Re: Galaxies without dark matter halos?

John Chandler <[EMAIL PROTECTED]> wrote in message
news:[EMAIL PROTECTED]
> greywolf42 <[EMAIL PROTECTED]> wrote:


> : The inclination of the orbit is one of those things that it is very
> : difficult to determine in astronomy.  We can't tell -- just by
> : looking -- whether we're looking nearly edge-on or straight
> : down onto the plane of the orbit.
>
> Actually, Kepler gave us more than the elliptical shape of orbits.
> He also discovered that the body treated as "at rest" occupies one
> focus of the ellipse and that the angular velocity varies inversely
> with the radial distance.

Historically, the latter is Newton, not Kepler.  Kepler discovered that
planets sweep out equal areas in equal time.  Similar, but not the same.

> Using those two additional properties of
> orbits allows us to tell what the inclination is.

The problem is that we don't know the 'angular velocity' of the orbit.  We
can only directly measure the radial portion of the speed projected in our
direction.

greywolf42
ubi dubium ibi libertas