Re: [Phys-l] New gravitational constant

[John Denker <jsd@av8n.com>]

On 01/22/2008 10:35 AM, LaMontagne, Bob wrote:
> I am always puzzled by the large uncertainties in G, given that we can
> measure the speed of interplanetary probes so accurately. When a probe
> does a fly-by of a planet, one can calculate the expected speed of the
> probe after the encounter. 

A flyby will tell you the value of the product GM, quite
accurately.  But if you don't know M, you don't know G
and vice versa.

As a corollary, we can accurately tell you the mass of the 
earth as measured in "solar masses" but not so accurately
as measured in kg.  And in astronomy the "solar mass" is 
in fact widely and routinely used, for this reason.  You
can compare astronomimcal masses and G drops out of the 
comparison.

On the other horn of the dilemma, the prototype kilogram
in Paris is too small for a convenient flyby.