A
Walk Through Time
©
by NIST
,
National
Institute of Standards and Technology (for
pictures to the text click the heads of each
paragraph), the keepers of atomic
time.
Abstract
This article
describes the history of time-keeping from the
perspective of arriving at a reliable
clock.
Contents
Ancient
Calendars
Early
Clocks
Revolution
in
Timekeeping
The
"Atomic Age"
World
Time Scales
NIST
Time
Calibration
------------------------------------------------------------------------
Ancient
Calendars
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.
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 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
not only on 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, such as our own, have
adopted a 365-day solar calendar with a leap
year occurring every fourth year.
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:
* 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 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.
A
Revolution In
Timekeeping
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. We have 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 of
Nuremberg. Replacing the heavy drive weights
permitted smaller (and portable) clocks and
watches. 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. These advances
in design were precursors to truly accurate
timekeeping.
Accurate
Mechanical Clocks
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.
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
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.
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.
The
"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 exactly like one a million years ago or in
another galaxy. Here was a potential "pendulum"
with a reproducible rate that could 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 the 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 existing standards, and
attention shifted almost immediately to
more-promising, atomic-beam devices based on
cesium.
In 1957 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.
In 1967 the
cesium atom's natural frequency was formally
recognized as the new international unit of
time: 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. The best primary cesium standards
now keep time to about one-millionth of a second
per year.
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, manufacturing,
electric power and many other technologies have
become dependent on super-accurate clocks.
Scientific research and the demands of modern
technology continue to drive our search for ever
more accurate clocks. The next generation of
cesium time standards is presently under
development at NIST's Boulder laboratory 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.
World
Time Scales
In the 1840s a
Greenwich standard time for all of England,
Scotland, and Wales was established, replacing
several "local time" systems. The Royal
Greenwich Observatory was the focal point for
this development because it had played such a
key role in marine navigation based upon
accurate timekeeping. Greenwich Mean Time (GMT)
subsequently evolved as the official time
reference for the world and served that purpose
until 1972.
The United
States established the U.S. Naval Observatory
(USNO) in 1830 to cooperate with the Royal
Greenwich Observatory and other world
observatories in determining time based on
astronomical observations. The early timekeeping
of these observatories was still driven by
navigation. Timekeeping had to reflect changes
in the earth's rotation rate; otherwise
navigators would make errors. Thus, the USNO was
charged with providing time linked to "earth"
time, and other services, including almanacs,
necessary for sea and air navigation.
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 in more than 25 countries
now contribute data to the international UTC
scale coordinated in Paris by the International
Bureau of Weights and Measures (BIPM). An
evolution in timekeeping responsibility from the
observatories of the world to the measurement
standards laboratories has naturally accompanied
this change from "earth" time to "atomic" time.
But there is still a needed coupling, the leap
second, between the two.
The
World's Time Zones
Time zones did
not become necessary in the United States until
trains made it possible to travel hundreds of
miles in a day. Until the 1860s most cities
relied upon their own local "sun" time, but this
time changed by approximately one minute for
every 12 miles traveled east or west. The
problem of keeping track of over 300 local
times was overcome by establishing railroad time
zones. Until 1883 most railway companies relied
on some 100 different, but consistent, time
zones.
That year, the
United States was divided into four time zones
roughly centered on the 75th, 90th, 105th, and
120th meridians. At noon, on November 18,
1883, telegraph lines transmitted GMT time to
major cities where authorities adjusted their
clocks to their zone's proper time.
On
November 1, 1884, the International
Meridian Conference in Washington, D. C.,
applied the same procedure to zones all around
the world. The 24 standard meridians, every
15 east and west of 0 at Greenwich,
England, were designated the centers of the
zones. The international dateline was drawn to
generally follow the 180 meridian in the
Pacific Ocean. Because some countries, islands
and states do not want to be divided into
several zones, the zones' boundaries tend to
wander considerably from straight north-south
lines.
NIST
Time and Frequency Services
Since 1923
NIST radio station WWV has provided
round-the-clock shortwave broadcasts of time and
frequency signals. A sister station, WWVH, was
established in 1948 in Hawaii. WWV's audio
signal is also offered by telephone: dial
(303) 499-7111 (not toll-free). A similar
service from WWVH is available by dialing
(808) 335-4363 in Hawaii.
Broadcast
frequencies are 2.5, 5, 10, and
15 megahertz 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 telephone lines.
In 1956
low-frequency station WWVB began broadcasting at
60 kilohertz. WWVB offers a direct path
signal of greater accuracy than WWV or WWVH, but
a special low-frequency receiver is required to
decode the time signal.
Since 1975
NIST time and frequency signals have been
relayed to most of the Western Hemisphere by
satellites positioned high above the equator.
The two GOES weather satellites operated by the
National Oceanic and Atmospheric Administration
broadcast a time code near 468 MHz that can
set suitable clocks to within
100 microseconds (millionths of a second)
of UTC time.
Today's
personal computer users can obtain the current
time through NIST's Time
and Frequency
Division
time servers:
NIST
web clock
- display the current time in your (Java
enabled) browser
INTERNET
Time Service
- synchronize Windows computers via the Internet
Automated
Computer Time
Service
- synchronize DOS and Windows computers via
modem