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Originally Posted by JohnD
All,
There is a lot of confused thinking about this (mine and the Bad Astronomer!)
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I disagree that the BA's explanation is confused
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I say that the BA is confused/confusing because his explanation starts with the point that the earth is so big that the Moon's gravity diminishes across its the diameter. This is clearly so, but he bases his argument on this effect, when "Tides" are a universal effect. The size of the satellite is irrelevant.
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Just as the gravitational effect is proportional to GMm/r^2, the tidal effect is proportional to GMd/r^3, where d is the diameter of the small body. So, the size is relevant.
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Originally Posted by Eroica
This means that in explaining a tidal bulge you can't cavalierly ignore the Earth's rotation. The point on the Earth's surface furthest from the Moon (ignoring the Sun for the moment) is moving through the spacetime continuum in a certain direction and at a certain speed. It should make no difference whether this movement is caused by the Earth's motion through space or its rotation on its axis.
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Actually, the rotation of the Earth would produce a bulge all around the Earth that is not related to tides at all. By focussing on the effect of that movement only on the near and far side, you are ignoring the effect on the sides.
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Originally Posted by [url=http://www.badastronomy.com/phpBB/viewtopic.php?p=155142#155142
Eroica[/url]]What is wrong with this interpretation? Is it not a perfect description of your different orbits theory?
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I agree that that more or less describes JohnD's argument--but it does not match Sawicki's.
Notice, in that argument though, that the Earth is rotating once per revolution. If you get rid of some of the rotation, you should get rid of all of it. Once you do, however, and you trace the path of each point, you find that every point on the Earth traces a similar path with the same radius. Each point experiences the same velocity and acceleration (not just in magnitude but even as vectors).
The only difference, from side to side, is the strength of gravity, as the BA says. Or, if you insist on using the more modern interpretation, the slope of the curvature of spacetime.