The nineteen-hour day

In more boring-billion news: the day was nnnnnineteen hours. Over the whole 2000-800 megayear span postHuronian.

We have some good calculation of, say, the 23:30 hour day in the late Cretaceous; but these records get sparse before all those trilobites. For the Precambrian, Ross Mitchell et al. were working with Milankovitch cycles, in this case precession and obliquity. It seems these data appear in the fossil-record, at least recently. Thence calculation of solar tides.

Earth did have an ocean back then. However when this blue marble was spinning 19 hours (we already knew it was spinning at 19 hours at a point) the tides are weaker. They were weaker still when this was a white marble, as in Cryogenian. Solar tides are in the realm of Milankovitch and these were stronger. So the sun pulled, as the moon was dragging (weakly): they canceled out Earth's spin. That point turned out to be the end of the boring-billion span.

This was already predicted in theory but, you know - cool theory bro. There wasn't the fossil record for it; other constraints could have counteracted Milankovitch. Now we know they didn't.

I suppose Milankovitch didn't cancel the moon's rise to a higher orbit. I'd thought that would have made the lunar tides even weaker. But the solar tides got weaker first. At 800 Mya the resonance broke (before Sturtian / Cryogenian) and, since then, the moon has been rising in pure concert with the day's lengthening.

On to constraining Theia (for real) now?