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On the spin history of Earth

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If you have a question about this talk, please contact Cleo Loi.

The semimajor axis of the Moon is increasing at the rate of 3.82cm per year, a result of the Lunar tides. The same tides reduce Earth’s spin angular momentum S while increasing the Lunar orbital angular momentum L. The smaller Solar gravitational torque also reduces S, in this case transferring the angular momentum to the orbital angular momentum of the Earth. The Solar gravitational tide is opposed by the Solar thermal tide, which exerts a torque T_th that tends to increase S and decrease the orbital angular momentum of Earth. Currently, T_th is smaller than the Lunar tidal torque by a factor of about 25. Global circulation models (GCMs) show that there is a resonance in the Earth’s atmosphere at a period of about 22 hours, roughly the length of day in the Cambrian, enhancing T_th by a factor of about 2. I will show, using geologic evidence, that the Lunar torque T_L was a factor of approximately 5 smaller in the Precambrian. I will also use the geologic record to show that the combination of larger thermal torque and reduced Lunar torque, acting over at least 2.5 billion years (Gyrs), produces an easily detected change in the number of days per month (a proxy for S + L) compared to that expected if only gravitational tides act. These calculations suggest it is unlikely that the thermal torque ever exceeded the magnitude of the lunar over the last 3 Gyrs, so that the length of day has been slowly increasing over that entire time.

This talk is part of the DAMTP Astro Mondays series.

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