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Earth Travel On Time, Again
burgburgburg writes "The NY Times has an interesting article about a rather puzzling phenomena: for the fifth year in a row, the Earth's travel through space is right on time. The rate that the Earth travels through space has slowed ever so slightly for millenia. To compensate for this, since 1972, scientists have added a "leap second" at the end of each year. The problem: Since 1999, the Earth has been on time. The recognition of a need for a leap second was an unintended consequence of the invention of the atomic clock. Suggested reasons for the unexpected punctuality: the tides, weather and changes in the Earth's core."
Scientists: Earth Travel Time on Schedule Scientists Say Earth Is on Schedule in Regards to Rate at Which It Travels Through Space
The Associated Press
BOULDER, Colo. Dec. 30 -- In a phenomenon that has scientists puzzled, the Earth is right on schedule for a fifth straight year. Experts agree that the rate at which the Earth travels through space has slowed ever so slightly for millennia. To make the world's official time agree with where the Earth actually is in space, scientists in 1972 started adding an extra "leap second" on the last day of the year.
For 28 years, scientists repeated the procedure. But in 1999, they discovered the Earth was no longer lagging behind.
At the National Institute for Science and Technology in Boulder, spokesman Fred McGehan said most scientists agree the Earth's orbit around the sun has been gradually slowing for millennia. But he said they don't have a good explanation for why it's suddenly on schedule.
Possible explanations include the tides, weather and changes in the Earth's core, he said.
The leap second was an unexpected consequence of the 1955 invention of the atomic clock, which use the electromagnetic radiation emanated by Cesium atoms to measure time. It is extremely reliable.
Atomic-based Coordinated Universal Time was implemented in 1972, superseding the astronomically determined Greenwich Mean Time.
Leap seconds can be a big deal, affecting everything from communication, navigation and air traffic control systems to the computers that link global financial markets.
Just add {In Space!} to anything.
NIST
According to http://tycho.usno.navy.mil/leapsec.html leap seconds compensate for changes in the earths rotational speed not the earths orbital speed.
The leap second is added to compensate for the slowing in earth's rotation, not its motion around the sun.
It is somewhat odd that the rotation has stopped slowing down. Some have speculated that as more and larger dams are built, creating large lakes far from the equator, that there's a net movement of mass closer to the axis -- causing the earth's rotation to increase in speed slightly.
On the other hand, global warming and the melting icecaps and warming oceans should move mass away from the axis, slowing down rotation.
It will be very interesting to see what happens over the next few years. I'd be curious if there's any relationship between the non-slowing of the earth's rotation and the decrease in the earth's magnetic field, mentioned in Slashdot a couple of weeks ago.
thad
I love Mondays. On a Monday, anything is possible.
There is a very comprehensive reference of currently used time standards over at wolfram research site. It came up yesterday while I was trying to figure out the difference between Universal Time (UT) and Coordinated Universal Time (UTC). In the last link I believe you will find that "Earth's rotation is irregular at the 0.1 second level" along with a diagram of the errors so far.
The IERS has a plot showing how the length of day has decreased over the past few years. Curiously, the current phase of accelerated rotation of the crust began right around the time we started adding leap seconds to UTC.
Do they use the background of stars
In a word, yes.
The World of Astronomy site at Wolfram.com is a bit out of date and does not include the most recent changes in time scales. I recommend this page which describes the history of various time scales.
According to Kepler's Third Law: the orbital period, T, and the semimajor axis, a are related by
T^2/a^3 = 4 * pi^2 / ( G * M),
where M is the mass of the Sun. Neglecting the gravitational attraction between the L1 mass and the earth (the L1 mass will be pulling the earth while its position remains constant since its pull towards earth is balance out by the pull toward the Sun), no change in Earth's mass will change its orbital period.
Suddenly, the hairy finger of a familiar monkey tapped me on the shoulder. It was time.--G. T.
According to http://tycho.usno.navy.mil/leapsec.html leap seconds compensate for changes in the earths rotational speed not the earths orbital speed.
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As someone pointed out earlier, the article is incorrect, and a leap second is based on the slowing of earth's rotation.
The dominant force behind the slowing is "tidal braking" from the moon. Basically, just as the moon exerts forces on the ocean, the ocean exerts forces on the moon. As a result, the moon is getting thrown gradually into higher and higher orbits because of force from the earth. The energy has to come from somewhere, and that somewhere is earth's rotational inertia.
Leap seconds were implemented as a result of branch of astronomy known as earth orientation. Basically, Earth Orientation is astronomy backwards. By looking at distant quasars constantly and monitoring atomic clocks, astronomers can see minute changes in earth's rotation. Quasars are observed because they are bright (in the radio part of the spectrum) and are far enough away that any physical motion over time would be negligible in the night sky. Correcting for leap seconds and other rotational issues like precession and nutation allows for the accurate functioning of GPS.
For more information, check out USNO's Earth orientation web site
but they know the error rate and its extremely low. They measure the particle count of cesium or some other radioactive material and the official time is an average of all atomic clocks.
Only the State obtains its revenue by coercion. - Murray Rothbard
The "punctuality" they're referring to (and the "leap second" correction) have to do with the rotational speed (length of a day) of the earth, not its speed of revolution around the sun (which leap days correct for).
