Why there will be a ‘leap second’ on Tuesday

Chuck Bednar for redOrbit.com – @BednarChuck

The typical day lasts 86,400 seconds, but Tuesday will be about one second longer as a way to account for the gradual slowing down of the Earth’s rotation, experts from the NASA Goddard Space Flight Center in Maryland explained in a statement late last week.

The duration of a second, they explained, is based on the extremely predictable electromagnetic transitions in cesium atoms, which are reliable enough that the cesium clock is accurate to one second in one second in 1,400,000 years. However, while the length of a day is 86,400 seconds, the actual mean solar day is actually about 0.0002 seconds longer than that.

As mentioned above, this phenomenon is the result of the gradual slowing of the Earth’s rotation due to what the US space agency calls “a kind of braking force” caused by a gravitational tug-of-war between the planet, sun, and moon. Scientists believe that the length of the mean solar day has actually been longer than 86,400 seconds since the year 1820.

Two milliseconds can add up over time

While the difference of two milliseconds seems completely insignificant, if repeated daily over the course of an entire year, it would add up to nearly one full second. This is not the case, as the length of an Earth day is influenced by many different factors (including seasonal and day-to-day weather variations) but over time the need to add a “leap second” does arise.

A technique known as Very Long Baseline Interferometry (VLBI) is used by scientists to track how long it takes the planet to complete one full rotation, and such measurements are conducted at a global network of monitoring stations. The VLBI measurements are used to create the time standard known as Universal Time 1 (UT1), which is not as uniform as the cesium clock. When the two drift too far apart, leap seconds are added to keep then within 0.9 seconds of one another.

A division of the International Earth Rotation and Reference Systems Service decides when to add leap seconds, and they are typically inserted on either June 30 or December 31, according to NASA. Usually, the clock moves from 23:59:59 to 00:00:00 the next day, but with the leap second on Tuesday, it will go from 23:59:59 to 23:59:60, then to 00:00:00 on Wednesday, July 1.

“In the short term, leap seconds are not as predictable as everyone would like,” said Chopo Ma, a geophysicist at Goddard and a member of the directing board of the International Earth Rotation and Reference Systems Service. “The modeling of the Earth predicts that more and more leap seconds will be called for in the long-term, but we can’t say that one will be needed every year.”


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