permitted
smaller (and portable) clocks and watches. Although they ran slower as the mainspring
unwound, they were popular among wealthy individuals due to their small size
and the fact that they could be put on a shelf or table instead of hanging on
the wall or being housed in tall cases. These advances in design were precursors
to truly accurate timekeeping.
1656, Christiaan Huygens, a Dutch scientist, made the first pendulum clock,
regulated by a mechanism with a "natural" period of oscillation. (Galileo
Galilei is credited with inventing the pendulum-clock concept, and he studied
the motion of the pendulum as early as 1582. He even sketched out a design for
a pendulum clock, but he never actually constructed one before his death in
1642.) Huygens' early 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 error to less than 10 seconds a day.
Around 1675, Huygens developed the balance wheel and spring assembly, still
found in some of today's wristwatches. This improvement allowed portable 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 per
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 developed new methods for 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 (worth more than
$10,000,000 in today's currency) 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 to win the prize.
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 contained two
pendulums, one a slave and the other a master. The slave pendulum gave the master
pendulum the gentle pushes needed to maintain its motion, and also drove the
clock's hands. This allowed the master pendulum to remain free from mechanical
tasks that would disturb its regularity.
The performance of the Shortt clock was overtaken
as quartz crystal oscillators and clocks, developed in the 1920s and onward,
eventually improved timekeeping performance far beyond that achieved using 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 an electric signal of relatively
constant frequency that can be used to operate an electronic clock display.
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, shape and
temperature. Thus, no two crystals can be exactly alike, with just the same
frequency. Such quartz clocks and watches continue to dominate the market in
numbers because their performance is excellent for their price. But the timekeeping
performance of quartz clocks has been substantially surpassed by atomic clocks.
"Atomic Age" of Time Standards
Scientists had long realized that atoms (and molecules)
have resonances; each chemical element and compound absorbs and emits electromagnetic
radiation at its own characteristic frequencies. These resonances are inherently
stable over time and space. An atom of hydrogen or cesium here today is (so
far as we know) exactly like one a million years ago or in another galaxy. Thus
atoms constitute a potential "pendulum" with a reproducible rate that
can form the basis for more accurate clocks.
The development of radar and extremely high frequency radio communications in
the 1930s and 1940s made possible the generation of the kind of electromagnetic
waves (microwaves) needed to interact with atoms. Research aimed at developing
an atomic clock focused first on microwave resonances in the ammonia molecule.
In 1949, NIST built the first atomic clock, which was based on ammonia. However,
its performance wasn't much better than the existing standards, and attention
shifted almost immediately to more promising atomic-beam devices based on cesium.
The first practical cesium atomic frequency standard was built at the National
Physical Laboratory in England in 1955, and in collaboration with the U.S. Naval
Observatory (USNO), the frequency of the cesium reference was established or
measured relative to astronomical time. While NIST was the first to start working
on a cesium standard, it wasn't until several years later that NIST completed
its first cesium atomic beam device, and soon after a second NIST unit was built
for comparison testing. By 1960, cesium standards had been refined enough to
be incorporated into the official timekeeping system of NIST. Standards of this
sort were also developed at a number of other national standards laboratories,
leading to wide acceptance of this new timekeeping technology.
The cesium atom's natural frequency was formally recognized as the new international
unit of time in 1967: the second was defined as exactly 9,192,631,770 oscillations
or cycles of the cesium atom's resonant frequency, replacing the old second
that was defined in terms of the Earth's motions. The second quickly became
the physical quantity most accurately measured by scientists. As of January,
2002, NIST's latest primary cesium standard was capable of keeping time to about
30 billionths of a second per year. Called NIST-F1, it is the 8th of a series
of cesium clocks built by NIST and NIST's first to operate on the "fountain"
Other kinds of atomic clocks have also been developed for various applications;
those based on hydrogen offer exceptional stability, for example, and those
based on microwave absorption in rubidium vapor are more compact, lower in cost,
and require less power.
Much of modern life has come to depend on precise time. The day is long past
when we could get by with a timepiece accurate to the nearest quarter-hour.
Transportation, communication, financial transactions, manufacturing, electric
power and many other technologies have become dependent on accurate clocks.
Scientific research and the demands of modern technology continue to drive our
search for ever more accurate clocks. The next generation of time standards
is presently under development at NIST, USNO, in France, in Germany, and other
laboratories around the world.
As we continue our "Walk Through Time," we will see how agencies such
as the National Institute of Standards and Technology, the U.S. Naval Observatory,
and the International Bureau of Weights and Measures in Paris assist the world
in maintaining a single, uniform time system.
In the 1840s a railway standard time for all of
England, Scotland, and Wales evolved, replacing several "local time"
systems. The Royal Observatory in Greenwich began transmitting time telegraphically
in 1852 and by 1855 most of Britain used Greenwich time. Greenwich Mean Time
(GMT) subsequently evolved as an important and well-recognized time reference
for the world.
