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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."
The Earth knows that we're watching now, so it's taking extra care to be punctual...
Suggested reasons for the unexpected punctuality: the tides, weather and changes in the Earth's core.
No. God just likes to screw with us.
.... we can speed it back up again.
Let's do it!
NIST
We decided that for a change this holiday we'd work on big iron - the earth. Using polar cap cooling and using two atoms instead of one in the atomic clock, we managed to accelerate the earth by 1 second. System is perfectly stable. Except in California. And Iran.
www.voiceofthehive.com - Beekeeping and Honeybees for those who don'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.
Don't you guys remember back in 1999 when the moon blasted out of earth's orbit? That would probably explain the change.
I live ze unknown. I love ze unknown. I am ze unknown.
OK so help me out here. Pardon the pun, but how on earth do they figure out that the earth is in the exact same position as it was a 'year' ago? Do they use the background of stars, or some other mechanism? How can they reduce the error in such a measurement so that they can be sure that a second has been 'lost'?
I really could have used that extra second of sleep after partying late on new years eve.
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.
It seems to me that physicists assume that their atomic clocks keep perfect time. But what if they don't? What if some key physical constants are changing in our neck of the universe. As an engineer I have found that most physical constants aren't (everything is a function of everything, its just an matter of the coefficient). In the case of the atomic clocks, a change of only 32 parts per billion would change the timebase by one second per year. Perhaps a particularly large, long-wavelength gravity wave has stretched spacetime and changed the clocks? Perhaps the four fundamental forces oscillate in undiscovered ways?
IANAP, so perhaps a professional could explain why the atomic clocks must be right -- why a 32 ppb variation in them is impossible (i.e. would manifest itself in other more obvious ways).
Two wrongs don't make a right, but three lefts do.
I got this value from the web site for the Time and Frequency division of NIST.
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.
I thought they fixed that core problem a few years back by blowing the shit out of it with a bunch of nukes! I saw a documentary on this recently.
You got it backwards. The "leap second" was needed because the Earth *was* taking a little longer. For some reason it has stopped being slow by a second. If anything, its the Democrats trying to get Bush out of office a little sooner.
They that can give up essential liberty to obtain a little temporary safety deserve neither safety nor liberty.
Ben
Is the Earth's *rotation* slowing or speeding up at all, though?
Let's do some math and see how big of a distance difference a leap second is. Rough estimate of 93 million miles on average for r. 2*pi*r = 584,309,935 miles for the circumference of the orbit -- assuming a circular orbit. 60*60*24*365.25 = 31557600 seconds on average per year. circumeference/seconds ~= 18 miles. Interesting. But hardly seems like much.
Tidal slowing is also magnitudes more important than anything you'd see from mountain building, earthquakes, or any other surface phenomenon. The earth is BIG. But tides take out a LOT of energy. Tides are the major reason the Earth's rotational period slows over geological time.
So right now, the Earth is not slowing; this must mean a shorter-acting phenomenon is supplying the rotational energy that the tides normally suck out. Again, there is only one thing big enough -- turbulence in the Earth's liquid core. Like the Earth itself the core is BIG so little changes in the fluid flow there can actually affect the Earth noticeably, and that flow is known to be chaotic -- because the magnetic field caused by that flow reverses periodically.
My money would be on a near-term magnetic field polarity reversal. Of course "near term" probably still means it will be ten thousand years before it's a problem. Sucks to be a man-made satellite, though, especially when flying over the South Atlantic, an area where the Earth's magnetic field is already starting to do strange things.
Brackets contain world's first nanosig, highly magnified:[.]
Well, I suppose Jupiter sacked Atlas for his continual tardiness and the new God is still all gung ho about the promotion to the new job. Eventually he'll get tired too...
Oh well, what the hell...
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.
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
I recently had to implement code to convert terrestrial time (TT) to martian solar day (MSD). Some interesting tidbits in that research follow.
As you might guess, the extra days in leap years help keep our calendars synchronized with our actual position about the sun (heliocentric longitude). This is called intercalation, and the general rules governing the gregorian calendar cover 400 year periods. Other methods exist which are in a sense more "accurate," but less useful for predicting future dates. Fortunately, the earth is pretty regular in its movement around the sun.
The 0 degree mark for heliocentric longitude occurs at the vernal equinox, an event that can be easily determined from earth, and has been for centuries. In the Iranian calendar, the new year begins on the day of the vernal equinox. Since this event occurs later in the day each year, eventually an extra day must be added. Such calendars are based on observation rather than rule-based model and consequently are implicitly self-calibrating.
Leap seconds, as pointed out, are an entirely different beast, and are meant to shore up the discrepency between our actual rotation and the atomic clocks we use. The current offset is 22 seconds slow officially. Oddly enough, a NASA document from 1997 uses a value of 63 seconds as the offset between TT (terrestial time) and UTC (Greenwich Mean Time). Another from 2000 shows a 32.184 second offset from TT to TIA (atomic). It doesn't exactly correlate or add up, and I'm not precisely sure why that is. Perhaps someone could enlighten me on the matter.
Curiously, our leap years follow the mathematical model while our leap seconds follow the observation method of calibration. Consequently, you can determine the correct date in the future, but not the correct second.
-Hope
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