Earth's Slowing Spin

This is for IAC's benefit, though he'll likely ignore it as he's done before:

"LEAP SECONDS
Civil time is occasionally adjusted by one second increments to ensure that the difference between a uniform time scale defined by atomic clocks does not differ from the Earth's rotational time by more than 0.9 seconds. Coordinated Universal Time (UTC), an atomic time, is the basis for civil time.
Historically, the second was defined in terms of the rotation of the Earth as 1/86,400 of a mean solar day. In 1956, the International Committee for Weights and Measures, under the authority given it by the Tenth General Conference on Weights and Measures in 1954, defined the second in terms of the period of revolution of the Earth around the Sun for a particular epoch, because by then it had become recognized that the Earth's rotation was not sufficiently uniform as a standard of time. The Earth's motion was described in Newcomb's Tables of the Sun, which provides a formula for the motion of the Sun at the epoch 1900 based on astronomical observations made during the eighteenth and nineteenth centuries. The ephemeris second thus defined is

the fraction 1/31,556,925.9747 of the tropical year for 1900 January 0 at12 hours ephemeris time.

This definition was ratified by the Eleventh General Conference on Weights and Measures in 1960. Reference to the year 1900 does not mean that this is the epoch of a mean solar day of 86,400 seconds. Rather, it is the epoch of the tropical year of 31,556,925.9747 seconds of ephemeris time. Ephemeris Time (ET) was defined as the measure of time that brings the observed positions of the celestial bodies into accord with the Newtonian dynamical theory of motion.

Following several years of work, two astronomers at the U.S. Naval Observatory (USNO) and two astronomers at the National Physical Laboratory (Teddington, England) determined the relationship between the frequency of the cesium atom (the standard of time) and the ephemeris second. They determined the orbital motion of the Moon about the Earth, from which the apparent motion of the Sun could be inferred, in terms of time as measured by an atomic clock. As a result, in 1967 the Thirteenth General Conference on Weights and Measures defined the second of atomic time in the International System of Units (SI) as

the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom.The ground state is defined at zero magnetic field. The second thus defined is equivalent to the ephemeris second.
The Sub-bureau for Rapid Service and Predictions of Earth Orientation Parameters of the International Earth Rotation Service (IERS), located at the USNO, monitors the Earth's rotation. Part of its mission involves the determination of a time scale based on the current rate of the rotation of the Earth. UT1 is the non-uniform time based on the Earth's rotation.

The Earth is constantly undergoing a deceleration caused by the braking action of the tides. Through the use of ancient observations of eclipses, it is possible to determine the average deceleration of the Earth to be roughly 1.4 milliseconds per day per century. This deceleration causes the Earth's rotational time to slow with respect to the atomic clock time. Thus, the definition of the ephemeris second embodied in Newcomb's motion of the Sun was implicitly equal to the average mean solar second over the eighteenth and nineteenth centuries. Modern studies have indicated that the epoch at which the mean solar day was exactly 86,400 SI seconds was approximately 1820. This is also the approximate mean epoch of the observations analyzed by Newcomb, ranging in date from 1750 to 1892, that resulted in the definition of the mean solar day on the scale of Ephemeris Time. Before then, the mean solar day was shorter than 86,400 seconds and since then it has been longer than 86,400 seconds.

The length of the mean solar day has increased by roughly 2 milliseconds since it was exactly 86,400 seconds of atomic time about 1.79 centuries ago (i.e. the 179 year difference between 1999 and 1820). That is, the length of the mean solar day is at present about 86,400.002 seconds instead of exactly 86,400 seconds. Over the course of one year, the difference accumulates to almost one second, which is compensated by the insertion of a leap second into the scale of UTC with a current regularity of a little less than once per year. Other factors also affect the Earth, some in unpredictable ways, so that it is necessary to monitor the Earth's rotation continuously.

In order to keep the cumulative difference in UT1-UTC less than 0.9 seconds, a leap second is added to the atomic time to decrease the difference between the two. 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. This pattern reflects the general slowing trend of the Earth due to tidal braking.

