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SUMMARY:On the spin history of Earth  - Norm Murray (CITA)
DTSTART:20190204T140000Z
DTEND:20190204T150000Z
UID:TALK118840@talks.cam.ac.uk
CONTACT:Cleo Loi
DESCRIPTION: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 moment
 um 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 orbi
 tal angular momentum of Earth. Currently\, T_th is smaller than the Lunar 
 tidal torque by a factor of about 25. Global circulation models (GCMs)  sh
 ow that there is a resonance in the Earth's atmosphere at a period of abou
 t 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 Lun
 ar 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 billio
 n 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 gravitati
 onal 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. 
LOCATION:MR14\,  Centre for Mathematical Sciences\, Wilberforce Road\, Cam
 bridge
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