Jurassic Sidereal Day
Name: Jerry E.
Date: May 2001
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
During the Jurassic period what was the length of a sidereal
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.?
research material on this subject is available?
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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