[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

Faster Earth rotation supporting large-animal weight?



Terry Colvin brought up the question of whether (in principle?)
a faster-rotating Earth in the past could have reduced the effective
surface gravity. Not a completely off-the-wall notion, since the
Moon is tidally braking our rotation to this day (but recall that
the Mesozioc wasn't all that long ago compared to the age of the
Earth/Moon system). However, in proposing a test Terry did fall
victim to a subtle error, the same one that Hal Clement fell for in
his Mesklin books. For an object whose shape is determined solely
by equilibrium between gravity and rotation, the effective surface
gravity is the same at all points. If it were not, the surface
fluid (and for large gravity that's anything) would flow toward the
regions of greater surface acceleration, which denies the original
assertion that this is an equilibrium situation. Look, for example,
at images of Jupiter or Saturn, which are rotationally flattened
to the tune of 10% for Saturn. If the gravitational acceleration
were greater at the pole, we'd see screaming poleward circulation, 
but the cloud bands point the other way (and since SL9 we know that the
above-cloud circulation on Jupiter does the same thing).

The net effective gravity at the surface _can_ be affected by rotation -
this is probably a way for whiite dwarfs and neutron stars to (tempoarily)
cheat the limits for gravitational collapse. But this is by now way
off topic...

Bill Keel                        Astronomy, University of Alabama