Civil time is occasionally adjusted by one second increments to ensure
that the difference between a uniform time scale defined by atomic clocks
does not differ from the Earth's rotational time by more than 0.9 seconds.
Coordinated Universal Time (UTC), an atomic time, is the basis for our civil
In 1956, following several years of work, two astronomers at the U. S.
Naval Observatory (USNO) and two astronomers at the National Physical Laboratory
(Teddington, England) determined the relationship between the frequency
of the Cesium atom (the standard of time) and the rotation of the Earth
at a particular epoch. As a result, they defined the second of atomic time
as the length of time required for 9 192 631 770 cycles of the Cesium atom
at zero magnetic field. The second thus defined was equivalent to the second
defined by the fraction 1 / 31 556 925.9747 of the year 1900. The atomic
second was set equal, then, to an average second of Earth rotation time
near the end of the 19th century.
The National Earth Orientation Service (NEOS) as the Sub-bureau for Rapid
Service and Predictions of the International Earth Rotation Service (IERS),
located at the U.S. Naval Observatory, monitors the Earth's rotation. Part
of its mission involves the determination of a time scale based on the current
rate of the rotation of the Earth. UT1 is the non-uniform time based on
the Earth's rotation.
The Earth is constantly undergoing a deceleration caused by the braking
action of the tides. Through the use of ancient observations of eclipses,
it is possible to determine the deceleration of the Earth to be roughly
1.5-2 milliseconds per day per century. This is an effect which causes the
Earth's rotational time to slow with respect to the atomic clock time. Since
it has been nearly 1 century since the defining epoch (i.e. the ninety year
difference between 1990 and 1900), the difference is roughly 2 milliseconds
per day. Other factors also affect the Earth, some in unpredictable ways,
so that it is necessary to monitor the Earth's rotation continuously.
In order to keep the cumulative difference in UT1-UTC less than 0.9 seconds),
a leap second is added to the atomic time to decrease the difference between
the two. This leap second can be either positive or negative depending on
the Earth's rotation. Since the first leap second in 1972, all leap seconds
have been positive. This reflects the general slowing trend of the Earth due to tidal braking.
Confusion sometimes arises over the misconception that the regular insertion
of leap seconds every few years indicates that the Earth should stop rotating
within a few millennia. The confusion arises because some mistake leap seconds
as a measure of the rate at which the Earth is slowing. The one-second increments
are, however, indications of the accumulated difference in time between
the two systems. As an example, the situation is similar to what would happen
if a person owned a watch that lost two seconds per day. If it were set
to a perfect clock today, the watch would be found to be slow by two seconds
tomorrow. At the end of a month, the watch will be roughly a minute in error
(thirty days of the two second error accumulated each day). The person would
then find it convenient to reset the watch by one minute to have the correct
This scenario is analogous to that encountered with the leap second.
The difference is that instead of setting the clock that is running slow,
we choose to set the clock that is keeping a uniform, precise time. The
reason for this is that we can change the time on an atomic clock while
it is not possible to alter the Earth's rotational speed to match the atomic
clocks. Currently the Earth runs slow at roughly 2 milliseconds per day.
After 500 days, the difference between the Earth rotation time and the atomic
time would be one second. Instead of allowing this to happen a leap second
is inserted to bring the two times closer together.
The decision to introduce a leap second in UTC is the responsibility
of the International Earth Rotation Service (IERS). According to international
agreements, first preference is given to the opportunities at the end of
December and June, and second preference to those at the end of March and
September. Since the system was introduced in 1972, only dates in June and
December have been used.
The official United States time is determined by the Master Clock at
the U. S. Naval Observatory (USNO). The Observatory is charged with the
responsibility for precise time determination and management of time dissemination.
Modern electronic systems, such as electronic navigation or communication
systems, depend increasingly on precise time and time interval (PTTI). Examples
are the ground-based LORAN-C navigation system and the satellite-based Global
Positioning System (GPS). Navigation systems are the most critical application
for precise time. The newest satellite navigation system, the Global Positioning
System (GPS), is used for navigating ships, planes, missiles, trucks and
cars anywhere on Earth. These systems are all based on the travel time of
electromagnetic signals: an accuracy of 10 nanoseconds (10 one-billionths
of a second) corresponds to a position accuracy of about 10 feet.
