[extropy-chat] Astronomical question

[BillK]

On Sun, 27 Feb 2005 22:35:36 -0800, spike wrote:
> 
> So after we get tidelocked to the moon, our day length
> will be a little longer than the current lunal month
> and the moon will still look about the same as it
> does now, or perhaps a *little* less dramatic when it
> rises over the horizon while you gaze into your
> sweetheart's eyes.
> 

You have to add in the effect of the Sun tides (about half as strong
as the Moon tides).
See: <http://www.jal.cc.il.us/~mikolajsawicki/tides_new2.pdf>

Quote:
This misalignment of the water spheroid causes the net gravitational
pull exerted by the water bulges to have a small component along the
direction of the Moon motion (i.e. a tangential component along the
Moon's path). As a result, the Moon moves into a higher orbit and
hence away from the Earth, at the rate of some 4 meters per 100 years,
and the orbital period of the Moon increases. However, since the
orbital period of the Moon increases at smaller rate than the length
of the day does, both periods will eventually match. The Earth will be
then tidally locked with the Moon, and the length of the day and the
month will both be equal to some 50 present days, with the same side
of Earth always facing the Moon. Note that the same side of the Moon
already always faces the Earth, as the tidal action of the Earth on
the Moon caused the Moon's original spin to slow down, and Moon became
tidally locked with the Earth a long time ago, in the sense that the
Moon spins once on it's axis for each revolution around the Earth.

Once the Earth becomes tidally locked with the Moon, the solar tides
will tend to slow the
Earth's rotation even more, so the day will be longer than the month
and the Moon will rise
in the West and set in the East. The water spheroid generated by the
Sun will cause the high tide to appear earlier than the time of
highest moon, a situation exactly opposite to that of Fig. 2. Then the
tidal force of Earth on the Moon will pull the Moon into a lower orbit
and eventually inside the Roche limit (18500 km), whereupon the Moon
will disintegrate,
producing a ring around the Earth. 
End quote.

BillK