Re: Accuracy, etc.--astronomical measurements

[Leigh Palmer <palmer@SFU.CA>]

> At 1:52 PM -0700 9/3/99, David Roberts wrote:
>
> In pre-Sputnik 1955, C.W. Allen in his _Astrophysical Quantities _ gave
> masses for the moon, the four Galilean satellites of Jupiter and 15 of the
> larger satellites of the outer planets to 2 or 3 sig. fig.  In all cases,
> wobbles of the primary planet, due to the orbiting satellite, could be
> observed.
>
> >                        Msun / Mp
> > Merc                 6023600
> > Ven                  408525.1
> > Earth                332946.043
> >    Earth + Moon         328900.555
> > Mars                 3098710
> > Jup                  1047.3492
> > Sat                  3497.91
> > Uran                 22902.94
> > Nep                  19434
> > Pl                         13 x 10 e 7
> >
> > Lang attributes these to J. Myles Standish, Jr. (1988) at JPL.  The large
> > number of sig. fig. quoted reflect the fact that these values depend on
> > the hundreds or thousands of position and time measurements needed to
> > establish accurate values of the orbital elements.

to which I responded:

> By 1988 all the planets had been visited by spacecraft which made
> very accurate mass determinations possible by doppler measurements.
> Of course the results of such measurements could be most accurately
> expressed in terms of a mass ratio, and the Sun's mass is standard.
> I believe that all of them are thought to be more accurate than the
> sort of analysis of conventional positional astronomy you suggest,
> but I would have to check the literature. My Lang is at home, and
> I probably don't have the journal in question (likely Icarus - lost
> in our library's budget cutting in 1985) at hand, but I would be
> glad to follow it up (in the fullness of time) if you haven't found
> it there at Bowdoin by then.

I'm embarassed to say that my Lang is older than Dr. Roberts's; it
is the 1980 edition. Even then Lang points out that the most accurate
value for GM(Earth) (six digits) is found from observations of
artificial satellites, citing Clemence in 1965. He lists the masses
of the Galilean satellites from astronomical observations (in units
of Jupiter's mass) as

                     Io                0.0000381 +/- 30
                     Europa            0.0000248 +/-  5
                     Ganymede          0.0000817 +/- 10
                     Callisto          0.0000509 +/- 40

While I do recognize the difficulty of making the observations on
which these pre-robot numbers are based, I don't consider them to be
highly precise, and their accuracy is not overwhelming either. The
Galileo values (which I calculated from data scattered through their
web pages, sources and uncertainties not attributed) are:

                     Io                0.0000471
                     Europa            0.00002612
                     Ganymede          0.0000777
                     Callisto          0.0000568

I do not mean to deprecate observational astronomy in any way. I've
done some myself*, though never any astrometry. Still, I think that
the JPL robot numbers are the best obtainable for data like these.

Leigh

*Photoelectric photometry