1830, the U.S. Navy established a depot, later to become the U.S. Naval Observatory
(USNO), with the initial responsibility to serve as a storage site for marine
chronometers and other navigation instruments and to "rate" (calibrate)
the chronometers to assure accuracy for their use in celestial navigation. For
accurate "rating," the depot had to make regular astronomical observations.
It was not until December of 1854 that the Secretary of the Navy officially
designated this growing institution as the "United States Naval Observatory
and Hydrographic Office." Through all of the ensuing years, the USNO has
retained timekeeping as one of its key functions.
With the advent of highly accurate atomic clocks, scientists and technologists
recognized the inadequacy of timekeeping based on the motion of the Earth, which
fluctuates in rate by a few thousandths of a second a day. The redefinition
of the second in 1967 had provided an excellent reference for more accurate
measurement of time intervals, but attempts to couple GMT (based on the Earth's
motion) and this new definition proved to be highly unsatisfactory. A compromise
time scale was eventually devised, and on January 1, 1972, the new Coordinated
Universal Time (UTC) became effective internationally.
UTC runs at the rate of the atomic clocks, but when the difference between this
atomic time and one based on the Earth approaches one second, a one second adjustment
(a "leap second") is made in UTC. NIST's clock systems and other atomic
clocks located at the USNO and in more than 25 other countries now contribute
data to the international UTC scale coordinated in Paris by the International
Bureau of Weights and Measures (BIPM). As atomic timekeeping has grown in importance,
the world's standards laboratories have become more involved with the process,
and in the United States today, NIST and USNO cooperate to provide official
U.S. time for the nation. You can see a clock synchronized to the official U.S.
government time provided by NIST and USNO at http://www.time.gov.
In the latter part of the nineteenth century, a
variety of meridians were used for longitudinal reference by various countries.
For a number of reasons, the Greenwich meridian was the most popular of these.
At least one factor in this popularity was the reputation for reliability and
correctness of the Greenwich Observatory's publications of navigational data.
It became clear that shipping would benefit substantially from the establishment
of a single "prime" meridian, and the subject was finally resolved
in 1884 at a conference held in Washington, where the meridian passing through
Greenwich was adopted as the initial or prime meridian for longitude and timekeeping.
Given a 24 hour day and 360 degrees of longitude around the earth, it is obvious
that the world's 24 time zones have to be 15 degrees wide, on average. The individual
zone boundaries are not straight, however, because they have been adjusted for
the convenience and desires of local populations.
Interestingly, the standard timekeeping system related to this arrangement of
time zones was made official in the United States by an Act of Congress in March
1918, some 34 years following the agreement reached at the international conference.
In an earlier decision prompted by their own interests and by pressures for
a standard timekeeping system from the scientific community - meteorologists,
geophysicists and astronomers - the U.S. railroad industry anticipated the international
accord when they implemented a "Standard Railway Time System" on November
18, 1883. This Standard Railway Time, adopted by most cities, was the subject
of much local controversy for nearly a decade following its inception.
Time and Frequency Services
Since 1923, NIST radio station WWV has provided
round-the-clock shortwave broadcasts of time and frequency signals. WWV's audio
signal is also offered by telephone: dial (303) 499-7111 (not toll-free). A
sister station, WWVH, was established in 1948 in Hawaii, and its signal can
be heard by dialing (808) 335-4363 in Hawaii.
Broadcast frequencies are 2.5 MHz (megahertz), 5 MHz, 10 MHz, and 15 MHz for
both stations, plus 20 MHz on WWV. The signal includes UTC time in both voice
and coded form; standard carrier frequencies, time intervals and audio tones;
information about Atlantic or Pacific storms; geophysical alert data related
to radio propagation conditions; and other public service announcements. Accuracies
of one millisecond (one thousandth of a second) can be obtained from these broadcasts
if one corrects for the distance from the stations (near Ft. Collins, Colorado,
and Kauai, Hawaii) to the receiver. The telephone services provide time signals
accurate to 30 milliseconds or better, which is the maximum delay in cross-country
1956, low-frequency station WWVB, which offers greater accuracy than WWV or
WWVH, began broadcasting at 60 kilohertz. The broadcast power for WWVB was increased
in 1999 from about 10 kilowatts to 50 kilowatts, providing much improved signal
strength and coverage to most of the North American continent. This has stimulated
commercial development of a wide range of inexpensive radio-controlled clocks
and watches for general consumer use.
Time signals are an important byproduct of the Global Positioning System (GPS),
and indeed this has become the premier satellite source for time signals. The
time scale operated by the USNO serves as reference for GPS, but it is important
to note that the time scales of NI
ST and USNO are highly coordinated (that is, synchronized to well within 100
nanoseconds, or 100 billionths of a second). Thus, signals provided by either
NIST or USNO can be considered as traceable to both institutions. The agreements
and coordination of time between these two institutions are important to the
country, since they simplify the process of achieving legal traceability when
regulations require it.
