Leap Seconds: The Why and the When-For

Readers may have noted last week that the International Telecommunication Union (ITU) nearly decided earlier this month whether to continue or discontinue use of the leap second, but in the end, agreed not to agree until 2015 (or later) given the deadlock between anti-leap forces and pro-leap forces.

In general, pro-leapers are led by astronomers and England, who point to the odd consequence that if official clocks are freed from Earth's rotational speed, the clocks of the distant future will eventually show twelve noon when common sense says it's time for dinner from the replicator or whatever. Why? Not because atomic clocks are inaccurate, but because Earth is. Here's one of the atomic troublemakers in Switzerland:

Anti-leapers fret over the risk that intermittent leap-second-insertion might pose to the stability of complex, safety-critical networks that depend on exact synchronization. Anti-leapers feel that atomic-based time needs to be freed from the cruel rule of the Earth's rotation. The debate has gone on for almost ten years, and no resolution is in sight. 

Tacking on leap seconds is one of the very few human events that happens simultaneously, worldwide. The first leap for mankind came in 1972.

The leap second is not connected to the leap year, which will add one day to February in 2012 and again at four year intervals (same as the US presidential election cycle.)

The following explanation is condensed from an article I wrote for Air&Space. The news hook was a leap second on New Year's Day, 2009.

At the direction of the International Earth Rotation Service (I love that name -- as grand as the Planetary Protection Officer at NASA) the world's timekeepers add leap seconds to line up the 86,400-second day as clocked by Coordinated Universal Time (abbreviated UTC) with how humans measure the length of a day: from the Sun's position.

UTC is tied to microwaves emitted by an isotope of cesium. Its basic unit, the second — defined in 1967 and one of seven standard units presided over by the International Bureau of Weights and Measures in Sevres, France — is how long it takes for 9,192,631,770 cycles of that specific energy to tick by. The atomic clocks that measure the frequency are the most accurate scientific instruments in existence, neither losing nor gaining a second over hundreds of millions of years.

Our home planet has been slowing down for eons. Earth went around so much faster in the Paleozoic era that a day back then was two hours shorter than today's day. The forces that affect Earth's rotational speed include seasonal effects on oceans and winds, the swirlings of molten metal deep in the core, a tightening of the middle latitudes that is making the planet slightly rounder than before, and thinning of glaciers caused by global warming. 

Occasionally, the Earth's RPM speeds up over short periods. If Earth’s densest molten rock settles closer to the core, all of us Earth-riders speed up—a little. This may sometimes counter the tidal action that slows us down. Here's a chart from Wiki's earth rotation page showing how the amount of slowing varies a good deal from year to year:

For reasons not yet clear, says Richard Gross, a geophysicist at NASA’s Jet Propulsion Laboratory in California, Earth ran unusually slow for a few months in 1912, making for the longest days in the 20th century. By contrast, on July 13, 2003, the Earth was speedy enough that it beat the clock by one millisecond, going around in 86,399.999 seconds flat. Still, on average, the days of our lives must get longer.

But no expert or computer can predict when forces will combine to require another second to straighten out the clocks. Although leap seconds usually come every year or two, Earth had something unusual going on in its core during a six-year stretch after the New Year’s of 1999. During that period, timekeepers tolled only one extra leap second.

How does anybody know that a given day in 2003 took a millisecond less than the standard day? The answer comes from radio antennas spaced on continents around the world. Together they make up the Very Large Baseline Interferometry, or VLBI, network. Signal processing and precision timing turn the global network into one giant antenna, thousands of miles in diameter. That size gives it very sharp vision in the radio spectrum.

The VLBI network was set up to plumb the depths of the distant universe, the farthest objects of which are quasars, giant galactic cores that blast radio waves and X-rays across billions of light years. Because they are so far away, quasars appear to receivers on Earth almost stationary, so astronomers use them as a fixed frame of reference. Using radio antennas to pick up signals from quasars, scientists can monitor the rotation of Earth with great precision. Here's a diagram:

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.

Jet Propulsion Laboratory needs to track 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.

The growing complexity of electrical transmission, broadcast, Internet, and telephone systems, all of which rely on precise synchronization, could make 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 in 2008 that leap seconds be discontinued.

Observatories, which rely on UTC when steering automated telescopes, have joined to fight off a proposal from anti-leap-seconders to drop the little leap second and make only big changes, perhaps once 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 the meantime, given the deep divisions, leap we must: the next leap second comes on June 30.