Precise time measurements are needed for the synchronization of clocks
at two or more stations. Such synchronization is necessary, for example,
for high speed communications systems. Power companies use precise time
to control power distribution grids and reduce power loss. Radio and television
stations require precise time (the time of day) and precise frequencies
in order to broadcast programs. Many programs are transmitted from coast
to coast to affiliate stations around the country. Without precise timing
the stations would not be able to synchronize the transmission of these
programs to local audiences. All of these systems are referenced to the
USNO Master Clock.
Very precise time is kept by using atomic clocks. The principle of operation
of the atomic clock is based on measuring the microwave resonance frequency
(9,192,631,770 cycles per seconds of the cesium atom. At the Observatory,
the atomic time scale (AT) is determined by averaging 60 to 70 atomic clocks
placed in separate, environmentally controlled vaults. Atomic Time is a
very uniform measure of time (one tenth of one billionth of a second per
The USNO must maintain and continually improve its clock system so that
it can stay one step ahead of the demands made on its accuracy, stability
and reliability. The present Master Clock of the USNO is based on a system
of some 60 independently operating cesium atomic clocks and 7 to 10 hydrogen
maser atomic clocks. These clocks are distributed over 20 environmentally
controlled clock vaults, to insure their stability. By automatic inter-comparison
of all clocks every 100 seconds a time scale is computed which is not only
reliable but also extremely stable. Its rate does not change by more than
about 100 picoseconds (.000000001 seconds) per day from day to day.
On the basis of this computed time scale, a clock reference system is
steered to produce clock signals which serve as the USNO Master Clock. The
clock reference system is driven by a hydrogen maser atomic clock. Hydrogen
masers are extremely stable clocks over short time periods (less than one
week). They provide the stability and reliability needed to maintain the
accuracy of the Master Clock System.
Very Long Baseline Interferometry (VLBI) is used to determine Universal
Time (UT) based on the rotation of the Earth about its axis. VLBI is an
advanced technique used for observing with radio telescopes. The information
gained using VLBI can be used to generate images of distant radio sources,
measure the rotation rate of the Earth, the motions of the Earth in space,
or even measure how the plates are moving on the surface of the Earth. Measuring
the Earth's motion is critical for navigation. The most accurate navigation
systems rely on measurements of satellites which are not tied to the Earth.
These systems can provide a position accurate to a few feet, but the position
of the Earth relative to the satellite must also be known to avoid errors
of hundreds of feet.
The U.S. Naval Observatory has been in the forefront of timekeeping since
the early 1800's. In 1845, the Observatory offered its first time service
to the public: a time ball was dropped at noon. Beginning in 1865 time signals
were sent daily by telegraph to Western Union and others. In 1904, a U.S.
Navy station broadcast the first worldwide radio time signals based on a
clock provided and controlled by the Observatory.
A time of day announcement can be obtained by calling 202-762-1401 locally
in the Washington area. For long distance callers the number is 900-410-TIME.
The latter number is a commercial service for which the telephone company
charges 50 cents for the first minute and 45 cents for each additional minute.
Australia, Hong Kong, and Bermuda can also access this service at international
direct dialing rates. You can also get time for your computer by calling
202-762-1594. Use 1200 baud, no parity, 8 bit ASCII.
..from the U.S. Naval Observatory FAQ page [http://www.usno.navy.mil/faq.html],
specifically the page [http://maia.usno.navy.mil/leapsec.html].
For a list of all announced leap seconds since 1972 (when they started)
see the web page [ftp://maia.usno.navy.mil/ser7/leapsec.dat].
The Time Service Department [http://tycho.usno.navy.mil/]
sets the official time standards for the U.S. There is a leap second scheduled
for the end of June 1997 [http://tycho.usno.navy.mil/leap.html]