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Thread: Measurement of Earth's slowing rotational speed

  1. #1
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    Measurement of Earth's slowing rotational speed

    It is said that a slowing of the Earth's rotation could provoke earthquakes next year.

    How do the scientists measure the length of an Earth day (as opposed to a sidereal day) so accurately as to detect this slowing?

    Or do they measure it directly with eg as in the Michelson-Gale experiment?

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    Quote Originally Posted by wd40 View Post
    It is said that a slowing of the Earth's rotation could provoke earthquakes next year.

    How do the scientists measure the length of an Earth day (as opposed to a sidereal day) so accurately as to detect this slowing?

    Or do they measure it directly with eg as in the Michelson-Gale experiment?
    First of all, who are you talking about here? I am not going to follow a blind link to what might be a woo site.

    The rotation period can be timed with great precision by watching the transits of stars. Over short times, meaning decades, short term fluctuations mask the long term slowdown. Determining the latter requires good records over a very long time. The rotation actually speeded up for a while during the 20th century, as shown by the reduction in the frequency of leap seconds needed to keep it roughly in step with atomic time.

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    "Upsurge in big earthquakes predicted for 2018 as Earth rotation slows"
    https://www.theguardian.com/world/20...ig-earthquakes

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    Quote Originally Posted by Hornblower View Post
    First of all, who are you talking about here? I am not going to follow a blind link to what might be a woo site.
    Hey, it's a respected something in the Indian subcontinent.Um, well, the India.com About Us:

    India.Com brings together partners in ZEE, PMC, and United Internet that own and operate successful content brands and distribution businesses across television, print, and the internet, and have technology.
    See? They have technology, so you know they must be suppliers of accurate information and not some clickbait factory. Don't misread me; I am not a regular reader.

    No disagreement about the rest of your answer!
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    Skepticism enables us to distinguish fancy from fact, to test our speculations. --Carl Sagan

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    But do they know diddly doink about geology?

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    Good point.

    OP:

    Wikipedia: Earth's rotation :: Measurement

    Measurement
    The permanent monitoring of the Earth's rotation is performed with very-long-baseline interferometry coordinated with the Global Positioning System, satellite laser ranging, and other satellite techniques. This provides an absolute reference for the determination of universal time, precession, and nutation.
    Ka-ching.
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    Skepticism enables us to distinguish fancy from fact, to test our speculations. --Carl Sagan

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    This article (by Erik Klemetti, an igneous petrologist) takes a more measured view of the prediction.

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    Quote Originally Posted by Eclogite View Post
    This article (by Erik Klemetti, an igneous petrologist) takes a more measured view of the prediction.
    Nice find Eclogite.

    And here is the abstract for the talk given at the GSA meeting.
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    Quote Originally Posted by Swift View Post
    Nice find Eclogite.

    And here is the abstract for the talk given at the GSA meeting.
    A well written abstract and a plausible argument. I have two reservations:
    • Correlation is not causation
    • One suspects the stresses associated with terrestrial angular acceleration are of the same order of magnitude as tidal stresses, yet no correlation has ever been found between tidal maxima and seismicity.

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    Quote Originally Posted by wd40 View Post
    How do the scientists measure the length of an Earth day (as opposed to a sidereal day) so accurately as to detect this slowing?
    As opposed? Wouldn't they be the same thing? Or, what distinction do you see there?

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    A sidereal day is the time for a star overhead to return to its exact position. It is 236 seconds shorter than a 24 hour solar day, which is the time for the Sun to return to its position.

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    Quote Originally Posted by wd40 View Post
    A sidereal day is the time for a star overhead to return to its exact position. It is 236 seconds shorter than a 24 hour solar day, which is the time for the Sun to return to its position.
    Actually, 235.90 seconds

    But that's what I mean, the measured slowing is in the sidereal day

    https://forum.cosmoquest.org/showthr...y-sidereal-day

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    Quote Originally Posted by wd40 View Post
    A sidereal day is the time for a star overhead to return to its exact position. It is 236 seconds shorter than a 24 hour solar day, which is the time for the Sun to return to its position.
    This is not an issue with respect to measuring variations in the spin rate. The sidereal day and the mean solar day are just two different expressions for the spin rate, in different coordinates.

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    I think the idea is not so much that the change in spin itself induces stresses (which might indeed be on the scale of tidal force variations), but rather, the change in spin is associated with stresses. All the verbiage about "triggering" quakes is from journalists who did not read the research carefully! There is not a claim that the change in spin is causing the quakes, only that they are presaging them. So it only requires correlation, and indeed the cause can be unknown but presumably stems from whatever is the source of changes in magma patterns (the authors speculate on some possibilities, but this has nothing to do with the validity of the correlation). If the correlation holds over the next few years, it would certainly be a huge step forward in global earthquake prediction. It's hard to hope that it will hold true though-- the Caribbean has had it tough enough as it is already.
    Last edited by Ken G; 2017-Nov-24 at 04:39 PM.

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    Is rotational motion considered an absolute in relativity? Is there any way that the speed could be found to be slowing wrt the sidereal day but not wrt the solar day, or vice versa?

    Quote Originally Posted by Hornblower View Post
    This is not an issue with respect to measuring variations in the spin rate. The sidereal day and the mean solar day are just two different expressions for the spin rate, in different coordinates.
    Last edited by wd40; 2017-Dec-06 at 12:37 PM.

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    If we slow down the rotation relative to the stars, then to keep the ratio to the length of the year unchanged we would have to slow down the Earth's angular velocity about the Sun, perhaps by having the Sun expel some matter in increased solar wind to reduce its gravity. We could in principle do that in a thought exercise, and perhaps it could really happen during the Sun's future red giant stage, where it will lose a lot of mass.

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