Question Why is GGRAV set to 6.67259e-11 in Orbiter?

[Linguofreak]

I'm not quite sure I am following. Your proposed model is

1) Msun := 1
2) yr := 1
3) AU := 1
4) G = 4pi^2 AU^3 yr^-2 Msun^-1 = 4pi^2

1), 2), 3) are definitions. Obviously you can define your units as whatever seems convenient. It doesn't change the underlying physics.

4) Is where the actual physics comes in. In this case, from Kepler's third law for an unperturbed 2-body orbit. Isn't the error you induce with this assumption already much higher than any uncertainty we have for G?

Edit: Incidentally, 1AU is not the semi-major axis of Earth's orbit. It is defined completely independently of any astronomical reference, as 149597870700 metres. So you are back to a metre-based unit.

If we're using VSOP87, that was formulated before the AU was defined in meters, which only happened in 2012. Previously, the AU was defined such that a massless, unperturbed particle orbiting a mass of 1 Msun at 1 AU would cover a defined number of radians per 86400 second day, which implicitly defined the year as 3.155819601539e7 seconds. When the AU was defined in terms of meters, it was done based on the 2009 data on the speed of light in AU (defined by the above method) per day, and the existing definition of c in m/s.

So my proposal is basically:

1) yr := 31,558,196.01539 s := 1
2) AU := 149597870700 m := 1
3) Msun := 1
4) G := 4pi^2
5) kg/Msun is determined from the above, and is subject to the uncertainty in the value of G in SI units.