The Earth's rotation is referenced to quasars at cosmological distances from us. Since they are so far away, they are for all intents and purposes located at fixed positions on the sky (unlike many nearby stars which show parallax and proper motion over the course of a year or more). The postions are measured using radio VLBI (Very Long Baseline Interferometry) that can provide astrometric positions on the sky to better than a milli-arcsecond.
For more info, browse here: http://hpiers.obspm.fr
Note: IAAA (I am an astronomer)
OK, that was a bad way of phrasing it. The 2-3 milliseconds is the rate at which the lag between UTC (earth) and UT1 (astronomical) time increases per day, not the change in length of each day relative to the last. The rest of what I said is still valid.
First, I believe "Global Dimming" refers to less visible light reaching the planet's surface, independent of any variation in the Sun's output. This is thought to be due to an increase in airborne particulates. I'd have to check, but I think average solar output has been relatively constant over the last 50 years. If solar output dropped 15.9% in 50 years, (as your 3%/decade number suggests), we'd all be popsicles by now.
Second, I'm pretty sure that the Earth is too massive for reemission of solar radiation to generate enough thrust to cause a 1 second per year change in our orbit.
Third, if the suggested causes involve changes in the Earth's core, the article is probably referring to changes in the length of the day (the Earth's period of rotation), and not the length of the year. I can think of no way, short of ejection of significant mass at escape velocity, that the Earth's core could affect the planet's orbit in any way.
Actually, GPS satellites are in high-Earth orbits, but are lower (closer) than satellites in geosync orbits. Their orbital period is about 12 hours and is therefore not even close to the rotational period of the Earth.
We count seconds by counting the transition between the hyperfine levels of the ground state of the cesium 133 atom. We take any handy cesium atoms, count about 9 billions transitions, or actually the radiation from them, and call that a second. We build a machine to do the counting, increment, and we have a very accurate clock. We take some other arbitrary cesium atoms, build the same machine, and find another good match. In fact over time the scientists who do this thing think that all these clocks match to better than 1 part in a trillion That means that there is not going to be a meaningful difference in the measurements of an experiment of normal duration. Over the course of the year, perhaps a millionth of a second.
In fact our second is not fixed. As we have more accurate ways to measure a second, the standard is updated. The new standard is as closes a possible to the old one, but not exactly the same.
As far as whether constants are changing, I think we have no real way of knowing that. In the spirit of science, the simplest explanation is accepted as the best explanation. In this case, the clocks are accurate and constant, and the earth has a slight variation in it's mechanics. Whether this explanation is 'the truth' is left to the philosophers. Assuming that constants are different in different parts of the universe or that constants are varying below out ability to measure them is an unnecessary complexity wrt the measurement of time. There are people who research these things, but only to make sure everyone else it kept honest. When the discover a constant that is not constant, they will let us know.
"She's a scientist and a lesbian. She's not going to let it slide." Orphan Black
A simpler explanation for those who got lost in the long words.
Each day, the sun rises and sets a little more to the north or little more to the south depending on the season. The days of the year where the sun reaches the most north or most south are solstices. When the sun crosses the middle, they're equinoxes. The official "spring equinox" is when the sun crosses the middle moving north. If you were to call that the first day of the year and beginning counting days, you will total up 365 days between equinoxes. After about four years of that though, you'll be off by one, so you'll need to add an extra day. This is called "intercalation."
One could make a rule to add an extra day every four years, but after 100 years or so, they would be foward one day too many. Skip the 100th year, and after 400 years, they'd be 1 day behind. The rule as it stands is every fourth year, except years ending in '00, plus every 400th year. Easy enough, but still not quite right.
Because the rule is not quite right, it will never be perfectly accurate. But if you follow the rule exactly, you can tell that January 1, 1601 was Monday for instance. You can also tell exactly how many days are between now and January 1, 2400 because you know which years are leap years.
The method of watching the sun and adding leap years as necessary is a great way to stay exactly on time, but really inconvenient if you need to predict exactly how every year will fall for the next 100 years or so.
Some people say so what, just live. Who cares if your birthday in 20 years is on a Tuesday. Tax collectors care... Money lenders care... Hallmark greeting cards cares... Calendar makers care... The Vatican cares... So we use the 400 year rule and call it the Gregorian Calendar. It works well enough.
As for TT, UT, UTC, TIA, ET, and a number of other time standards, well... the important thing is that we're now using very accurate clocks for counting seconds and we've determined that the earth does not spin all the way around in exactly 24 hours no matter how closely we've measured it. In fact, it had slowed down for awhile and now seems to have gotten back up to speed.
We determine the difference between the atomic clock and the earth by watching the stars go by, and after spinning, spinning, spinning, we watch the atomic clock and the sky, and if it doesn't come out just right, we assume the clocks are right and the earth is wrong. To make up the difference, we throw in an extra second once every 6 months as necessary. It hasn't been necessary since 1999 which was the crux of the article.
-Hope
Leap seconds have nothing to do with orbit, and everything to do with corrections due to the speed of the earth's rotation. Read the NIST article, the reporter who wrote the one posted on slashdot is clueless.