Earth's rotation rate, again

[kilopi]

Just found this while browsing an all-about-science book in the sale rack at Borders:

Is it true that the rotation speed of the Earth Varies?

The rotation speed is at its maximum in late July and early August and at its minimum in April; the difference in the length of day is about 0.0012 second. Since about 1900 the Earth's rotation has been slowing at a rate of approximately 1.7 seconds per year.

The generally accepted value for tidal slowing is that the day lengthens about 2 milliseconds per century. I'm not even certain what that last sentence is talking about, but it appears to be off by a factor of one hundred thousand.

[wedgebert]

Apparently the Earth's rotional speed is constantly increasing or decreasing.

Hmm, that would explain why I fall down a lot. And to think, I was blaming the alcohol.

[Visitor]

There were 22 obey leap seconds since 1972.
Those are meant to keep UTC the same value as TAI, so this should represent roughly the slowing of the earth's rotation, i guess.

[tracer]

I thought leap seconds were inserted because the Earth's orbit around the Sun is slowly decaying, not because the length of the Sidereal day is changing.

[The Shade]

Apparently the Earth's rotional speed is constantly increasing or decreasing.

Hmm, that would explain why I fall down a lot. And to think, I was blaming the alcohol.

"To alcohol! The cause of, and solution to, all of life's problems!"

[bobjohnston]

The seasonal change results from exchange of angular momentum between the Earth and the upper atmosphere, as upper atmospheric winds change with the seasons. See http://www.physics.uoguelph.ca/summe...les/scor42.htm

[Visitor]

tracer,
Sorry i have no link to a english website available, but according to some papers from the PTB (shortcut for the german term federal physical-technical agency) the declining rotation speed cause the need for the leap seconds. By the way, the difference between siderian day and mean sun day is 4 minutes, IIRC. Any astronomer online?

[swansont]

I thought leap seconds were inserted because the Earth's orbit around the Sun is slowly decaying, not because the length of the Sidereal day is changing.

No. The dominant term is the tidal friction with the moon. This causes the moon to recede at ~4cm/yr. The transfer of angular momentum slows us down.

[kilopi]

I thought leap seconds were inserted because the Earth's orbit around the Sun is slowly decaying, not because the length of the Sidereal day is changing.

No. The dominant term is the tidal friction with the moon. This causes the moon to recede at ~4cm/yr. The transfer of angular momentum slows us down.

The main thing causing us to have leap seconds is that the standard day was defined as a bit shorter than it actually is.

It is defined as the average day of 1900--and tidal slowing has had a hundred years to lenghthen the day. So, the standard day is too short by about 2 msec. This gives rise to a leap second about every 500 days.

At least, it used to.

[Odinoneeye]

I thought leap seconds was part of the Government Conspiracy to hide the rotation stoppage due to PX.

Of course, I think everything is part of the Government Conspiracy to hide the rotation stoppage due to PX.

Life is just plain easier that way.

[Powa]

Odinoneeye you are a government agent trying to lead us off track by joking about Government Conspiracy so we would laugh about it and dismiss it as a joke. But it is all true.

On the other hand I could be a government agent trying to... er...
Never mind... My head hurts.