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Astronomy and Cosmology
Astronomy and Astrophysics
What Could Cause the Earth's Rotation to Dramatically Slow Down?
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[QUOTE="Ken G, post: 6860984, member: 116697"] That is an excellent article, containing a chart that shows clearly everything we are talking about. It also justifies your statement that the spin (of the surface) has actually sped up a little, even though we have not removed any leap seconds, because the spinup did not persist long enough to span the interval between when leap seconds are applied. The chart also shows what I've been pointing out, which is the big difference between secular advances (the yellow trend in the chart) and the oscillation/fluctuations (the green curve showing the instantaneous spin rate). So that chart really puts things in perspective, my point is that I believe we should not expect anything that can increase the spin to come from external astrophysical sources, the fluctuations seem more likely to be internal to the Earth (especially its internal or atmospheric rotation rate as compared to surface rotation, and anything that can change its moment of inertia). Moreover, if there are changes that are of astrophysical origin, they should probably be a periodic cycle rather than a secular trend. One obvious signal to look for is the monthly period in which the Sun's tidal forces either assist or resist the spindown effect of the Moon, and I think we see that in the Wiki article you cited, but they average out so quickly they don't really matter. The culprit does seem to be angular momentum exchange with the atmosphere, but we must not forget the curious result from the last few years that there is less angular momentum being contained in the spin of the core. That acts the wrong way, as it would transport angular momentum out and seem to increase the surface spin, but we must also account for changes in the rotational inertia. I wouldn't be quick to rule out internal changes within the Earth, there's certainly a lot more angular momentum and rotational inertia in there than there is in the atmosphere! But that Wiki article does clearly hone in on atmospheric effects as the cause of the change. I would say it's even possible that we have another indicator of global climate change there, as it seems the downward trend is likely to correlate well with CO2 concentration. I was going to suggest that warmer air might lead to an effect on the way to Venus, but that works in the wrong direction-- as you point out, Venus' atmosphere carries the opposite sign of angular momentum, so if our atmosphere was starting to act in that way, it would speed up our surface rotation, not slow it down. What is very cool about all this is that it's not clear if the slowing trend will continue for decades into the future. But if it does, that will really give an interesting challenge to astronomy classrooms, who have to explain why the Moon's orbit is getting farther away with time because of the slowdown of the Earth's spin, but oh by the way, the Earth's spin isn't actually slowing down lately! (I can't wait to see what the Young Earth folks will make of that seeming contradiction...) [/QUOTE]
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What Could Cause the Earth's Rotation to Dramatically Slow Down?
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