[GourmetDan]

"Depends a little on what we're trying to predict, eh?"

No it doesn't.

": Retrograde motion, epicycles, and all that."

Don't you have to assume that Einstein, Hoyle, Born and Ellis didn't know about retrograde motion in order to make that statement? Is that a reasonable assumption?

"Some coordinate systems are chosen to simply the calculations *greatly*."

So, you are saying that we should take our definition of reality from ease of calculation? But that would mean that we should define reality as geocentric within the earth-moon system and geokinetic within the solar-system. You then end up with two versions of reality depending on your choice for ease of calculation. Does that make sense to you?

"And, if we are talking the orbit of Mercury, classical mechanics won't cut it to more than an approximation."

Actually, we are talking about models within GR. Please try to be informed before you post.

"Cheers!"

Have a wonderful day!

[grey_whiskers]

I'll reply tomorrow, but it looks like you misunderstood the thrust of my statements...maybe I should have been more explicit.

Have a wonderful day!

And this is *why* I'll reply tomorrow...

It's my wife's birthday, and the kids are out on sleepovers and school dances until much later.

So a good day is pretty likely, despite my insomnia.

Cheers!