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[sci.astro] Solar System (Astronomy Frequently Asked Questions) (5/9)
Section - E.14.3 Is the Moon moving away from the Earth? (and why is Phobos moving closer to Mars?)

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Top Document: [sci.astro] Solar System (Astronomy Frequently Asked Questions) (5/9)
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See reader questions & answers on this topic! - Help others by sharing your knowledge
Author: Richard A. Schumacher <schumach@convex.com>,
	Michael Dworetsky <mmd@zuaxp0.star.ucl.ac.uk>,
	Joseph Lazio <jlazio@patriot.net>

Yes, at a rate of about 3--4 cm/yr.

The tidal bulges on the Earth (largely in the oceans), raised by the
Moon, are rotated forward (ahead of) the Earth-Moon line by the
Earth's rotation since it is faster than the Moon's orbital motion.

Using a similar picture as from the previous question, we'd see
(looking down from the north pole):

  Earth 					Moon
    ____	
   /    ) 					___	    ^
  /    /				       /   \	    |
 (____/ 				       \___/	 Moon's orbit &
                                                         Earth's rotation
 (Ocean) Tidal bulge				         this way
*greatly* exaggerated.

The gravity from these leading and trailing bulges impels the Moon
mostly forward along the direction of its motion in orbit (the Moon's
orbit is not exactly in the plane of the Earth's equator).  This force
transfers momentum from the rotating Earth to the revolving Moon,
simultaneously dragging the Earth and accelerating the Moon.

In addition to causing the Moon to recede from the Earth, this process
also causes the Earth's rotation to slow and days to become longer (at
a rate of about 0.002 seconds every century).  Eventually the result
will be that the Earth will show only one face to the Moon (much like
the Moon only shows one face to the Earth).  A lower limit to how long
it will take for the Earth and Moon to become tidally locked is 50
billion years, at which point the month and the Earth's "day" will
both be approximately 50 (of our current) days long.  However, this
estimate is based on the assumption that liquid water seas would be
present on Earth's surface to provide the tidal interactions
necessary.  But as the Sun evolves, the seas will evaporate and tidal
interactions will be much slower (solid planet distortions only).  The
oceans will evaporate about 1--2 billion years from now, so the actual
time will probably be much longer.

Considerably more detail on the evolution of the Earth-Moon system can
be found in an article by J. Burns in the book _Planetary Satellites_
(ed. J. Burns [Tucson: University of Arizona]) and in Sir Harold
Jeffries' _The Earth_, 3rd ed (Cambridge Univ Press, 1952).

It is also interesting to consider what would happen if a satellite
orbits its planet *faster* than the planet rotates.  This is not the
case for the Earth and Moon, but it is true for Mars and Phobos.  In
this case, Phobos also raises (crustal) tides on Mars.  But now,
Phobos is in front of the tidal bulge, so the gravitational action of
the tidal bulge slows Phobos and Phobos moves *inward*.  Thus, at some
point in the future, Phobos will hit Mars.  The most recent estimate
is that the impact will occur in 40 million years, by A. T. Sinclair
(1989, Astronomy & Astrophysics, vol. 220, p. 321).

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Top Document: [sci.astro] Solar System (Astronomy Frequently Asked Questions) (5/9)
Previous Document: E.14.2 Why does the Moon always show the same face to the Earth?
Next Document: E.14.4 What was the origin of the Moon?

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