Re: [LEAPSECS] War of the Worlds

From: John Cowan <cowan_at_CCIL.ORG>
Date: Wed, 11 Jan 2006 23:44:13 -0500

Rob Seaman scripsit:

> I don't have an envelope large enough, but there are various issues
> to consider. The Hurtling Moons of Barsoom are much smaller than our
> own and should have a negligible tidal breaking effect. (See http://
>, for instance, for their
> interesting history.) And do the Earth's oceans mediate our Moon's
> breaking or is that a crustal phenomenon? (The Earth-Moon system
> should better be regarded as a double planet, than planet and
> satellite.) On the other hand, Mars passes much closer to Jupiter,
> the 800 pound gorilla of the solar system, but then it is further
> from King Kong - the Sun, that is - and tides are an inverse cube
> effect. But Mars is much smaller and has a smaller moment of inertia
> in the first place - but then Mars is much smaller and the "lever
> arm" to grapple with it is less pronounced.
> Taken all together, one suspects that LOD(Mars) is many orders of
> magnitude more constant than LOD(Earth). One would not be
> flabbergasted to be utterly wrong, however.

I wouldn't be too quick to dismiss tidal braking from Phobos. It's
awfully close to Mars, and tidal braking is as you say an inverse-cube
effect. The formula (kai Wikipedia) is (2GMmr)/R^3, where M and m are
the masses, r is the radius of the primary, and R is the orbital radius
of the secondary. The mass of the Earth-Moon system is eight orders of
magnitude larger than the Mars-Phobos system, and the radius of Earth
is only twice the radius of Mars, but the ratio of the cubed orbital
radii is five orders larger for Phobos than for the Moon. So the tidal
acceleration of the Moon toward the Earth is only some three orders larger
than Phobos's toward Mars. That puts the effect in the same ballpark.
How much difference in actual slowing can be attributed to Earth's ocean
and Mars's lack of one I don't know.

(See for
the relevant masses and radii.)

Eric Raymond is the Margaret Mead               John Cowan
of the Open Source movement.          
        --Bruce Perens,               
          some years ago              
Received on Wed Jan 11 2006 - 20:46:13 PST

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