Re: Yes, I'm back

Christophe Grandsire writes:
 > En réponse à Peter Clark <peter-clark@...>:
 > >         The second is a "simple" matter of physics. If you assume
 > > that the star has about the same mass as the sun, you can apply
 > > Kepler's third law to its distance from the sun and base your
 > > calculation off of Earth's orbit.  However, if the mass of the
 > > star in question is different, then you'll need some slightly
 > > more complicated math.
 > >
 >
 > Why? Kepler's third law *includes* the Sun's mass in its
 > formula. Replace it with the star's mass and you have it.
 >

No it doesn't.  Kepler didn't know the mass of the sun.  All Kepler's
third law states is that the periods of the planets are proportional
to the 3/2 powers of the major axis lengths of their orbits.  You
can't relate that constant of proportionality to the mass of the sun
until you have Newton's law of gravitation.  (Okay, you can still call
the derived Newtonian version "Kepler's third law" if you like, but
Kepler never wrote it, and it's not what Peter was referring to.)

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