Skip to comments.How will you spend your bonus time on June 30?
Posted on 06/30/2012 6:28:49 AM PDT by upchuck
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 turna dayor, 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 time 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 NASA 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 scientific measurements 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 leap second to be added and the first since 2008.
For more information about NASA's Space Geodesy Project, including VLBI, visit,
Great...now I’ll be one second early for all my appointments.Or is it one second late?
So much for my threat to leave the country if Obama is in power one second more than four years.
i lost it responding.
U mean one se
An extra second? Is that good or bad? I don’t know whether to blame Bush or praise Obama!
Newton’s Law....and he did it without the benefit of computers.
Obviously, we are devolving, as well as the speed of the earth’s rotation.
Not only is Obama slowing down the economy, he is slowing down the rotation of the planet.
I was going to reply but forgot what I was going to say. Give me a sec...
The planet was slowed down by the first Wookie shaking her ass in the whitehouse.
Bush the moon.
Step one: arbitrarily pick a technology to define the year in order to keep our technological society synchronized.
Step two: refine the technology and realize that the year varies in a length according to the technology, which may itself be a little variable.
Step three: act as if it matters — like a metric system built on powers of ten which nevertheless initially defined the meter using ONE QUARTER of the Earth’s circumference. Sort of.
This is an ‘extra, extra’ ping to some lists, hope your weekend is great, and long.
Dang, I would have thanked you up there, but didn’t see yours in time. Thanks upchuck!
He was wrong--one day this year is the longest day, and it's today.
In the shadow of the Moon
New Scientist | 30 January 1999 | editors
Posted on 08/31/2004 8:42:25 AM PDT by SunkenCiv
I would think we should praise 0bama. He should make campaign ads... I'm Barack 0bama and I'm so awesome, I just gave everyone an extra second of life. What the hell, he doesn't have anything else to run on.
An extra second of sleep!