The Why Files The Why Files --

Second to none

White-faced clock with black numbers ticks time away.The Romans wrote the modern calendar, modestly naming July and August after a brace of glory-starved Caesars. But their calendar didn't quite line up with reality, and the months slipped against the natural seasons, so in 1582, 1752 and 1918, the European nations carved out some days so the dates would correspond to the seasons.

Could leap seconds wind up alongside wind-up clocks in the dustbin of history? Photo: Manchester City Council

Since then, we've added leap days every four years or so to keep time and reality in synch. But in 1972, after the invention of the hyper-accurate atomic clock, timekeepers began slipping in a leap second every few years, so atomic-clock time would jibe with time derived from Earth's position in space.

Now, the time gods (housed in a gaggle of uber-nerd international organizations) are thinking about deep-sixing the leap second. Throwing the leap second to the sharks, they say, will make it easier to develop and maintain global positioning satellites (GPS) and computer communications.

Man and machine teeter on edge of cliff above cloud line
Operators of Global Positioning System (GPS) systems say their life will be easier if we jettison the leap second. GPS was developed for the military, but it has plenty of civilian uses. U.S. Geological Survey scientist Michael Poland collects information from a GPS station on Mount St. Helens after the 1980 explosion. Another volcano, Mount Adams, appears in the distance. Photo: USGS

But astronomers, who also need accurate time, are "irate," says Michael Hapgood, secretary of Britain's Royal Astronomical Society, because they would need to rewrite bags of software just to make life easier for other fields -- which seem to be doing fine with the present jury-rigged system.

The discrepancy would start out small -- but eventually, daybreak could happen at midnight. That would be confusing, but then, time is confusing. So let's leap back a second for some background.

Whether you know it or even care, we now stagger under three systems of time:

International Atomic Time is based on atomic clocks.
Universal Time 1 (AKA solar, or astronomical time) is time that places the stars in the same relationship to Earth on any given day of the year.

Coordinated Universal Time (UTC) is a kind of compromise between atomic and solar time.

The roots of the time problem arise from our doddering planet, which is spinning ever slower (where are the "spin doctors" now that we need them?). Friction from the tides and the planet's molten core is devouring rotational energy, and every century, a day gets about 0.0014 seconds longer.

Trivial? Perhaps, but it adds up. Timekeepers now fix the growing discrepancy by slipping in an extry second (a "leap" second), to create UTC, which holds solar time within one second of atomic time. Since UTC was adopted in 1972, 22 leap seconds have been added.

Here's one way to think about leap seconds: Mechanics use shims to accurately adjust the position of a part. Timekeepers use leap seconds to "adjust" time. Leap seconds are shims for time.

Large satellite dish points towards endless blue sky.Alarming clock
The global timekeepers are thinking about jettisoning those shims to rely exclusively on atomic time. They say people who operate GPS and other time-critical gadgets have trouble accounting for leap seconds, which are somewhat unpredictable and thus tough to factor into software.

The Goldstone radio telescope near Barstow, Calif., was used to track a number of U.S. spaceships. If we ditch the leap second, would that kind of tracking be easier -- or harder? Photo: NASA

But astronomers worry that if leap seconds wind up alongside wind-up clocks in the trash, they will have trouble aiming telescopes. People who point radio receivers at satellites could find themselves listening to empty space.

Furthermore, time and astronomy have a long, worthy friendship, as we read in "The Leap Second..." (see below):

"From remote antiquity, the celestial bodies -- the Sun, Moon and stars -- have been the fundamental reckoners of time. The rising and setting of the Sun and the stars determine the day and night; the phases of the Moon determine the month; and the positions of the Sun and stars along the horizon determine the seasons."

And if astronomers helped make the rules, they also played by them, Hapgood says. "The objection from astronomers is that they have stuck to the international standard by building a lot of software that uses the time relationships specified in the standard. The people who want to change the standard are the people who haven't followed the standard, and they want to dump the extra work on the people who have followed the standard."

Time for compromise?
However, the "scrap the leap second" side stresses that shim seconds are getting more common as our elderly Earth slows. They worry that more time shims could bollix up computer-to-computer communications, which are at the heart of modern technology. Shims could also flummox transport systems that rely GPS. Airlines, for example, are considering a change to GPS-based navigation.

realistic illustration of silver and white panelled instrument surrounded by stars.GPS satellites contain atomic clocks. Accurate time is the key to this modern location-finding system. Artistic rendition from NASA

But Hapgood points out that changes in gravity and satellite motion both make clocks run differently on satellites compared to Earth. Without compensating for relativity, GPS would amass an error of 10 kilometers per day. But, Hapgood notes, the GPS industry is already smart enough to compensate for these time glitches, and it should be smart enough to continue coping with leap seconds.

With so many working brains in the discussion, he favors meeting in the middle. "We are not against change, but ... you need to look after both types of time. It should not be an either-or issue; we should come up with a compromise solution that fits everyone."

After all, he says, it's not only astronomers who have a stake in traditional time. "The link between time on the clock and the solar clock is deeply embedded in the way we understand time."

-- David Tenenbaum

The Leap Second: Its History and Possible Future, R. A. Nelson et al, Metrologia, 2001, 38, 509-529
The leap-second argument.

Related Why Files
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Radio astronomy

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