...удовлетворить малограмотную аудиторию...
Это он Вас имел в виду, г. Уникум?
Не...а
Он иммел ввиду тех невежд, которые требуют приводить величины масс планет в абсолютных единицах какой-либо стандартной системы единиц, напримр в СИ в кг. ...
А стандартно в небесной механике применяется GM, т.к. массы планет могут быть определены только относительно друг друга...
discussion Uncertainty of gravitational constant
http://groups.google.com/group/sci.astro/msg/3768f28170aab861?dmode=source&hl=enhttp://groups.google.com/group/sci.astro/browse_frm/thread/e54e403f41e29502/3768f28170aab861?lnk=st&q=&rnum=99&hl=en#3768f28170aab861"Message from discussion
Re; Uncertainty of gravitational constant
From:
jcob...@bnr.ca (Jim Cobban)
Subject: Re; Uncertainty of gravitational constant
Date: 1998/11/20
Message-ID: <734g05$92h$1@bcarh8ab.ca.nortel.com>#1/1
X-Deja-AN: 413974510
Organization: Bell-Northern Research Canada
Newsgroups: sci.astro
As pointed out by this discussion, the methodology of determination of
the masses of solar system objects is significantly different between
the two eras.
Prior to the space age all that could be determined was the angular
position of solar system objects. It was impossible to directly measure
distance. Assuming the applicability of Newton's laws it was possible
to calculate the absolute space position of various objects in terms of
astronomical units, that is setting the distance between the Earth-Moon
center of gravity and the center of gravity of the solar system to 1.0.
Once you have done this you can calculate, highly accurately, a
constant, traditionally called the Gaussian gravitational constant k, which
represents the gravitational
influence of the Sun on the objects in the solar system. Further, by
solving the system so as to minimize deviations between prediction and
future positions it is possible for each of the major planets to
calculate the ratio between their values of k and the value of k for the
Sun. It is NOT possible to calculate the "mass" of any of the objects
in this system. As an additional aid in reducing the massively parallel
calculations, observations of the motion of planetary satellites, in
angular terms, can provide independent measurements of the value of k
for those planets which have moons. That excludes Mercury, Venus,
Pluto, and even to some extent the Earth. That is because we cannot
trivially calculate the distance between the Earth and the Moon in
astronomical units because the Moon is the one object that we cannot
observe from different spots on the Earth's orbit.
I repeat: Using only angular measurements it is impossible to measure
the "mass" of any object. However you can measure the relative mass of
any two objects.
With spaceprobes it is possible to accurately measure their DISTANCE at
any instant in terms of the time it takes light to cross from them to
the observer. You could also measure their angular positions, for
example using a long baseline radio interferometer, but the accuracy of
the distance measurement, in terms of light time, is orders of magnitude
more accurate. Since the length of the SI metre is defined in terms of
the time it takes for light to cross it, or alternatively the speed of
light is now a defining constant of the SI system, we can accurately, to
a matter of centimetres in fact, determine the exact distance to any
spaceprobe in metric units. This permits us, for each object which a
probe passes relatively close to, to determine with significant
accuracy the value representing the strength of the gravitational field
of the object in terms of metric units.
But, once again, you cannot measure the "mass" of any of the objects.
Just as with the older methods, all you can measure is the strength of
the gravitational field. In gravitational theory the strength of the
gravitational field is proportional to the mass and the constant of
proportionality is labelled G. While the value of the field strength is
frequently known to 7 or 8 digits, the value of the constant of
proportionality (in metric units) is only known to about 4 digits.
Note that it is possible to close the loop to some extent. Since we
have now measured the length of the AU in metric units to about 9
digits, and since we know the strengths of the gravitational fields of
many of the solar system objects to 7 or 8 digits, it is possible to
plug this knowledge back into the traditional model. When we do so we
find that there is no significant change except for the orbits of Uranus
and Neptune. In the old model the predictions for these planets drifted
unless a fudge factor was introduced (called planet X). However once
the space probe determined gravitational field strengths are introduced,
that fudge factor disappears and the observed orbits of Uranus and
Neptune are accounted for over the last 200 years.
However any time you see a mass for any object published in terms of
kilograms (or Petatonnes as one source quotes) then you know that the
author is fudging his results to satisfy a semi-literate audience.
--
Jim Cobban |
jcob...@nortel.ca | Phone: (613) 763-8013
Nortel Networks (MED) | FAX: (613) 763-5199 "
