LEAP SECOND

ATOMIC CLOCK TAKES QUANTUM LEAP
By Staff Sgt. Don Branum, 50th Space Wing Public Affairs

SCHRIEVER AFB CO (AFPN) 1/6/2006 - A leap second on the clock that doesn’t come along very often, and it’s a subject of debate between astronomers and clock watchers. Outside the debate is the job of making sure everyone who relies on the leap second receives it.

That job, in the hands of the 2nd Space Operations Squadron here and the U.S. Naval Observatory, went without a hitch Dec. 31 as the USNO clock ticked to 23:59:60 Universal Time Coordinate.

Leap seconds came about as the result of the United States adopting atomic clocks as time standards, said Bill Bollwerk, head of the USNO Detachment Colorado here. “When the U.S. went to atomic clocks, we found they were more stable than the earth’s rotation.”

Atomic clocks, which use the “ticking” of cesium-133 atoms, are about 4,000 times more accurate than the Earth’s rotation. Because of this, the United States defined a second in 1958 as the time it takes for an atom of cesium-133 to tick through 9,192,631,770 cycles.

The difference between an atomic second and an astronomical second seems small — usually a second or less during a year, which is 31.5 million seconds long. Over several years, however, the fractions of a second add up. When they add up to 0.9 second, the International Earth Rotation Service, located at the USNO in Washington, D.C., adds a leap second to the calendar.

The Naval Observatory ole of squadron is to ensure everyone in the Department of Defense receives the leap second correctly.

“(The USNO’s) role is the time standard for DOD,” Mr. Bollwerk said. “We maintain correct time. Our responsibility is to put DOD time in conformance with the UTC standard.”

Proper handling of leap seconds is crucial for navigation and satellite communications, Mr. Bollwerk said. Global positioning system timing signals help synchronize the position of the earth relative to the satellites, ensuring the best possible accuracy for military and civil users.

Power companies use the USNO master clock’s precise timing to control power distribution and reduce power loss, according to the USNO’s Leap Second fact sheet. In addition, radio and television stations require precise timing in order to broadcast and synchronize nationwide transmissions to local audiences.

The Dec. 31 leap second was the first since Dec. 31, 1998 — the longest span of time without a leap second since they were implemented in 1972.

Leap seconds have sparked controversy between astronomers and those who rely on the stability of the atomic clock. Some researchers believe leap seconds will become more frequent, occurring twice per year, as the earth’s rotation slows over the next century.

Astronomer Dennis McCarthy, a retired director of the USNO Directorate on Time, advocates abolishing the leap second. However, both astronomers and time experts agree that little data supports the assertion that leap seconds have been problematic.

Until or unless scientists can agree on a different solution for the difference between atomic and astronomical time, 23:59:60 may become ever more familiar to clock watchers and Schriever’s space professionals.