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It is well known that the Earth's rotation is slowing down and that millions of years ago there was a point in time where there was only a mere 20 hours in a day on Earth.

My question is in two parts. Firstly why is the Earth's rotation slowing given that angular momentum is conserved, and secondly given that understanding, will the Earth either (a) stop rotating in the future, or (b) eventually begin rotating in the reverse direction to what it is now or will it (c) reach a constant rotational velocity in the same direction it is rotating now and why?

I think the answer to the question will depend on whether the Earth's rotation is slowing at a decreasing rate or at a constant rate.

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    $\begingroup$ How do you know Earth is slowing down, and that there was only 20 hours a day millions ago? $\endgroup$ – amorfis Apr 16 '14 at 23:51
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    $\begingroup$ If I recall correctly, Earth will eventually be tidally locked with the moon, with a day lasting exactly a month. But I leave it to others to answer because my knowledge is based entirely on popular science. $\endgroup$ – gerrit Apr 16 '14 at 23:55
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    $\begingroup$ The Earth's rotation is gradually slowing due to tidal drag. Oddly enough, there's a nice explanation of it (and leap seconds) on the xkcd blog: what-if.xkcd.com/26 In a nutshell, rotation wouldn't ever completely stop, the Earth would just eventually become tidally locked with the moon just like the moon is tidally locked with the Earth (i.e. the same side of the earth would always face the moon and vice versa). (I can't back enough of that up to make a complete answer, though.) $\endgroup$ – Joe Kington Apr 17 '14 at 2:04
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    $\begingroup$ This question appears to be on-topic because it fits the scope of the proposal as defined in area51. Voted to reopen. $\endgroup$ – Kenshin Apr 20 '14 at 1:20
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    $\begingroup$ Even if it falls short of "expert," this seems like a deep, complex question that considers multiple scenarios. $\endgroup$ – Tom Au Apr 20 '14 at 20:53
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The short answer is "Probably Yes".

The longer answer is that it is debatable whether Earth's rotation will become tidally locked to the moon due to tidal drag, at which point it will not be rotating relative to the moon, or whether the sun's exhaustion of hydrogen and the fusion of Helium will result in the sun's expansion vaporizing the earth first. As tidal drag slows the earth's rotation, the moon moves further away, reducing its effect on the earth's rotation, and making the calculations fairly complex.

In either case, unless something really unexpected happens to the earth first, such as it being ejected from the solar system, which is completely improbable, it will stop rotating, if only due to it ceasing to be a planet at all.

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  • $\begingroup$ Do you have rough time estimates for when these events would occur? (rotation stopping and sun consuming earth) $\endgroup$ – Phylliida Sep 4 '18 at 21:40
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Strictly speaking, the Earth will never cease to rotate in the technical sense... not while Earth is intact at least. No matter what the Earth might eventually become tidally locked with, whether the Moon or the Sun, it will be rotating, at the same rate as either the Moon's or the Sun's orbital period. For example the moon is tidally locked with the Earth, which means its rotation rate is equal to its orbital period, so it always shows (roughly) the same face, although it wobbles on a regular pattern during each orbit, showing somewhat closer to 60% of its surface area to us rather than a flat 50%.

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  • $\begingroup$ Seems to me that this is a matter of frames of reference :-) But, a fair point in an intuitive sense. $\endgroup$ – Semidiurnal Simon May 25 '18 at 8:29
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Strangely, the hypothesis (well supported) that Earth's rotation is slowing got its modern start from Immanuel Kant of all people ironically, who pointed out that tidal friction must be dissipative (we might presume by heat transfer) with geophysical consequence, which Kant claimed to be a lengthening of the day. His then novel idea was evidently supported by none other than Lord Kelvin. I am no physicist, but Earth's orbit has many nuances and influences, such as from other planets and its eccentricity, precession, axis motion in three dimensions and so forth. As the Sun burns away, it loses mass (extremely slowly) and all orbits increase, including the Moon's. So over a few billion years, a calculated effect of the role of any drift toward lunar tidal lock is exceedingly complicated. Perhaps a future supercomputed simulation will shed light on a model that is actually verifiable by physical measurements from a satellite (think ESA Hipparcos). Then we will know something about this question.

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    $\begingroup$ This all seems very speculative and doesn't actually help to answer the question. This is not a discussion forum. $\endgroup$ – bon Apr 17 '16 at 19:35
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    $\begingroup$ This answer would be improved by a reference or two, at least to Kant's Allgemeine Naturgeschichte or whatever the source is. I think it would also be useful to state categorically that your answer is "We don't know.". Split the answer into a couple of paragraphs, and I think it's a better answer than the accepted one. $\endgroup$ – kwinkunks Apr 17 '16 at 23:03

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