"Leap Second" to be Added to World Clocks
Written by Nancy Atkinson
The US Naval Observatory operates 70 cesium atomic clocks. Credit: USNO
If you ever feel like you need more time, here's some great news: you're actually going to get it. On December 31, 2008 a “leap second” will be added to the world’s clocks at 23 hours, 59 minutes and 59 seconds Coordinated Universal Time (UTC). This corresponds to 6:59:59 pm Eastern Standard Time, when the extra second will be inserted at the U.S. Naval Observatory’s Master Clock Facility in Washington, DC. This is the 24th leap second added to UTC, a uniform time-scale kept by atomic clocks around the world, since 1972. Coincidentally, Fraser and Pamela's most recent episode of Astronomy Cast is about time, so if you want to know more about time and the atomic clocks used to provide precise timekeeping, check it out.
Historically, time was based on the mean rotation of the earth relative to celestial bodies and the second was defined in this reference frame. However, the invention of atomic clocks defined a much more precise “atomic time” scale and a second that is independent of the earth’s rotation. In 1970, an international agreement established two timescales: one based on the rotation of the earth and one based on atomic time.
Atomic clocks do not use radioactivity, but they use the exact frequency of the microwave spectral line emitted by atoms of the element cesium, in particular its isotope of atomic weight 133 ("Cs-133"). The integral of frequency is time, so this frequency, 9,192,631,770 hertz (Hz = cycles/second), and this provides the fundamental unit of time, which are measured by cesium clocks.
The problem is that the earth’s rotation is very gradually slowing down, which necessitates the periodic insertion of a “leap second” into the atomic timescale to keep the two within 1 second of each other. The International Earth Rotation and Reference Systems Service (IERS) is the organization which monitors the difference in the two timescales and calls for leap seconds to be inserted or removed when necessary.
Since 1972, leap seconds have been added at intervals varying from six months to seven years, with the last being inserted on December 31, 2005. The U.S. Naval Observatory is charged with the responsibility for the precise determination and dissemination of time for the Department of Defense and maintains its Master Clock. The U.S. Naval Observatory, together with the National Institute of Standards and Technology (NIST), determines time for the United States.
Source: US Naval Observatory
Filed under: Science
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December 9th, 2008 at 7:36 am
Yes, the Earth is slowing down, so according to general relativity, the cesium clock muss be affected as well, Is this taken in account?
December 9th, 2008 at 7:38 am
It's a pity they couldn't insert the extra second NOW…. I'm working on a deadline job to be finished by 5 o'clock this afternoon…
December 9th, 2008 at 7:45 am
I know it's likely millions and millions of years away, but how slow would the Earth have to be rotating for it's gravity to be weakened to the point where any living being would not be able to stay firmly planted on solid ground?
December 9th, 2008 at 7:47 am
~ ~ ~ ~ ~ ~ ~ ~
Feenixx Says:
December 9th, 2008 at 7:38 am
It's a pity they couldn't insert the extra second NOW…. I'm working on a deadline job to be finished by 5 o'clock this afternoon…
~ ~ ~ ~ ~ ~ ~ ~
Then what are you doing on here???
No wonder employee production is down.
Thanks alot Universe Today!
December 9th, 2008 at 7:47 am
Miguel V. asks
"Yes, the Earth is slowing down, so according to general relativity, the cesium clock muss be affected as well, Is this taken in account?"
I reckon it's either taken into account, or irrelevant.
Aren't all measurements of "stuff" involving time made RELATIVE to the cesium clock?
December 9th, 2008 at 7:54 am
Eric Near Buffalo Says
"Then what are you doing on here???
No wonder employee production is down"
coffee break, which actually increases my productivity, and it's my only chance to get the job done…. btw, I'm self employed….
December 9th, 2008 at 8:02 am
Wow! A full extra free second! Let's see what I can do with this extra time on my hands…
Oops… just wasted it…
December 9th, 2008 at 8:51 am
I extra second for me to rob all the worlds banks.
MUAHAHAHA
But seriously, the time problem is nothing compared to the month and years problem.
The current setup is just bad, there are plenty of good alternatives out there.
