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Name: Jerry E.
Status: Other
Age: 50s
Location: N/A
Country: N/A
Date: May 2001


Question:
The Earth's rotational rate is decreasing slightly each year. Eons ago, around the time the moon appeared, the rotational rate was reportedly around 3 times faster than today (8hr sidereal day).
During the Jurassic period what was the length of a sidereal day?
What effect did the shorter but more frequent days have on plant and animal life?
How did it effect average temperatures, winds, ocean currents, plant and animal growth rates, rate of water evaporation, weight (centripetal force), tectonic plate movements, etc.?
What research material on this subject is available?



Replies:
Jerry,

Assuming that the rotation rate of Earth was greater, although the amount greater could be debated, there would be some effects on weather patterns and thus on climate.

A rotation rate as large as you suggest, 3 times the present, would surely spawn more energetic ocean currents and weather patterns, with greater vorticity (spin rate). Low pressure systems would have been more intense, high pressure systems deeper but perhaps smaller, wind speeds would have been higher, ocean currents stronger and more far reaching, and all of these systems would have moved around the Earth faster. Hurricanes, if they could exist, would have been more devastating.

However, this scenario would also have meant that the transfer of energy from the tropics to the poles (both by ocean currents and by the atmosphere) would have been more efficient, probably resulting in the global climate being less diverse; the poles would probably have been warmer than today and the tropics perhaps slightly cooler.

However, again, all of this may have been tempered by the shorter day length, which would not allow the Earth to warm as much while exposed to the Sun or to cool as much when in shadow. It would require a model to be able to get an idea if having three "days" within what is now one day would have made a difference in global temperature or seasons, evaporation and heat exchange between the Earth and the atmosphere, and thus what effects there would have been on plant and animal life.

During the period of the Earth we are speaking of, it is generally assumed that the land masses were not in the same place or the same shape or size; so, ocean currents and atmospheric motions could have been significantly different from what they are now as a result of these differences alone.

Tectonic motions surely would have been enhanced, resulting in ,perhaps, more and stronger earthquakes and greater volcanic activity.

Since centrifugal forces would have been greater, and assuming that the mass of the Earth was the same, everything would "weigh" slightly less, as gravity would be slightly overcome. Gravity has less effect at the equator (0.0035%) than at the poles because there is no centrifugal force at the poles. If we consider the effect of an Earth spinning 3 times as fast, at the equator the radial velocity of the Earth would be three times what it is now (1395 m/s compared to the present value of 465 m/s). The centrifugal acceleration is determined as the square of the radial velocity divided by the Earth's radius (~6400 km). The faster spinning Earth decreases our "weight" by only 0.028% at the equator, not enough to notice. However, the increased rotation rate would slightly increase the diameter of the Earth at the equator and slightly decrease the diameter at the poles.

David R. Cook
Atmospheric Research Section
Environmental Research Division
Argonne National Laboratory



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