Here’s how they do it: Using atomic clocks, geodetic researchers measure the slight time differences between the arrival of a quasar’s signals at several widely separated radio telescopes. The delays in arrival times change as the Earth rotates. Knowing the fixed positions of the telescopes and the changes in the time differences makes it possible to calculate the rate of the Earth’s rotation.
The Jet Propulsion Laboratory needs to keep a close eye on Earth’s rotation because it uses tracking measurements taken by telescopes located on the rotating Earth to help spacecraft navigate around the solar system. That’s why the lab has a geophysicist—Richard Gross—among its astrophysicists.
Because Earth’s movements are so unpredictable, atomic clockmakers can’t helpfully pre-program leap seconds into new clocks. Nobody knows about the next one more than six months in advance.
What timekeepers do know is that the number and frequency of leap seconds required can only increase. “My guess is that we may need more than one in five or 10 years,” says Bill Klepczynski, the former director of time at the U.S. Naval Observatory, “but nobody knows for sure.” The growing complexity of electrical transmission, broadcast, Internet, and telephone systems, all of which rely on precise synchronization, makes frequent insertions risky. The 2005 leap second revealed a programming problem at the Swiss time-broadcasting station HBG, and some “network time protocol” servers on the Internet suffered computer hiccups. Such dangers have prompted several scientific organizations, including the U.S. Naval Observatory, to recommend that leap seconds be discontinued.
Others believe that leap seconds should no longer be inserted in the broadcast time scale, to which society’s sensitive machines are tuned, but should continue to be inserted in global civil time. Observatories, which rely on UTC when steering automated telescopes, have joined to fight off a proposal from anti-leap-seconders to drop the leap second and make a change only every 600 to 900 years, by inserting a full hour instead. “Civil time that tracks the sun means that we keep a conventional meaning of time that is consistent with all of human history,” argues researcher Steve Allen of the University of California’s Lick Observatory. In any case international discussions about changing time take lots of time, so leap seconds are in our future through 2019 and probably longer.