But of course, changing everything over in this world we live in now would be near impossible…
December 9th, 2008 at 10:01 am
@Eric Near Buffalo:
If the Earth stopped spinning the gravitational effect of the Earth would increase not decrease.
Like putting a cork on a spinning record turntable (which I remember doing when I was a child) makes the cork fly outwards away from it's centre point of rotation.
December 9th, 2008 at 10:21 am
Relativistic effects on time are infinitesimally small at the speeds involved. Earth orbits at 0.01% of the speed of light which isn't slow, but the change in speed that results in the a one second adjustment once every six months or so is much much smaller than that.
So, while I am sure that relativity does make a difference, I suspect that for that difference to add up to a second or more, we'd have to wait millions of years.
December 9th, 2008 at 10:37 am
Don't they realize how much hassle this is going to be for programmers?? Bah humbug.
December 9th, 2008 at 10:49 am
"When the world is running down you make the best of what's still around" – The Police
December 9th, 2008 at 11:54 am
Gravity is a myth. Electromagnetism was practically unknown in the time of Newton. And Neptune and Mercury are totally in the wrong places according to gravity. Elements in the atmosphere do not sort themselves according to their specific weights which defies gravity. Ozone is heavier than oxygen but defies gravity. And so too does water vapor and the clouds which are heavier than air defy gravity.
"What we call mass would seem to be nothing but an appearance, and all inertia to be of electromagnetic origin." — Henri Poincaré, physicist, 1908
"Gravitation is an electromagnetic phenomenon." — Immanuel Velikovsky, cosmologist, 1946
Velikovsky, I., Cosmos Without Gravitation, 1946
For more information about the leap second and the retardation of the rotation of the Earth due to the expansion of the Earth you can see below:
McCarthy, D.D., Precision Time and the Rotation of the Earth, Transits of Venus: New Views of the Solar System and Galaxy, Proceedings IAU Colloquium, Number 196, 2004
McCarthy, D.D., et al., The Physical Basis of the Leap Second, The Astronomical Journal, Issue 5, Nov 2008
December 9th, 2008 at 3:00 pm
"I know it's likely millions and millions of years away, but how slow would the Earth have to be rotating for it's gravity to be weakened to the point where any living being would not be able to stay firmly planted on solid ground?"
Earth's (or any other planet's) gravity has nothing to do with its rotation, only its mass. (and density matters for the force of gravity at its *surface,* but not at a distance).
However, centrifugal force (yes, I know that strictly speaking it's not a 'force') does mean that you'd be very slightly lighter standing at the equator, compared to the poles, and as the planet gradually spins down over tens of billions of years (through tidal interactions with the Moon, that also cause its orbit to enlarge) that difference will fall toward zero…
Jupiter however, its outer parts being made mostly of its low density atmosphere, and turning once in about 11 hours, is stretched into a visibly oblate spheroid.
December 9th, 2008 at 5:10 pm
24 seconds in 36 years seems like a fast rate of slowing. Are there any estimates for rotation rates of Earth as a function of time? Oh, say maybe over the last billion years or so? I assume the rate is not linear. Also what would cause the slowdown? More dams holding water at higher elevations on the planet (a small effect no doubt)? Energy dissipated by tidal action in the oceans?
December 10th, 2008 at 1:47 am
Cynthia Says:
December 9th, 2008 at 5:10 pm
24 seconds in 36 years seems like a fast rate of slowing. Are there any estimates for rotation rates of Earth as a function of time? Oh, say maybe over the last billion years or so? I assume the rate is not linear. Also what would cause the slowdown? More dams holding water at higher elevations on the planet (a small effect no doubt)? Energy dissipated by tidal action in the oceans?
—–
Reply to Cynthia:
Technically speaking, anything that is accelerated by coming near the earth is slowing down Earth's rotation – which means that all of those missions that come back past Earth to do a slingshot to increase their speed are slowing down the planet. On the other side, this effect is negligible based on the masses involved. In theory, there are probably a large number of things that may affect Earth's rotation but again it comes down to a matter of the masses involved and most of them are negligible.
