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Is the speed that the Earth spins on its axis constant over the course of 24 hours? As opposed to does it turn faster and slower during different hours over any given day even by a very small variation. Who has made such measurements and how were they made?

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    $\begingroup$ Leaving the moon and other external sources aside, the moment o inertia is ruled by how the mass of earth shiftes to and fro its center. A major earthquake may shift enough mass to alter the length of day by a few microseconds. Summer lengthens the day by an infinitely amount, as there are more trees in the northern hemisphere, growing leaves some few meters above t ground and then shed them to ground level. Psibly (citationNeeded) there are also very, very,very minor effects on heating and cooling of air, unequal to the hemisph, that cause air mass t extend far o closer to center o gravity. $\endgroup$ Jan 7 at 22:33
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    $\begingroup$ xkcd.com/162 $\endgroup$
    – John Doty
    Jan 8 at 0:20
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    $\begingroup$ The Earth's rotation is monitored and documented by the International Earth Rotation and Reference Systems Service (IERS). See iers.org/IERS/EN/Science/EarthRotation/EarthRotation.html & en.wikipedia.org/wiki/… $\endgroup$
    – PM 2Ring
    Jan 8 at 0:32
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    $\begingroup$ cambridge.org/core/services/aop-cambridge-core/content/view/… $\endgroup$
    – BowlOfRed
    Jan 8 at 16:54
  • $\begingroup$ Related: The Shortt–Synchronome clock in 1926 was the first clock accurate enough to detect seasonal changes in Earth's rotation rate. $\endgroup$
    – qwr
    Jan 9 at 4:24

2 Answers 2

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The rotation of the Earth (~73 µrad/s) does speed up and slow down by a few picoradians per second during the day because of solar/lunar tidal forces acting on the asymmetric distribution of the Earth's mass, and these variations can now be observed in real time by ring laser gyroscopes.

Because of angular momentum conservation, the Earth's rotation changes whenever its moment of inertia changes, which can happen because of earthquakes rearranging the tectonic plates, or more regularly because of earth, ocean, and atmospheric tides. The Earth's mass is not precisely spherically symmetric, e.g., the continents and oceans are asymmetrically distributed, so the size the Earth's tidal bulges (and hence its moment of inertia) changes depending on which parts of the Earth are closest to the Sun or Moon. This causes semidiurnal (~12 hour) and diurnal (~24 hour) changes in the Earth's rotation speed due to tidal cycles and a daily wobble in the direction of the Earth's axis.

As discussed in a recent article on "Variations in the Earth’s rotation rate measured with a ring laser interferometer", the best modern instruments can resolve 12 prad/s in just a one second measurement and are sensitive down to a fraction of a prad/s for changes with periods of a few hours. The expected variations in the Earth's rotation speed with periods of ~12 and ~24 hours are seen.

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The answer is that it depends on how technical you want to get.

At first approximation, no, the speed of the Earth's rotation is constant.

Getting more technical, the speed of rotation of the Earth depends on a lot of different factors: tides, distributions of atmospheric and oceanic masses, etc... A big factor in the Earth's rotation (and orbit) is the interaction with the Moon. This is known as tidal braking or tidal acceleration. Halley was the first to suggest this with considerations of the Earth's orbit and Laplace was the first to give a theoretical framework for changes in the Earth's orbit due to this interference. I know that this isn't exactly to Earth's rotation, but it is very much related.

The result from all of this is that the rotation of Earth is slowing, mainly due to this effect. Days are longer by around 2 milliseconds since the last century.

Another less important cause of Earth's rotational retardation is due to tidal acceleration from heat dissipation; however this is a very small effect.

Some of the measurements regarding the rotation speeds of Earth are made though very-long-baseline-interferometry (VLBI). Very briefly, telescopes in multiple different places (on the surface or in space) receive radio waves from natural source deep in space. The time differences in when these telescopes received these sources can be used to make quite a few measurements regarding Earth's natural parameters including LOD fluctuations as well measurements regarding Earth's surroundings.

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    $\begingroup$ The LOD fluctuations link is very informative. Thank you for addressing my question. It indicates there are fluctuations occurring even within several weeks. $\endgroup$ Jan 7 at 15:12

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