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Re: [Phys-l] earth's rotation

Apologies. The day would continue to get longer as the Earth's rotation
slows. I forgot angular velocity and acceleration were both reversed
(instead of just one) when I said days would get shorter.

But in the reversed-Earth scenario the Moon would be closer than it is
and Moon-caused tides would be stronger than they are now. The stronger
tides possibly would have affected life and the rate of evolution.

Thanks for the correction, John.

On a side note, I'm not sure what makes mentioning this aspect more
brave than, say, wind patterns, Hawaiian weather, or developing a
rigorous vector-related definition of "reversed." But I'm new to the
list and have only been reading since December.

Perhaps I misinterpreted the question. I assumed the question was "How
would the situation on Earth have been different if the Earth had always
been spinning oppositely?" while others may have interpreted the
question to be "What would happen if the Earth suddenly switched
rotation directions today?"

David Vakil
Astronomy & Physics Professor
El Camino College
310-660-3134

-----Original Message-----
From: phys-l-bounces@carnot.physics.buffalo.edu
[mailto:phys-l-bounces@carnot.physics.buffalo.edu] On Behalf Of John
Mallinckrodt
Sent: Monday, January 07, 2008 10:10 AM
To: Forum for Physics Educators
Subject: Re: [Phys-l] earth's rotation

David,

You're a brave man for talking about tidal bulges around here, but
you raise an interesting point.

It seems to me, however, that the effect would still be to make the
days longer for awhile (a LOOOOONG while) until the Earth is
essentially in (temporary) synchronous rotation about the Sun with an
attendant (semi)permanent day or night. Then the days would begin to
grow shorter again as the Earth begins to rotate in the opposite
direction (i.e., the current direction). The process would
eventually conclude with the Earth and moon locked in synchronous
rotation.

A quick calculation suggests that the final orbital radius of the
Moon would be about 200 thousand km (about half its current value)
and that the length of the day (and month) would be about 11
(current) days.

John Mallinckrodt
Cal Poly Pomona

On Jan 7, 2008, at 8:58 AM, Vakil, David wrote:

One important change in the Earth-Moon system hasn't been
mentioned. If
the Earth's rotation changes directions but all other orbital
parameters
(e.g. Moon's orbit) don't change, tides would change the nature of the
Earth-Moon system.

Currently, the tidal bulges on Earth rotate ahead of the Moon,
relative
to the Moon's revolution around the Earth because a day is shorter
than
a month. Over the long term, this leads to the Moon slowing the
Earth's
rotation down as it pulls the tidal bulges back towards alignment. The
slowing occurs primarily via friction between water and the ocean
floor.
Similarly, these tidal bulges cause the Moon to recede from the Earth
approximately 4cm/yr. Over approximately 100 million year, this
process
has slowed the Earth's rotation from 22hr/day to the current 24hr/day,
and has increased the size of the Moon's orbit accordingly
(conservation
of angular momentum).

Reversing the direction of the Earth's rotation and not the Moon's
revolution direction would reverse this effect. Days would get shorter
and the Moon would get closer. Over a few hundred million years, this
would have a noticeable effect on both the Earth and the Moon. For
example, tides would be stronger. This could significantly alter
coastal
areas, independent of weather or climate patterns.

Neptune's moon Triton is experiencing this effect because its orbit is
retrograde. Mars' Moon Phobos is also getting closer to Mars
because its
revolution rate is faster than Mars' rotation rate. It will actually
crash into Mars or be tidally disrupted in approximately 50 million
years.

David Vakil
Astronomy & Physics Professor
El Camino College
310-660-3134
_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l

A. JOHN MALLINCKRODT
Professor of Physics, Cal Poly Pomona
http://www.csupomona.edu/~ajm
Acting Editor, AMERICAN JOURNAL of PHYSICS
http://www.kzoo.edu/ajp

Professional/Personal email: ajm@csupomona.edu
Journal-related email: ajp@csupomona.edu
Phone: 909-869-4054
FAX: 909-869-5090

Physics Department
Building 8, Room 223
Cal Poly Pomona
3801 W. Temple Ave.
Pomona, CA 91768-4031


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