Newsgroups: sci.space.history
From: Henry Spencer <henry@zoo.toronto.edu>
Subject: Re: Earliest Satellite Proposal?
Date: Tue, 11 Nov 1997 16:52:00 GMT

In article <6433i0$7ks@lace.colorado.edu>,
Frank Crary <fcrary@rintintin.Colorado.EDU> wrote:
>>the first measurement of the Gravitational constant was in 1798, by Henry
>>Cavendish.
>
>The measurement of the gravitational constant isn't necessary for satellites.
>Newton calculated GM, the gravitational constant times the mass of the Earth,
>and that's all you need for objects in orbit around the Earth...

Side note on this:  it's also the *preferred* form of the data for work
with orbits, because it can be measured precisely.  The gravitational
forces produced by objects which can be manipulated in the lab are so weak
that measuring G to high precision is very difficult.  A single spacecraft
flyby will measure a planet's GM to seven decimal places or so, but after
two centuries of trying, G is known to only about four.
--
If NT is the answer, you didn't                 |     Henry Spencer
understand the question.  -- Peter Blake        | henry@zoo.toronto.edu