WASHINGTON,DC — On June 30, 2015 a “leap second” will be added to the world’s clocks at
23 hours, 59 minutes and 59 seconds Coordinated Universal Time (UTC). This corresponds to
7:59:59 pm Eastern Daylight Time, when the extra second will be inserted at the U.S. Naval
Observatory’s Master Clock Facility in Washington, DC.
Historically, time was based on the mean rotation of the Earth relative to celestial bodies and the
second was defined in this reference frame. However, the invention of atomic clocks defined a
much more precise “atomic” timescale and a second that is independent of Earth’s rotation. In
1970, international agreements established a procedure to maintain a relationship between
Coordinated Universal Time (UTC) and UT1, a measure of the Earth's rotation angle in space.
The International Earth Rotation and Reference Systems Service (IERS) is the organization
which monitors the difference in the two time scales and calls for leap seconds to be inserted in
or removed from UTC when necessary to keep them within 0.9 seconds of each other. In order
to create UTC, a secondary timescale, TAI, is first generated; it consists of UTC without leap
seconds. When the system was instituted in 1972, the difference between TAI and UTC was
determined to be 10 seconds. Since 1972, 25 additional leap seconds have been added at
intervals varying from six months to seven years, with the most recent being inserted on June 30,
2012. After the insertion of the leap second in June, the cumulative difference between UTC and
TAI will be 36 seconds.
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. This is
because some mistake leap seconds to be 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.
The decision as to when to make a leap second is determined by the IERS, for which the USNO
serves as the Rapid Service/Prediction Center. Measurements show that the Earth currently runs
slow, compared to atomic time, at about one millisecond per day. These data are generated by
the USNO using the technique of Very Long Baseline Interferometry (VLBI). VLBI measures
the rotation of the Earth by observing the apparent positions of distant objects near the edge of
the observable universe. These observations show that after roughly 1000 days, the difference
between Earth rotation time and atomic time would be about one second. Instead of allowing
this to happen a leap second is inserted to bring the two time-scales closer together. We can
easily change the time of an atomic clock, but it is not possible to alter the Earth's rotational
speed to match the atomic clocks.
The U.S. Naval Observatory is charged with the responsibility for the precise determination and
dissemination of time for the Department of Defense and maintains DoD’s Master Clock. The
U.S. Naval Observatory, together with the National Institute of Standards and Technology
(NIST), determines time for the United States. Modern electronic navigation and
communications systems depend increasingly on the dissemination of precise time through such
mechanisms as the Internet-based Network Time Protocol (NTP) and the satellite-based Global
Positioning System (GPS).
The U.S. Naval Observatory is the largest single contributor (at approximately 30 percent of the
weighted average) to the international timescale UTC, which is computed in Paris, France, at the
International Bureau of Weights and Measures. This international prominence in atomic
timekeeping is due to the large number of atomic clocks operated by the Observatory and the
fidelity to which they are measured and maintained. Moreover, the U.S. Naval Observatory’s
principal role in keeping track of changes in the “Earth clock” (i.e., Earth rotation) and its
dissemination of this information as the Rapid Service/Prediction Center for the IERS attests to
the fact that globally, as well as nationally, the U.S. Naval Observatory remains the leader in
precise time.
Information concerning the U.S. Naval Observatory, its mission, history, and programs is
available from our World Wide Web site at www.usno.navy.mil/USNO.