NASA explains why clocks will get an extra second on June 30

Jun 29, 2012
With this antenna at Kokee Park on the Hawaiian island of Kauai, NASA makes regular VLBI (Very Long Baseline Interferometry) measurements that are used in the time standard called UT1 (Universal Time 1). Credit: U.S. Navy/PMRF

If the day seems a little longer than usual on Saturday, June 30, 2012, that's because it will be. An extra second, or "leap" second, will be added at midnight to account for the fact that it is taking Earth longer and longer to complete one full turn--a day--or, technically, a solar day.

"The solar day is gradually getting longer because Earth's rotation is slowing down ever so slightly," says Daniel MacMillan of NASA's Goddard Space Flight Center in Greenbelt, Md.

Scientists know exactly how long it takes Earth to rotate because they have been making that measurement for decades using an extremely precise technique called Very Long Baseline Interferometry (VLBI). VLBI measurements are made daily by an international network of stations that team up to conduct observations at the same and correlate the results. NASA Goddard provides essential coordination of these measurements, as well as processing and archiving the data collected. And NASA is helping to lead the development of the next generation of VLBI system through the agency's Space Geodesy Project, led by Goddard.

From VLBI, scientists have learned that Earth is not the most reliable timekeeper. The planet's rotation is slowing down overall because of tidal forces between Earth and the moon. Roughly every 100 years, the day gets about 1.4 milliseconds, or 1.4 thousandths of a second, longer. Granted, that's about 100 or 200 times faster than the blink of an eye. But if you add up that small discrepancy every day for years and years, it can make a very big difference indeed.

"At the time of the dinosaurs, Earth completed one rotation in about 23 hours," says MacMillan, who is a member of the VLBI team at Goddard. "In the year 1820, a rotation took exactly 24 hours, or 86,400 standard seconds. Since 1820, the mean solar day has increased by about 2.5 milliseconds."

By the 1950s, scientists had already realized that some and technologies demanded more precise timekeeping than Earth's rotation could provide. So, in 1967, they officially changed the definition of a second. No longer was it based on the length of a day but on an extremely predictable measurement made of electromagnetic transitions in atoms of cesium. These "atomic clocks" based on cesium are accurate to one second in 1,400,000 years. Most people around the world rely on the time standard based on the cesium atom: Coordinated Universal Time (UTC).

Another time standard, called Universal Time 1 (UT1), is based on the rotation of Earth on its axis with respect to the sun. UT1 is officially computed from VLBI measurements, which rely on astronomical reference points and have a typical precision of 5 microseconds, or 5 millionths of a second, or better.

"These reference points are very distant astronomical objects called quasars, which are essentially motionless when viewed from Earth because they are located several billion light years away," says Goddard's Stephen Merkowitz, the Space Geodesy Project manager.

For VLBI observations, several stations around the world observe a selected quasar at the same time, with each station recording the arrival of the signal from the quasar; this is done for a series of quasars during a typical 24-hour session. These measurements are made with such exquisite accuracy that it's actually possible to determine that the signal does not arrive at every station at exactly the same time. From the miniscule differences in arrival times, scientists can figure out the positions of the stations and Earth's orientation in space, as well as calculating Earth's rotation speed relative to the quasar positions.

Originally, leap seconds were added to provide a UTC time signal that could be used for navigation at sea. This motivation has become obsolete with the development of GPS (Global Positioning System) and other satellite navigation systems. These days, a leap second is inserted in UTC to keep it within 0.9 seconds of UT1.

Normally, the clock would move from 23:59:59 to 00:00:00 the next day. Instead, at 23:59:59 on June 30, UTC will move to 23:59:60, and then to 00:00:00 on July 1. In practice, this means that clocks in many systems will be turned off for one second.

Proposals have been made to abolish the leap second and let the two time standards drift apart. This is because of the cost of planning for leap seconds and the potential impact of adjusting or turning important systems on and off in synch. No decision will made about that, however, until 2015 at the earliest by the International Telecommunication Union, a specialized agency of the United Nations that addresses issues in information and communication technologies. If the two standards are allowed to go further and further out of synch, they will differ by about 25 minutes in 500 years.

In the meantime, leap seconds will continue to be added to the official UTC timekeeping. The 2012 leap second is the 35th to be added and the first since 2008.

Explore further: Space sex geckos at risk as Russia loses control of satellite

add to favorites email to friend print save as pdf

Related Stories

Leap second to be added

Jun 29, 2012

A leap second will be introduced on 30 June 2012 following a decision made by the International Earth Rotation and Reference Systems Service (IERS) earlier this year. This could potentially be one of the last ...

Scientists meet to discuss usefulness of GMT

Nov 03, 2011

Leading scientists from around the world are meeting in Britain from Thursday to consider a proposal that could eventually see Greenwich Mean Time relegated to a footnote in history.

'Leaping' into the realm of science

Mar 01, 2012

Yesterday was Feb. 29, the extra day we add to the calendar in leap years. But why do we need this extra day, and what is the science behind it? And what about the lesser-known leap second – which delegates ...

Recommended for you

Video: A dizzying view of the Earth from space

6 hours ago

We've got vertigo watching this video, but in a good way! This is a sped-up view of Earth from the International Space Station from the Cupola, a wraparound window that is usually used for cargo ship berthings ...

NEOWISE spots a comet that looked like an asteroid

6 hours ago

Comet C/2013 UQ4 (Catalina) has been observed by NASA's Near-Earth Object Wide-field Infrared Survey Explorer (NEOWISE) spacecraft just one day after passing through its closest approach to the sun. The comet ...

What the UK Space Agency can teach Australia

6 hours ago

Australia has had an active civil space program since 1947 but has much to learn if it is to capture a bigger share of growing billion dollar global space industry. ...

Discover the "X-factor" of NASA's Webb telescope

6 hours ago

NASA's James Webb Space Telescope and Chandra X-ray observatory have something in common: a huge test chamber used to simulate the hazards of space and the distant glow of starlight. Viewers can learn about ...

User comments : 2

Adjust slider to filter visible comments by rank

Display comments: newest first

Falcon41
not rated yet Jun 30, 2012
I have several atomic timepieces so I found this article very interesting. I have, however, noted slight discrepancies between them and I am curious as to how that second will affect them. If time is an illusion, I insist on a precise illusion.
The Singularity
5 / 5 (1) Jun 30, 2012
You know those people who say "there are never enough hours in the day" well there you go, an extra second just for them. They will be pleased to know that our days are getting longer.