U.S. Naval Observatory Earth Orientation Department

What is a Leap Second?


Civil time is occasionally adjusted by one-second increments to ensure that the difference between a uniform time scale defined by atomic clocks (TAI) does not differ from the Earth's rotational time by more than 0.9 seconds. Coordinated Universal Time (UTC), an atomic time, is this adjusted time scale and it forms the basis for our civil time.

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 Rapid Service/Prediction Center 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 ocean tides. Through the use of ancient observations of eclipses, it is possible to determine the deceleration of the Earth to be roughly 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 about 1 century since the defining epoch (i.e., the duration since 1900), the difference has accumulated to roughly 2 milliseconds per day. Other factors also affect the Earth's dynamics, 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 inserted periodically in the atomic UTC time scale 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 (click here for a list of all announced leap seconds). This reflects the general slowing trend of the Earth due to tidal braking.

Confusion sometimes arises over the misconception that the occasional 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 time again.

This scenario is analogous to that encountered with the leap second. The difference is that instead of resetting the clock that is running slow, we choose to adjust the clock that is keeping a uniform, precise time. The reason for this is that we can change the time of 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 of when 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. GPS, in particular, is widely 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 3 meters (or 10 feet).

Precise time measurements are needed for the synchronization of clocks at two or more sites. 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 their transmissions. 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 day).

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 ensure 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 (.0000000001 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 (UT1) based on the rotation of the Earth about its axis. VLBI is an advanced astronomical technique of observing extra-galactic sources (typically quasars) with radio telescopes. The information gained using VLBI can be used to generate images of the distant radio sources, measure the rotation rate of the Earth, the motions of the Earth in space, or even measure how the tectonic plates where the telescopes are located are moving on the surface of the Earth. Measuring the Earth's rotational motion is critical for navigation. The most accurate navigation systems rely on measurements using satellite systems which are not tied to the Earth's surface. These systems can provide a position accurate to a about a meter (few feet), but the position of the Earth relative to the satellites must also be known to avoid potentially far larger errors.

The U.S. Naval Observatory has been in the forefront of timekeeping since the early 1800s. 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.

[IERS] [IERS Rapid Service/Prediction Center] [current leap second announcement] [past leap seconds] [USNO EO Dept.]

Last modified: 24 October 2001 Approved by EO Dept. Head, USNO