Another Second to Celebrate on New Years Eve!!
The keepers of the nation's primary standard of time and frequency, the NIST-7 atomic clock at NIST's Boulder (Colo.) laboratories, will be adding a leap second (as will the operators of all other standard clocks around the world) on Dec. 31. This is the 22nd such adjustment to the world's time scale, as decreed by the International Bureau of Weights and Measures in Paris. Leap seconds are needed to keep our clocks (the best of which are now so accurate that they lose or gain less than a millionth of a second per year) in step with the spinning Earth, which varies several thousandths of a second per day. Since we can't speed up the Earth, we have to slow down the clocks every one to two years to keep them "in synch." This is done by stopping them for exactly one second so that the Earth can catch up. This year's leap second will be inserted at 23:59:60 Coordinated Universal Time (7 p.m. EST) on Dec. 31, 1998. Therefore, the official last minute of 1998 will actually be 61 seconds long.
The Naturalist Net has a direct link to the MASTER CLOCK (there will be a network delay but it is really connected to the Master Clock)
U.S. NAVAL OBSERVATORY WASHINGTON, D.C. 20392-5420
July 23, 1998
No. 64 TIME SERVICE ANNOUNCEMENT
SERIES 14
UTC TIME STEP
1. The International Earth Rotation Service (IERS) has announced the introduction of a time step to occur at the end of December, 1998.2. Coordinated Universal Time (UTC) will be retarded by 1.0s so that the sequence of dates of the UTC markers will be: 1998 December 31 23h 59m 59s 1998 December 31 23h 59m 60s 1999 January 01 0h 0m 0s
3. The difference between UTC and International Atomic Time (TAI) is: from 1997 01 Jul, UTC to 1999 01 January, UTC: TAI-UTC= +31s from 1999 01 Jan, UTC until further notice: TAI-UTC= +32s
4. The insertion of one leap second will be evident by the change of sign of the DUT1 correction which will become positive. Extrapolated values of DUT1 are distributed weekly in the IERS Bulletin A.
5. All coordinated time scales will be affected by this adjustment. However, Loran-C and GPS will not be adjusted physically. Times of Coincidence for LORAN-C are available on the Time Service Web Page (http://tycho.usno.navy.mil/loran.html). For GPS, the leap second correction contained within the UTC data of subframe 4, page 18 of the navigation message transmitted by satellites will change.
Before the leap second GPS-UTC = +12 (i.e., GPS is ahead of UTC by twelve seconds)
After the leap second GPS-UTC = +13s (i.e., GPS will be ahead by thirteen seconds)
DENNIS D. McCARTHY
Director
Directorate of Time
Time measured by the rotation of the Earth is not uniform when compared to the time kept by atomic clocks. In fact, radio telescopes now observe the most distant objects in the universe, known as quasars, to determine the irregularities in the Earth's rotation. As a result of such irregularities, the atomic clocks get out of sync with the Earth.
In 1972, by international agreement, it was decided to let atomic clocks run independently of the Earth, keep two separate times, and coordinate the two. In order to keep the difference between Earth time and atomic time within nine tenths of a second as the two times get out of sync, leap seconds are added to the atomic time scale. The International Earth Rotation Service (for which the U. S. Naval Observatory provides the rapid service) is the organization which monitors the difference between the two time scales and calls for leap seconds to be inserted when necessary. Since 1972, leap seconds have been added at intervals varying from six months to two-and-one-half years -- this leap second is eighteen months since the last one. Leap seconds are added because the Earth's rotation tends to slow down. If the Earth were to speed up, a leap second would be removed.
The U. S. Naval Observatory is charged with the responsibility for precise determination and management of time dissemination, and as such provides the Master Clock for the Department of Defense and the entire nation. 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).
These systems are all based on the travel time of the electromagnetic signals: an accuracy of 10 nanoseconds (ten one-billionths of a second) corresponds to a positional accuracy of about 10 feet. In fast communications, time synchronization is equally important. All of these systems are referenced to the U. S. Naval Observatory Master Clock.
The present Master Clock, accurate to better than a billionth of a second per day, is based on a system of over 50 independently operating cesium atomic clocks and 10 hydrogen maser atomic clocks. These clocks are distributed over 12 environmentally controlled clock vaults, to insure their stability. By automatic hourly intercomparison of all clocks, a time scale can be computed which is not only reliable but also extremely stable. On the basis of this computed time scale, a clock reference system is steered to produce clock signals which serve as the U. S. Naval Observatory Master Clock.
The U. S. Naval Observatory's success in its time standard function is evident in the fact that it is the largest single contributor to the international time scale (UTC), which is computed in Paris, France, at the International Bureau of Weights and Measures. Moreover, its principal role in keeping track of the change in the "Earth clock" (i.e., Earth rotation) and its dissemination of this information as the Rapid Service Sub-bureau for the International Earth Rotation Service attests to the fact that globally, as well as nationally, the U. S.Naval Observatory remains the leader in precise time.
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