[VadeRetro]

Apparently there is a source of error from astronomical perturbations of the Jovian moons by each other and other bodies. Not that you can't do it at all (Roemer did it in the late 1600s) but that it's only so accurate.

[PatrickHenry]

Apparently there is a source of error from astronomical perturbations of the Jovian moons by each other and other bodies. Not that you can't do it at all (Roemer did it in the late 1600s) but that it's only so accurate.

Right. It's just a rough measurement. Greater precision can be achieved, but not without better equipment and some extra work. I suppose you could get a cleaner data set with one of the planets that has fewer moons. Mars would probably do, but then you'd need a better telescope. I suspect this gets done all the time. RA would know if we do it with the signals from the Martian probes.