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Every second counts
Lifestyle

Every second counts

Sunday, 05 July 2015
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Christophe Roulet
Editor-in-chief, HH Journal

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3 min read

During the night of June 30th, a “leap second” was added to time. The twenty-sixth since 1972, its purpose is to make up for the slowing of Earth’s rotation, which is partly due to warming oceans.

While this “jump” in time may be small compared with Marty McFly’s fast forward in Back to the Future, it shouldn’t be underestimated. During the night of June 30th to July 1st, a “leap second” was added to time, the twenty-sixth since 1972. While one second’s extra sleep is hard to appreciate, scientists at the International Earth Rotation Service (IERS) in France wouldn’t have had it any other way. Indeed, the purpose of this extra second is to recalibrate two time scales: atomic time which continues unperturbed and time based on Earth’s fluctuating rotation.

A little history: metrologists long measured time based on astronomical data. One day corresponded to the interval between two transits of the Sun above the Greenwich meridian at noon. Defined as Greenwich Mean Time (GMT), this duration could be subdivided, using the sexagesimal system bequeathed to us by ancient civilisations, into 24 equal hours, then into 1,440 minutes and 86,400 seconds. All that changed, however, in 1955 and the advent of caesium atomic clocks. They measure time with unprecedented accuracy, varying by a single second every three million years. And so international atomic time (TAI, from the French temps atomique international) supplanted astronomical time. In 1967, the International Time Bureau at the Paris Observatory introduced the atomic hour based on the atomic second. Since then, some five hundred atomic clocks spread between different laboratories have helped define it.

Ocean warming causes water to move from the poles to the equator, which modifies Earth's kinetic energy and contributes to its slowing.
Michel Grenon
A consequence of global warming

These “time lords” were soon confronted with a problem: Earth’s rotation does not have the same diabolical precision as an atomic clock. Instead it tends to slow down due to various phenomena such as the tides, earthquakes and the concentration of ocean water. “Ocean warming causes water to move from the poles to the equator, which modifies Earth’s kinetic energy and contributes to its slowing,” explains Michel Grenon of the Geneva Observatory, quoted in Le Monde. At a rate of two milliseconds a century, Earth is hardly slamming on the brakes. However, this desynchronisation with atomic clocks was deemed sufficient to introduce the leap second in 1972. Suspending atomic time compensates Earth’s drift to give Coordinated Universal Time (UTC) or international civil time, which is atomic time adjusted for the Earth’s rotation.

Exactly when these leap seconds make their appearance is up to IERS, which studies multiple geophysical parameters. Indeed, unlike a leap year, a leap second cannot be planned in advance as the slowing of the Earth’s rotation is an irregular and unpredictable occurrence. As soon as the assumed gap exceeds 0.9 second, IERS announces that a leap second will be added within six months, always on either June 30th or December 31st.

Not everyone is a fan of the leap second. Opponents include the business and finance sectors. They point to the disruptive effect a leap second has on computing systems, a problem made worse by the irregular nature of these manipulations. Scientists, and astronomers in particular, take a very different view. “[Doing away with the leap second] would be to ignore our link with the natural environment,” rails Michel Grenon. “Our lives run to solar time. There is no reason to abandon the leap second. Technology must serve people, not the other way round.” Verdict in November, at the next assembly of the International Telecommunication Union where the matter is to be decided.

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