Re: Accuracy, etc.--astronomical measurements

[Leigh Palmer <palmer@SFU.CA>]

> By the same token,  the ratios of planet/Sun
> mass can be found with great precision and accuracy.

That's not true, and the degree of accuracy with which those ratios
are known is much less than the accuracy to which GM anything is
known.

If that were true, then by extension one could measure the masses of
the satellites of planets and, indeed, of artificial Earth satellites
for which very accurate orbits can be determined (e.g. the Global
Positioning System). (The latter orbits are known more exactly than
are any planetary orbits.) Planetary satellite masses were not known
until experiments were done with spacecraft.

> At this point, the
> astronomers can go no farther---  a value of G is required to convert
> from, say, solar masses to kg.  But this value of G has had to be
> determined by Earth-based experiment, e.g. that of Cavindish.  Thus the
> ratio of GM(observed) / G(Cavindish) is limited by the uncertainty in G,
> and thus astronomical masses expressed in kg are relatively poorly
> determined.
>
> A further remark:  a highly accurate value of GM is just what you want for
> sending rockets to Mars, etc.
>
> We sometimes say the physicists can do experiments, but astronomers must
> wait for them to happen.  An important distinction, I think.

That is no longer true, of course. Many scientists involved in the
exploration of space consider themselves to be astronomers.
Experimental astronomy dates back to the eighteenth century when
science at last accepted the idea that stones can fall from the sky,
and meteorites could be analyzed.

I prefer to consider all interrogation of Nature as being in one
category. I refer to this body of knowledge as empirical knowledge,
and the methods of acquisition, observation and experiment together,
as "empiry". That avoids the petty union job description battles
that ensue if one really takes that distinction to be important!

Leigh