Official U.S. Government time, as provided by NIST
and USNO, is available on the Internet at http://www.time.gov.
NIST also offers an Internet
Time Service (ITS) and an Automated
Computer Time Service (ACTS) that allow setting of computer and other clocks
through the Internet or over standard commercial telephone lines. Free software
for using these services on several types of popular computers can be downloaded
there. Information about these services can be found on the Time
and Frequency Division Web site.
More information about NIST time and frequency standards and research can be
obtained by contacting:
Time and Frequency Division
NIST - MC 847.00
Boulder CO 80305-3328
Copyright 2001 www.time.gov
Andrewes, William J.H., editor, The Quest for Longitude, Cambridge,
Massachusetts: Harvard University, 1996.
Audoin, Claude, and Bernard Guinot, The Measurement of Time: Time, Frequency,
and the Atomic Clock, Cambridge: Cambridge University Press, 2001.
Bartky, Ian R., "The Adoption of Standard Time," Technology and
Culture, Vol. 30 (Jan. 1989), pp. 25-56.
Breasted, James H., "The Beginnings of Time Measurement and the Origins
of Our Calendar," in Time and its Mysteries, a series of lectures
presented by the James Arthur Foundation, New York University, New York: New
York University Press, 1936, pp. 59-96.
Cowan, Harrison J., Time and Its Measurements, Cleveland: World Publishing
Dohrn-Van Rossum, Gerhard, History of the Hour: Clocks and Modern Temporal
Orders, Chicago: University of Chicago Press, 1998.
Garver, Thomas H., "Keeping Time," American Heritage of Invention
& Technology, Vol. 8, No. 2 (Fall 1992), pp. 8-17.
Goudsmit, Samuel A., Robert Claiborne, Robert A. Millikan, et al. Time,
New York: Time Inc., 1966.
Hawkins, Gerald S., Stonehenge Decoded, Garden City, N.Y.: Doubleday,
Hellwig, Helmut, Kenneth M. Evenson, and David J. Wineland, "Time, Frequency
and Physical Measurement," Physics Today, Vol. 23 (December 1978),
Hood, Peter, How Time Is Measured, London: Oxford University Press, 1955.
Howse, Derek, Greenwich Time and the Discovery of the Longitude, London:
Philip Wilson Publishers, Ltd, 1997.
Itano, Wayne M., and Norman F. Ramsey, "Accurate Measurement of Time,"
Scientific American, Vol. 269 (July 1993), pp. 56-65.
Jespersen, James and D. Wayne Hanson, eds., "Special Issue on Time and
Frequency," Proceedings of the IEEE, Vol. 74, No. 7 (July 1991).
Jespersen, James and Jane Fitz-Randolph, From Sundials to Atomic Clocks:
Understanding Time and Frequency, 2nd (revised) edition, Mineola, New York:
Dover Publications, 1999.
Jones, Tony, Splitting the Second, Bristol, UK: Institute of Physics
Landes, Davis S., A Revolution in Time: Clocks and the Making of the Modern
World, Cambridge, Massachusetts: Harvard University Press, 1985.
Lombardi, Michael A., NIST Time and Frequency Services, NIST Special
Publication 432*, revised 2002.
Mayr, Otto, "The Origins of Feedback Control," Scientific American,
Vol. 223, No. 10 (October 1970), pp. 110-118.
Merriam, John C., "Time and Change in History," Time and Its Mysteries,
(see Breasted above), pp. 23-38.
Millikan, Robert A., "Time," Time and Its Mysteries, (see Breasted
above) pp. 3-22.
Morris, Richard, Time's Arrows, New York: Simon and Schuster, 1985.
Needham, Joseph, Wang Ling, and Derek J. deSolla Price, Heavenly Clockwork:
The Great Astronomical Clocks of Medieval China, Cambridge: Cambridge University
Priestley, J. B., Man and Time, Garden City, New York: Doubleday, 1964.
Seidelmann, P. Kenneth, ed., Explanatory Supplement to the Astronomical Almanac,
Sausalito, Calif.: University Science Books, 1992.
Shallis, Michael, On Time, New York: Schocken Books, 1983.
Snyder, Wilbert F. and Charles A. Bragaw, "In the Domains of Time and Frequency"
(Chapter 8), Achievement in Radio, NIST Special Publication 555*, 1986.
Sobel, Dava, Longitude, New York: Penguin Books, 1995.
*Most NIST Special Publications are available from the Superintendent, U.S.
Government Printing Office, Washington, DC 20402, and in Federal Depository Libraries.
SP432 may also be downloaded from http://www.boulder.nist.gov/timefreq/general/pdf/1383.pdf