standard gravitational parameter


by Sanjay87
Tags: gravitational, parameter, standard
Sanjay87
Sanjay87 is offline
#1
Nov12-08, 08:53 PM
P: 20
Hi,

Is it true that the standard gravitational parameter of an object (G*M) is more accurately known than the the gravitational constant (G)? If so, why? Any references would be much appreciated.

Thanks,
San
Phys.Org News Partner Physics news on Phys.org
Physicists design quantum switches which can be activated by single photons
'Dressed' laser aimed at clouds may be key to inducing rain, lightning
Higher-order nonlinear optical processes observed using the SACLA X-ray free-electron laser
D H
D H is offline
#2
Nov12-08, 09:04 PM
Mentor
P: 14,456
Yes, its true. We can measure the standard gravitational parameter of some object by observing other objects orbits about the object in question. With hundreds of years of observing planets in orbit about the Sun, we have the standard gravitational parameter for the Sun nailed down extremely well (11 significant digits). We know the standard gravitational parameter for the Earth to 9 or 10 significant digits. While we can measure G*M very precisely, untangling G (or M) from G*M is a much harder task. We only know G to 4 or 5 significant digits.
rbj
rbj is offline
#3
Nov12-08, 11:12 PM
P: 2,265
i think what this also means (and i dunno which is cause and effect) is that we don't know the mass of the earth precisely. i know it's true that G*M is known to 10 digits and that G only to 5 (by measurement with a Cavendish-like experiment) but i find it unexpected that knowing the dimensions and composition of the Earth well, of the mutual orbit of these (unequal) twin planets around their common center of gravity, that with years of astronomical observation of the Moon, that we wouldn't have gotten that more precisely.

Vanadium 50
Vanadium 50 is offline
#4
Nov13-08, 01:26 AM
Mentor
Vanadium 50's Avatar
P: 15,601

standard gravitational parameter


Why do you think we know the composition of the earth well? (Better than 10 parts per million, which is what 5 digits of accuracy means) Most of it is underground. :)
D H
D H is offline
#5
Nov13-08, 07:26 AM
Mentor
P: 14,456
Quote Quote by rbj View Post
i think what this also means (and i dunno which is cause and effect) is that we don't know the mass of the earth precisely.
That is exactly right. The uncertainty in the Earth's mass is for, all practical purposes, entirely due to the uncertainty in G.
but i find it unexpected that knowing the dimensions and composition of the Earth well
Vanadium 50 already asked the key question here. Piling on, one of the key sources of insight into the dimensions and composition of the Earth comes from remote sensing projects such as GRACE. The problem here is that these remote sensing experiments are sensitive only to the product G*Me.
... of the mutual orbit of these (unequal) twin planets around their common center of gravity, that with years of astronomical observation of the Moon, that we wouldn't have gotten that more precisely.
What these observations give us insight into is the Earth's standard gravitational parameter, the Earth/Moon mass ratio, and the Sun/(Earth+Moon) mass ratio. They do not give direct insight to the Earth's mass unencumbered with G.


Register to reply

Related Discussions
Statistic vs Parameter Set Theory, Logic, Probability, Statistics 2
[SOLVED] What are the standard units for inertial mass and gravitational mass? Introductory Physics Homework 10
lattice parameter Materials & Chemical Engineering 0
Elimination of parameter Calculus & Beyond Homework 3
which parameter. General Math 0