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 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 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,
http://space-geodesy.nasa.gov/
Bump! Love your tagline.
>>Solution: Blame the moon.
Good thinking. If in doubt, blame the biggest white thing you can see. I’m getting the hang of this “livin’ in the Obamanation” thing.
First paragraph:
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 ....
— Midnight in which time zone? Actually the leap second is added at midnight UTC or at 19:59:60 EDT.
— the third to last paragraph does mention that the extra second is inserted into UTC at 23:59:60.
Second to Last paragraph:
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.
— this leap second will be the 25th since 1972 and only the first since 2008
the Wikepedia report does a better job:
http://en.wikipedia.org/wiki/Leap_second
http://tycho.usno.navy.mil/leapsec.html
This leap second can be either positive or negative depending on the Earth's rotation. Since the first leap second in 1972, all leap seconds have been positive and there were 23 leap seconds in the 34 years to January, 2006.
That’s just more time for Hussein to destroy America.
Bill Clinton got a whole extra day in office to plot harm to the country. February 29, 2000, was the first time since the ratification of the Constitution that February had an extra day in a year ending in two zeroes.
I’m going to make love.
The solar day fluctuates very slightly due to complications in the earth’s rotation. Most of these fluctuations affect both solar and sidereal days; I think the eccentricity of Earth’s orbit also affects solar time.
[For those who are unfamiliar with this, siderial time is measured by Earth’s rotation against the (almost) fixed stars, whereas solar time, which is measured by UT1, is measured by its rotation against the Sun. Because the Earth also orbits the Sun in the same direction as its rotation, it takes one additional revolution to complete a solar year than it does a siderial year. This causes the solar day to be about four minutes longer than a siderial day.]
The sidereal day is stretching out quite predictably, and does not include leap seconds. So in sidereal time, the longest day of the year is December 31. I think this is also true of solar time in years without leap seconds. Note that most years do now have a leap second (35 of them since 1967).
I know this good sex therapist who may be able to help.
≤}B^)
The precession of the equinoxes is the reason why the "first point of Aries" is now in Pisces, and why astrological signs are one constellation off from where the sun actually is in a given part of the year. It was first discovered by the ancient Greek astronomer Hipparchus--which is why you'll never hear Obama talk about it (can't credit Muslim scientists for the discovery).
It’s going to be the longest birthday of my life - lol.
I’m going to use that extra second to count how many rights and freedoms we still have in this country.
We had to account for these leap seconds in the Navigation Timepiece Rate Book where we kept track of the ship’s chronometers.
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