|Re: WP34 again|
Message #20 Posted by Dave Shaffer (Arizona) on 23 July 2011, 2:36 p.m.,
in response to message #19 by Paul Dale
Do you have a link to the IAU's definitions of these? Or better yet, send me a file with them :-)
Here's a link to the IAU 2009 system of constants:
Note that in any such system, there are generally NO masses given for planets, etc. The value that counts for calculating things like orbits is the product of the object's mass times the gravitational constant (capital "G") - or GM. This is because we don't really know what the mass of anything large (like the Earth) is! You get it by doing something like determining the period of an orbit, and then plugging in a value of G. G is VERY HARD to measure! You measure the actual gravitational attraction between masses, which isn't very big.
On the other hand, though, you get the product GM from the orbit period directly, which you can measure to much higher accuracy. In the IAU table, "G" is given to 6 digits, whereas GM(E) has 10 digits
Hence, serious orbit or geodetic or other scientific calculations use the product GM.
On the IAU page, you will see lots of mass ratios - gotten from orbit periods.
So, putting in an Earth mass (or for other objects), while convenient, perhaps, isn't quite right for maximum accuracy.