Re: Galactic Gravitation...

Wayne Throop wrote:

Or, if the radius and period are correct, that's pretty direct
indication of what acceleration the sun experiences from the
galactic gravitation. For something inside the disk.

I'm not quite sure what you mean here -- the instantaneous centripetal acceleration _is_ the overall gravitational field strength at the position of the Sun, less peculiar local gravitational fields like those of the Sun's planetary system and nearby stars, both of which are negligible. So the figure you quoted -- which checks out with the distance and orbital period -- _is_ the gravitational field strength.

How much the direction of the acceleration is away from the galactic
center for something ouside the disk... well, it's not completely
negligable, but the mass of the whole disk (as I understand it) is
something like a tenth that of the halo. But the disk is much closer,
so... well, I'm not sure what the direction would be. If it were only
the disk, and if it were uniform, and the position is only a couple tens
of lightyears above it, the direction due to the disk would be pretty
much paralel to the axis of the disk. So the issue is, what's the force
of the disk compared to that of the center. Hrm. It may be more offset
than I was thinking. Can't estimate it off the top of my head though.
Can you provide numbers?

I'm not sure exactly what the numbers are; I'd have to dig through books to find out. As you say, the mass ratio between the halo and the disk is about 9:1, but gravity falls off as the inverse square. The halo gravitational field component is roughly constant at any given distance from the center of the Galaxy, since it's spherical and thus only depends on the halo mass inside it, but the component for the disk depends on your distance from it, and with 1/r^2 gravity, that won't be insignificant for points which are close to the disk, which includes much of the visible matter of the Galaxy, excluding stray stars and things like globular clusters.

But my point was, the centripetal acceleration for the Solar System around the Galaxy -- and galactic rotation curves in general -- already take this correction into account, since they're based on how galactic disk matter is rotating, rather than some theoretical notion of the total gravitational field of the halo material. (As you well know, the rotation curves are the measurable quantity here, and the flat rotation curves for pretty much all known galaxies are what led astrophysicists to conclude that there is uniformly-distributed halo dark matter to explain them.)
--
Erik Max Francis && max@xxxxxxxxxxx && http://www.alcyone.com/max/
San Jose, CA, USA && 37 20 N 121 53 W && AIM, Y!M erikmaxfrancis
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change back -- Chante Moore
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