Confusion sometimes arises over the misconception that the regular insertion of leap seconds every few years indicates that the Earth should stop rotating within a few millennia. The confusion arises because some mistake leap seconds for a measure of the rate at which the Earth is slowing. The 1 second increments are, however, indications of the accumulated difference in time between the two systems. (Also, it is important to note that the current difference in the length of day from 86,400 seconds is the accumulation over nearly two centuries, not just the previous year.) As an example, the situation is similar to what would happen if a person owned a watch that lost 2 seconds per day. If it were set to a perfect clock today, the watch would be found to be slow by 2 seconds tomorrow. At the end of a month, the watch will be roughly a minute in error (30 days of 2 second error accumulated each day). The person would then find it convenient to reset the watch by one minute to have the correct time again.

This scenario is analogous to that encountered with the leap second. The difference is that instead of setting the clock that is running slow, we choose to set the clock that is keeping a uniform, precise time. The reason for this is that we can change the time on an atomic clock, while it is not possible to alter the Earth's rotational speed to match the atomic clocks! Currently the Earth runs slow at roughly 2 milliseconds per day. After 500 days, the difference between the Earth rotation time and the atomic time would be 1 second. Instead of allowing this to happen, a leap second is inserted to bring the two times closer together.
International Atomic Time (TAI) is a statistical atomic time scale based on a large number of clocks operating at standards laboratories around the world that is maintained by the Bureau International des Poids et Mesures; its unit interval is exactly one SI second at sea level. The origin of TAI is such that UT1-TAI is approximately 0 (zero) on January 1, 1958. TAI is not adjusted for leap seconds. It is recommended by the BIPM that systems which cannot handle leapseconds use TAI instead.

Coordinated Universal Time (UTC) is defined by the CCIR Recommendation 460-4 (1986). It differs from TAI by the total number of leap seconds, so that UT1-UTC stays smaller than 0.9s in absolute value. The decision to introduce a leap second in UTC is the responsibility of the International Earth Rotation Service (IERS). According to the CCIR Recommendation, first preference is given to the opportunities at the end of December and June, and second preference to those at the end of March and September. Since the system was introduced in 1972, only dates in June and December have been used. TAI is expressed in terms of UTC by the relation TAI = UTC + dAT, where dAT is the total algebraic sum of leap seconds.

The first leap second was introduced on June 30, 1972. The historical list of leap seconds can be found here.

The Global Positioning System (GPS) epoch is January 6, 1980 and is synchronized to UTC. GPS is NOT adjusted for leap seconds.

As of 1 January 2006,
TAI is ahead of UTC by 33 seconds.
TAI is ahead of GPS by 19 seconds.
GPS is ahead of UTC by 14 seconds.
Until 1960, Universal Time (UT) was taken as the independent variable of astronomical ephemerides. UT was then replaced by Ephemeris Time (ET), based on the motion of the sun. However, ET did not include relativistic effects, such as corrections for the gravitational potential and velocity, as required by advances in the accuracy of time comparisons. Thus ET was superseded in 1981 by Terrestrial Dynamical Time (TDT) and Barycentric Dynamical Time (TDB), which distinguish coordinate systems with origins at the center of the Earth and the center of the solar system, respectively, and are consistent with the general theory of relativity. In the language of general relativity, TDT is a proper time while TDB is a coordinate time. In 1991, TDT was renamed simply Terrestrial Time (TT) and two additional relativistic time scales, Geocentric Coordinate Time (TCG) and Barycentric Coordinate Time (TCB) were adopted. Definitions of these time scales are given in Systems of Time.
Terrestrial Time (TT) is a uniform atomic time scale, whose unit is the SI second, that replaces Ephemeris Time and maintains continuity with it. TT may be regarded as the time that would be kept by an ideal atomic clock on the geoid. To convert a TT value to a prediction of UT1, it is necessary to know the difference dT = TT - UT1. Values of dT are tabulated in the Astronomical Almanac. For example, mathematical predictions of lunar and solar eclipses in the distant past and future depend sensitively on estimates of dT. The computed path of a solar eclipse that occurred 2000 years ago would be in error by about 3 hours, or some 45 degrees in longitude to the west, on the assumption that the rate of rotation of the earth were uniform. Conversely, records of well documented ancient eclipses, together with modern telescopic observations of occultations, Very Long Baseline Interferometry, satellite laser ranging, lunar laser ranging, and other measurements correlated to atomic time scales since 1955, have provided the data on which long term trends and short term fluctuations have been derived. Since dT was approximately 32.184 seconds at the origin of TAI in 1958, a practical realization of TT is TT = TAI + 32.184 seconds. Although this expression gives TT in terms of TAI, in practice TT is obtained from the relation TT = UTC + dAT + 32.184 seconds for a known value of UTC and a given number of leap seconds."

