fillGradient coordinates

[Jacques Hausser]

Hello the Gurus

I'm new to gradients... and for any reason, I'm not able to get the  
keys of the Fillgradient property of such-and-such graphic to analyse  
it in details.
My main problem just now is: what kind of coordinates the properties  
hereunder are using ?
It would just be fantastic if they where graphic-centered coordinates,  
but probably they are windows coordinates...
Please confirm (and I would very happily welcome additional  
information). Dictionnary information is:

fillGradient["from"]
	A coordinate specifying the starting point of the gradient

fillGradient["to"]		
	A coordinate specifying the end point of the gradient

fillGradient["via"]		
	A coordinate specifying the intermediate point of the gradient  
(affects scaling and shearing of the gradient)

Very concise...

If you are curious (you ARE, usually), I'm wanting some precisions to  
use gradients to adjust the light-shadow (night-day) areas of the  
earth and the moon running around each other and around the sun in a  
little application demonstrating the components of sea tides. Yes,  
even land-locked Swiss biologists deal with maritime problems ! -  
although our local heroes have had a bad start in the "Vendée Globe" .o(

Thank you in advance

Jacques (with a s, don't mix)