By Patricia Daukantas
2008 is shaping up to be a really long year – especially with the addition of a “leap second” to the last day of the year, as announced by the U.S. Naval Observatory (USNO) in Washington, D.C. But how do the world’s timekeepers know that our calendars, already tweaked by a leap day every four years, need such a small adjustment?
As it turns out, the Naval Observatory and its colleagues in the international scientific community increasingly rely on optical technology to distinguish between the time based on the rotation of the Earth and the time based on the atomic standard. (After all, the second is defined as exactly 9,192,631,770 times the period of a certain hyperfine transition of the cesium-133 atom.) The leap second will keep the two kinds of time synchronized.
USNO is responsible for monitoring the variations in Earth’s rotation. Its time scale is governed by a combination of 52 atomic clocks – 16 hydrogen masers and 36 cesium clocks. The observatory’s Time Service, a member of the International Earth Rotation and Reference Systems Service, has provided data on past leap seconds, ongoing variability of the Earth's rotation and other cool facts about time measurement.
The observatory has been working on cesium fountain clocks and rubidium fountain clocks and recently dedicated a new Master Clock Facility building for its present and future time technology.