If you wanted to point a finger at the largest culprit however, it would be the moon which is slowly being pushed by the Earth away from us so in a way you are close with the concept of the tidal action in the oceans – dry land also experiences a very minor flexing as well as the oceans but since it is solid it is hard to notice today (according to the collision moon formation theory it would have been more noticeable billions of years ago when the moon was much closer to the Earth). At any rate, the moon's gravity causes a minor bulge in the Earth, which moves slightly ahead of the moon due to the Earth's rotation and this causes a gradual acceleration of the moon. Theoretically, in billions of years, the moon should make its way out of Earth orbit.
December 10th, 2008 at 2:06 am
To Hunnter.
What alternatives are you talking about?
December 10th, 2008 at 6:10 am
Thanks for the answers Sci-Fi-Si and Frank Glover, they cleared that up for me. I was always under the impression that the Earth's gravity was mostly a product of it's centrifugal force. Now that I know it's more about mass, it changes my way of thinking.
December 10th, 2008 at 6:44 am
W ko?cu si? wy?pi?
December 10th, 2008 at 6:54 am
How does this affect our computers? Will we be receiving software updates with patches that include this extra second?
December 10th, 2008 at 5:36 pm
Well ShadowDancer, via the web there seems to be some consensus that the length of an Earth day might have been on the order of 14 hours 4 billionish years ago, or after the moon was formed. More recent geological evidence points to 21 or 22 hour days a mere 400 to 600 million years ago. In the long run the Moon will continue to spiral away from the Earth until a lunar month is equal to an Earth day, which will be something on the order of over 1000 hours. Fortunately that would take a trillion years and surely the Earth and Moon would be consumed by a bloated Sun long before then, perhaps in a billion years or so.
But getting back to the original question, what would the rotation rates of the Earth be for the last 4 billion years? I'd guess that the slow-down rate is somewhat proportional to the tidal forces, but that the tidal forces are not linearly related to the change in the Earth-moon distance. Move the moon twice as far away from the Earth and the tidal forces should drop by a factor greater than two. But what's the relationship? 1/radius^2 ? 1/radius^3 ? I suppose the shape of the curve as a function of time is pretty much a decaying exponential, more or less, and that would mean the change in rotation rate was quicker in the past and will asymptotically approach a final value in the future.
December 10th, 2008 at 10:20 pm
Reply to Cynthia:
The transformation of the sun into a red giant is approximately 5 billion years from now according to our current understanding of the sun and how it is changing. As for the moon, the theoretical prediction for reaching an orbit that matches the rotation of the Earth would be in about 50 billion years. As for the Earth's rotation, it should indeed be a decaying change in rotation over time due to the moon moving farther away (i.e. the Earth slowed down more when the moon was closer). As far as gravity is concerned, it is inversely proportional to the square of the distance between two bodies. The math for changing tidal forces is more complicated though.
As for the leap second, that has more to do with keeping the atomic clocks in sync with the background stars due to the fact that the Earth doesn't rotate in exactly 24 hours. What this really means is that inserting leap seconds has to do with the current rotational speed of the Earth as opposed to the slowing of the rotation of the Earth, even though that is the technical cause of the leap seconds being inserted. The actual change in Earth's rotation is only about 2 microseconds since 1820. A good explaination can be found here: http://novan.com/earth.htm
December 23rd, 2008 at 5:14 am
It's my understanding that the the Earth's rotation will never actually stop. It may slow down substantially over the millenia but never halt. Look at Venus – no moon but it still spins on its axis. Ditto Mars and the other planets. The moons of the others barely register as brakes on rotation and of course barring Mars they're all gas/ice giants not rocky/metallic ones.
February 3rd, 2009 at 2:12 pm
The 24 extra seconds in the last 37 years…
If we take the idea to the absurd and we could ride along with Einstein holding on to the tail of a photon at the speed of light, time would stop. With this adding of the occasional
second because of slowing velocity does that mean that if we could stop our rotation and orbit and expansion through the universe that likewise time would stop? If time is just a function of mass and velocity can it work both ways,… that, ultimate velocity and none at all, both yield some kind of crazy stasis?