If the Earth stopped rotating...

<blockquote id="quote"><font size="2" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote"><i>Originally posted by Astrodelugeologist</i>
<br />If the Earth suddenly stopped rotating, what effect would that have on the orbit of the moon, if any? Would the moon's tidal interaction with a suddenly motionless Earth alter its orbit?<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">The exact answer depends on your definition of "stopped rotating". Do you mean that Earth would keep the same face toward the Moon, as the Moon does toward Earth? Or do you mean no rotation with respect to the distant stars?

Either way, not much happens to the Moon in the short term. But the Moon's slow recession from Earth at the rate of about 1 cm/yr would cease. If Earth always faced the Moon, tides on the Earth would end, so tidal interchanges with the Moon's orbit would also end.

It would be more interesting for us if Earth stopped rotating with respect to the stars. Then lunar tides would have a week to build up instead of just six hours. A 1-meter tide would likely become a 30-meter tide sweeping in at the rate of about 30 miles/hour -- a speed that humans could not outrun. Continent outlines would become completely redefined. As for the Moon, the tidal acceleration would become a deceleration, and the Moon's orbit would start shrinking instead of expanding. -|Tom|-

Tom,

When I said "stopped rotating", I was thinking of the Earth keeping the same face towards the Sun (though now I realize that this would be a one-year rotation period for the Earth).

What would happen to the Moon's orbit in this case? And would the Earth's own orbit be affected?

--Astro

<blockquote id="quote"><font size="2" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote"><i>Originally posted by Astrodelugeologist</i>
<br />What would happen to the Moon's orbit in this case? And would the Earth's own orbit be affected?<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">The second question is easiest. Tidal forces between Sun and Earth are negligible, and would still be under the hypothesis posed. The Sun must have a much larger radius than at present for tides to be significant. During the Sun's "T-Tauri" phase when the planets first formed, its radius was much larger -- perhaps by two orders of magnitude soon after Venus and Earth were formed. Orbital tidal acceleration varies with roughly the 7th power of (distance divided by solar radius), so the Sun's tidal effect on Earth's orbit has decreased by perhaps 14 orders of magnitude since that time.

As for the Moon's orbit in today's circumstances but with Earth rotating once per year, that would allow ocean tides to flow in one direction for three months at a time. That would result in quite massive pile-ups of water against the continents. But the effect of those tides on the Moon would average to zero. Only the tides raised by the Moon could produce a consistently lagging bulge on Earth that would slowly rob the Moon of angular momentum and cause its orbit to shrink. But the rate of that shrinkage would be very small, even as compared with today's rate of expansion of the Moon's orbit.

BTW, the principles of tidal theory I am using to figure out these answers are the ones explained in chapter six of <i>Dark Matter, Missing Planets and New Comets</i>. -|Tom|-

<blockquote id="quote"><font size="2" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote">It would be more interesting for us if Earth stopped rotating with respect to the stars. Then lunar tides would have a week to build up instead of just six hours. A 1-meter tide would likely become a 30-meter tide sweeping in at the rate of about 30 miles/hour -- a speed that humans could not outrun. Continent outlines would become completely redefined. As for the Moon, the tidal acceleration would become a deceleration, and the Moon's orbit would start shrinking instead of expanding. -|Tom|-<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">

Haha"Interesting". thats one way to put it.
Which takes me to another question, is there anyway we know of that could stop the earths rotation, Could an planetary body in close proximity make such drastical changes as to stop the rotation totaly?

<blockquote id="quote"><font size="2" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote"><i>Originally posted by PheoniX_VII</i>
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is there anyway we know of that could stop the earth's rotation, Could a planetary body in close proximity make such drastical changes as to stop the rotation totally?<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">It would require a very close approach lasting a very long time to remove all Earth's spin, which represents an enormous pool of energy. A well-placed collision by a Mars-sized body might do it, but nothing in the biosphere would survive. -|Tom|-

Tom,

Wat do you think the effect will be of a Space Elevator on the rotation of the Earth? I've heard disturbing stories that such a massive elevator could change the rotation speed, thereby influencing the delicate balance of seasons etc.