Re: [Phys-l] earth's rotation

[John Mallinckrodt <ajm@csupomona.edu>]

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