Re: Accuracy, etc.--astronomical measurements

[Leigh Palmer <palmer@SFU.CA>]

At 1:52 PM -0700 9/3/99, David Roberts wrote:

> I was surprised and offended by Mr. Palmer's dismissal of my contribution
> yesterday on the issue of determining planetary masses.  I've been a happy
> camper on this list since 1992, have a Ph.D. in Astronomy, and have been
> teaching physics and astronomy for 30 years.  I received authoritative
> bluster based on a superficial understanding of the physics and of the
> observational problem.  God help this man's students!

I did not mean to give offense, and I certainly am glad to see that David
Roberts was not stifled by my "dismissal" of his earlier statement:

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

I've been teaching physics for a similar period and astronomy since 1980.
I never worry about God helping my students, but I have proctors to keep
them from helping one another on exams. My students seem to succeed
despite my heavy hand, and many of them return to visit me after leaving
the university.

What Dr. Roberts says about determination of these orbiting masses is
true for binary stars, but it is decreasingly correct (by which I mean
decreasing precision and accuracy) for bodies in larger mass ratio
systems. The effect is, I believe, undetectable for Mercury and may
be also for Venus. (Zero significant digits is both imprecise and
inaccurate.)

> 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.

It would be helpful here (at least I would be very interested because
my Allen's is much newer than yours) if you would tabulate the masses
and uncertainties given for the Galilean satellites together with their
Voyager or Galileo equivalents.

>                        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.

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.

Leigh