A one-second argument

Here it comes again, one of those oddly named "leap seconds." It will be added to 2005 at the end of the year. This will keep the length of the day, as measured by civil atomic time, in sync with Earth's rotation.

It also may foreshadow the outbreak of a major debate over whether or not we should monkey with time this way.

Astronomers and others who want their clocks to match the natural day-night rhythm of Earth's rotation love the leap second. It helps them keep track of where the Sun and other astronomical objects are in the sky. Electronic engineers who need the unvarying rhythm of atomic time hate it. Introduction of a rogue second can mess up modern communications and other tightly synchronized electronic operations.

Some of these systems act as though they had choked trying to swallow a 61-second minute. A review of the issue in the journal Metrologia explained that leap-second haters wonder if "it would be better simply to let atomic time run freely and accept that the world's civil time scale will slowly diverge from the rotation of the Earth."

This long-simmering clash of priorities may come to a head in Geneva next month when the International Telecommunications Union (ITU) is to take up a formal proposal to abolish the leap second.

Already, Britain's Royal Astronomical Society has asked the ITU to drop the proposal and seek a wide discussion among all users of civil time as to how to deal equitably with the competing needs. The society notes: "The idea that clock time follows solar time is deeply embedded in contemporary technical culture through a wealth of literature, ... and in the skills of working scientists and engineers around the world."

At one time we took for granted that there were 86,400 seconds in a solar day. That's how we divided up the day. Now we define and measure seconds with increased precision. One second is the duration of 9,192,631,770 cycles of the frequency of radiation from cesium atoms. We no longer divide the solar day into 86,400 parts to define and measure a second. We simply say there are 86,400 of the new seconds in a day. Yet Earth's rotation is gradually slowing down. The natural day is getting longer by about one second per year on average. It takes more of those precisely defined seconds to fill it out. That's why International Atomic Time, in which day length never changes, is out of sync with Sun time.

Civil time is also based on atomic clock time. However, leap seconds are added whenever necessary to keep it in step with solar time to within 0.9 seconds. Civil time has gained 21 leap seconds since the scheme started in 1972.

The Royal Astronomical Society accuses those eager to drop leap seconds of trying to solve the problem of precision timing by "exporting problems" to those who use clock time as a measure for solar time. "These include astronomers, satellite operators, and potentially all who study environmental phenomena related to the rising and setting of [the] Sun," the group adds.

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