The above is from: http://tycho.usno.navy.mil/leapsec.html

Anyway, I was mistaken, it was not roughly 150 years ago, it was roughly 187 years ago that the Earth had 86,400 seconds in a day. Of course, IAC will probably complain about why we scientists didn't properly measure the exact length of the day back then with an atomic clock.

And now, for the final numerology explanation:

Why was 6 afraid of 7?

Because 7 8 9, of course. (not 4 3 2)

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Yes, was their measurment of seconds back then as accurate as a .002 error?

 

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Hint: It's NOT 432.

 

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I don't know then, tell me.

 

You honestly don't know?

 

Hi IAC,

I want to try to explain what these guys fail to explain to you.
I think I know what lead you to false conclusion. It is your
understanding about the definition of an atomic clock. However,
my English is really limited. But let me try as simple as I can.

Let say there is no such things as clock. If I say, lets meet at X Club, 1 hour
from now, how do I make sure that my understanding of 1 hour is equal
to your understanding of 1 hour? We need the SAME 1 HOUR REFERENCE.

If I need 1 hour to walk from Y to Z, and I set the definition "1 hour
is equal to the time required by me to walk from Y to Z", it will be difficult,
isn't it? Because when I feel tired I will walk slower. And if I died, the
reference is gone at all.

So, we need a reference which is continually reproducible to measure.

Now those scientist want to define 1 SECOND; and they define it as the
time needed by a nice Caesium-133 atom to complete a transition
between two energy levels of its ground state. Why? because
everywhere around the world, today or tomorrow, this Caesium-133 atom
needed the same that much time for that transition. So now we could use it
as the SECOND REFERENCE without bias.


Now lets define 60 seconds = 1 minute.

And then define 60 minutes = 1 hour.

We are now agree that 1 hour = 60 x 60 = 3,600 SECONDS.

So now, if I say, lets meet at Club X ONE HOUR from now,
it means we will meet after a time equal to the time needed by
Caesium-133 atom for 3,600 times mentioned transition. Agree with me?

Now we have the same clock. And now we want to measure by that clock,
one day is actually consist of how many seconds? Apparently, the
number is not even.

After 24 hours, which is = 24 x 3,600 = 86,400 seconds, it is not fully one day YET.
Apparently it consist of 86,400.002 seconds.
Which means, 1 day is equal to the time needed by that Caesium-133 atom
to complete 86,400 energy transition, PLUS 0.002 seconds.

But, if we say 1 day is equal to 86,400.002 seconds, it is IMPRACTICAL,
isn't it? So, they "remove" this 0.002 seconds per day.

HOWEVER, after one year, this removal will become 365 x 0.002 = 0.73 seconds.
(Well it is actually 0.9 seconds, because it is not exactly 0.002 seconds).
And this 0.9 second now looks more significant. Therefore, they adjust
the clock, by adding 0.9 second by the end of the year.

And so it does not mean that the earth spins slower half a second every year.

I like colourful text. Hope that helps.

 

I just want to see if you know.

 

Uh, inzomnia, the length of the second was established by the atomic clock through the measured duration of one Earth spin divided by 86,400.

 

And the Earth spin keeps slowing down, so they have to add leap seconds.

 

No, it is not. I am not trying to be smart here, whatsoever,
trust me. This is the basic of your false conclusion. Please
check here: http://en.wikipedia.org/wiki/Atomic_clock

----- has defined the second as the duration of 9 192 631 770 cycles of the
radiation which corresponds to the transition between two energy levels of
the ground state of the caesium-133 atom.

They make that time period which is needed by that atom for once
energy transition as the reference. So they do not 'create' the
atomic clock. They took from natural phenomena and make that
as a basis. More or less. I over simplify it.

 

It is the atomic clock measured second length in 1956, so they've added 23 leap seconds since.

 

Yes, if u take it from 1956, it will be around 23 leap (cumulatively).
But for what reason? For the reason which i write in the long post.
Please take time to carefuly read it, really.

