NASA's leap second image (Credit: NASA)

NASA Explains Why June 30 Will Get Extra Second


NASA’s leap second image (Credit: NASA)

It hardly matter if June 30 is longer by one “second” but NASA explains that the day will be officially longer as a “leap” second will be added to it.

Daniel MacMillan of NASA’s Goddard Space Flight Center in Greenbelt, Maryland attributes it to the eart’s rotation whch is gradually slowing down a bit thus adding leap seconds. “Earth’s rotation is gradually slowing down a bit, so leap seconds are a way to account for that,” he said.

Otherwise, usually a day has 86,400 seconds and it takes eons to make a significant change in the day and night times on Earth. The Coordinated Universal Time, or UTC or “atomic time” will add one second due to extremely predictable electromagnetic transitions in atoms of cesium. These transitions are based on the cesium clock that is accurate to even one second in 1,400,000 years, said NASA scientist McMillan.

The mean solar day or the average length of a day, based on how long it takes Earth to rotate is about 86,400.002 seconds long, slowing down Earth’s rotation, due to braking force caused by the gravitational tug of war between Earth, the moon and the sun. NASA scientists estimate that the mean solar day hasn’t been 86,400 seconds long since the year 1820 or so, thus pointing out clearly a slowing down of Earth’s rotation.

Though the difference at 2 milliseconds is hardly noticeable, it is repeated daily throughout the year, then it adds up to a second. However, this is due to mean average and not impacting the the length of each day’s variation.

The variation in a day time varies due to several factors including seasonal and daily weather variations, dynamics of the Earth’s inner core, variations in the atmosphere and oceans, groundwater, and ice storage, and oceanic and atmospheric tides, explains McMillan.

Even atmospheric variations due to El Niño can slow down Earth’s rotation, increasing the length of day by 1 millisecond, or a thousandth of a second.

Scientists monitor how long it takes Earth to complete a full rotation using an extremely precise technique called Very Long Baseline Interferometry (VLBI), with a worldwide network of stations, with Goddard providing essential coordination the data collected.

Usually, a leap second is added either on June 30 or December 31. Normally, the clock would move from 23:59:59 to 00:00:00 the next day. But with the leap second on June 30, UTC will move from 23:59:59 to 23:59:60, and then to 00:00:00 on July 1. The systems are turned off for one second to make the switch.

In modern computers, previous leap seconds have generated some calls to abandon them altogether as the the need to add a leap second cannot be anticipated in advance.

“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.”

From 1972, when leap seconds were first implemented, through 1999, leap seconds were added at a rate averaging close to one per year. Since then, leap seconds have become less frequent. This June’s leap second will be only the fourth to be added since 2000.

“The next-generation system is designed to meet the needs of the most demanding scientific applications now and in the near future,” says Goddard’s Stephen Merkowitz, the Space Geodesy Project manager.

Despite proposals to abolish the leap second, no decision has been made by the International Telecommunication Union, a specialized agency of the United Nations that addresses issues in information and communication technologies, explains Elizabeth Zubritsky of NASA’s Goddard Space Flight Center.

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