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RocketGator
2006-Dec-13, 05:13 PM
Say, about half its current distance, how high would tides be? Would the Earth be noticably more egg-shaped?

Fazor
2006-Dec-13, 05:41 PM
As I understand it, if the moon was closer it'd have to rotate faster in order to avoid spiraling into the earth. But, lets say all the conditions are right for a closer orbit.

I don't know that the earth would be noticably much more "egg shaped", as the tide effect on the earth is realatively small (if i'm not mistaken). I think the tides would be quite a bit higher, but they would also be alot more violent due to the more rapid orbit of the moon.

Oh, and I think the earth's atmosphere would smell considerably more like moon-cheese, but i could be wrong about that. Does smell travel through a vaccuume? :)

grant hutchison
2006-Dec-13, 06:10 PM
Tidal forces vary as the inverse cube of distance. So a first approximation would be that if the moon were at half the distance, we could expect our tides to be eight times higher.
That wouldn't make a difference to the shape of the Earth that you could see with the naked eye.

Grant Hutchison

antoniseb
2006-Dec-13, 06:15 PM
As I understand it, if the moon was closer it'd have to rotate faster in order to avoid spiraling into the earth.

Grant's post is correct. Fazor doesn't understand it correctly. The Moon would still be spiralling out if it orbited 120,000 miles from the Earth.

Peter Wilson
2006-Dec-13, 07:42 PM
Title of thread should be When the Moon was closer, because moon is spiraling outward, and "billions and billions" of years ago, it was half the distance, or even closer.

Tides, so they say, were spectaculary high...except the only spectators at that time were one-cell organisms.

grant hutchison
2006-Dec-13, 08:42 PM
Grant's post is correct. The Moon would still be spiralling out if it orbited 120,000 miles from the Earth.I wonder if Fazor just means, correctly, that the moon would need to orbit the Earth more quickly if it were closer in.
Period squared varies with radius cubed, so the moon, at half its present distance, would have an orbital period 0.35 of what it is at present: a sidereal month would last 9.66 days. That would mean the Earth would need to rotate farther to "catch up" with the moon each day: the moon would rise 2.5 hours later on successive days, rather than just 50 minutes later as it does now.
Fazor, that means the tidal bulge would actually move more slowly around the Earth because of the shorter month.

Grant Hutchison

RocketGator
2006-Dec-13, 11:16 PM
What about a geosynchronous Moon?

BISMARCK
2006-Dec-13, 11:22 PM
It'd have to be further away, but that seems to be what will eventually happen.

grant hutchison
2006-Dec-13, 11:30 PM
What about a geosynchronous Moon?A geosynchronous moon would produce a tidal bulge that never moved: there would be no lunar tides, because sea-level wouldn't rise and fall, other than in response to solar tides.
BISMARCK seems to be thinking of a situation in which the Earth's rotation eventually slows to match the month: that's something a long way down the tidal evolution of the Earth-moon system, with a very slowly rotating Earth and a distant moon.

But maybe you're wondering about a moon that goes around the Earth once a day?

Grant Hutchison

RocketGator
2006-Dec-13, 11:44 PM
But maybe you're wondering about a moon that goes around the Earth once a day?

My mistake, I meant a Moon orbiting about 25,000-ish miles up. Not exactly geosynchronous, but close to it (so there is a moving tidal bulge).

The same questions as in the original post for this distance too, also.

BISMARCK
2006-Dec-13, 11:49 PM
Yeah, I was thinking about it being tidally locked.

grant hutchison
2006-Dec-14, 12:42 AM
Same story. A ninth the distance means tides 93 = 729 times higher. So there would a tidal bulge ~500m washing very slowly around the Earth: again, nothing that would make the shape of the Earth look any different to the naked eye.
The tide in the open ocean at present is said to be somewhere between half a metre and a metre, but of course we see much higher tides on some shores. These are generated by resonant sloshing of the ocean waters in enclosed areas. I presume the slow movement of the tidal bulge in your scenario would reduce the amount of such sloshing and perhaps make the tides more uniform. But maybe there would be other, different resonances, raising tides kilometres high? I dunno.
There's also the matter of whether the solid earth would have the time and flexibility to respond completely to the slower movement of the tidal potential: if the slow tides allowed the solid surface to rise and fall to the same extent as the liquid surface, then those 500m bulges would pass through without changing the water level!

Grant Hutchison

RocketGator
2006-Dec-14, 01:34 AM
There's also the matter of whether the solid earth would have the time and flexibility to respond completely to the slower movement of the tidal potential: if the slow tides allowed the solid surface to rise and fall to the same extent as the liquid surface, then those 500m bulges would pass through without changing the water level!

Grant Hutchison

That sounds totally awesome, and I'm not sure why.

Thanks! :)

TriangleMan
2006-Dec-14, 04:57 AM
Same story. A ninth the distance means tides 93 = 729 times higher.
You need to factor in that part of the tidal force we currently experience is due to the Sun. If we assume that the Sun accounts for 1/3 of the tides then tides would be 'only' be around 729*2/3 = 486 times higher.

hhEb09'1
2006-Dec-14, 05:15 AM
You need to factor in that part of the tidal force we currently experience is due to the Sun. If we assume that the Sun accounts for 1/3 of the tides then tides would be 'only' be around 729*2/3 = 486 times higher.No, that's not right. The Sun also would have to be 1/9 of its current distance in order for that to happen. If the Sun stays at the same distance it is now, then its tidal contribution will be the same, about 1/3 of what we see now.

OOPS, sorry, I didn't see what you were getting at. My mistake.

grant hutchison
2006-Dec-14, 10:04 AM
You need to factor in that part of the tidal force we currently experience is due to the Sun. If we assume that the Sun accounts for 1/3 of the tides then tides would be 'only' be around 729*2/3 = 486 times higher.Well, that solar contribution sometimes adds to (spring tides) and sometimes subtracts from (neap tides) the lunar contribution. The figure of half a metre to a metre is the mean lunar tide in mid-ocean, and that's the number that would be 729 times higher (to a first approximation, anyway). The solar contribution of quarter to half a metre would then be pretty much lost against the size of this huge lunar tide.

Grant Hutchison

Fazor
2006-Dec-14, 02:46 PM
I wonder if Fazor just means, correctly, that the moon would need to orbit the Earth more quickly if it were closer in.

Fazor, that means the tidal bulge would actually move more slowly around the Earth because of the shorter month.

Grant Hutchison


That's exactly what I meant, just had the results wrong :) But this post grew so much overnight I guess it's irrelevant now. Thanks.

publiusr
2006-Dec-22, 05:19 PM
It would be a nice idea for a TV show--to depict 500 m tides.