 

Not to the Earth:

 

The Sub-bureau for Rapid Service and Predictions of Earth Orientation Parameters of the International Earth Rotation Service (IERS), located at the US Naval Observatory, monitors the Earth's rotation. Part of its mission involves the determination of a time scale based on the current rate of the rotation of the Earth. They estimate that the Earth's rotation is slowing at about 1.4 milliseconds per solar day per century which roughly agrees with the rate of rotation of the Earth has actually slowed down since 1820.

Tracing these tiny milliseconds back for 4.5 billion years adds up to a very significant amount of time for a solar day. The author has determined that the day/night rotation was 63,000 seconds shorter than the present 86,400 seconds it is today. This would put the Earth's rotation at about 6.5 hours per day/night cycle, when it was created, 4.5 billion years ago. (This is a much faster rate of rotation than the Cassini-Huygens mission (2003 to 2004) determined Saturn's 10.5 hours rotation period to be.)

That's pretty fast.
http://novan.com/earth.htm

 

But that slowing down has nothing to do with the adjustment
of atomic clock, has that?

http://novan.com/earth.htm

While we know the Earth's rotation is slowing that is not the main reason why the extra "Leap Second" was added by our official time keepers this year. The reason for adding a leap second is that the planet does not rotate exactly once every 24 hours (86,400 seconds). The rotation actually takes 86,400.002 seconds so that each day this little difference builds up between the atomic clock and the earth's rotation. When the difference builds up enough (.9 seconds), the time keepers must add another second (leap second) to keep the stars location, relative to the planet's rotation, in exact sync with the superaccurate atomic clocks.

The Earth's rotation is slowing but at a much slower rate than 1 leap second every so many years. The length of time it takes the Earth, at the present time, to rotate once is 86,400.002 seconds compared to 86,400 seconds back in 1820. The rotation has slowed roughly only by 2 milliseconds since 1820. That seems like an insignificant amount of time BUT over the course of the planet's entire lifetime, it has had very profound effects on the geophysics of the planet.

 

Inzomnia:

No, the adjustment is exactly as you noted. But please note that IAC doesn't like to read long paragraphs, or he certainly has a hard time understanding them.

Spidergoat:

We covered this before too. Thus, during the dinosaur days, the earth's rotational speed was closer to 22 hours, not 24. Think of how that would affect the climate!

IAC:

Please look at the bold and highlight of my previous post.

Way back in 1956, they decided to base the atomic second on what they believed was the number of seconds on January 1, 1900 A.D., which they defined to be 86,400.00 seconds. Turns out they were somewhat off, and the earth had 86,400.00 seconds way back around 1820 A.D., not 1900 A.D., as we've since determined by better astronomical measurements with satellites, etc., not available in 1956.

They were apparently uncertain as to whether the then-observed very slow slowing of rotation would continue at the same rate, and so they arbitrarily used the year 1900 A.D., as they then calculated it astronomically, to base their atomic second, rather than the then current year (1956). That's not the way I would have done it (I would have used the then current year, or better still 1950, as noted below) but that's the way they did it. Their astronomy was also somewhat off, and we've since learned that their definition of 86,400.00 seconds = one day fits best for the years circa 1820, not 1900. Anyway, that's the system they developed, and that we've used ever since, to define one second, now defined not by astronomical observations, but by the Cesium atom.

Because they started way off on the length of the day (off by about .002 seconds) right at the beginning when they started the atomic clock system, almost immediately they began having to add a 'leap second' every year or two. You can see the dates as to when they were added in the web pages I cited, though they typically add them at New Years. It's about every 500 days or so nowadays. A few thousand years from now, we'll be adding quite a few leap seconds every year due to the rotational slowing; or else we'll go back and re-define the second.

Your misunderstanding is apparently a common mis-perception, engendered by the fact that they STARTED OFF the atomic clock timing system with a day that was off by .002 seconds, i.e 86,400.002 seconds was the length of the day in 1956. If the earth ever speeds up (unlikely, unless we get a huge asteroidal impact in the right direction, or other unlikely event), the length of the day might go back to what it was in 1900 or 1820.

Anyway, I think it would have been preferable if they had used the year 1950 A.D. (my birth-year), rather than 1900 A.D., don't you agree?