That is the direction of gravity, not the "position" of gravity.
(0, -1) means gravity pulls downwards. (0, -1000) means gravity pulls downwards 1000x as strong. (1, 1) means gravity would pull to the upper right, etc.
If you want to implement planetary gravity, you'll need to create your own velocity integration function. I have a C example of that in the vanilla Chipmunk distribution:
https://github.com/slembcke/Chipmunk-Ph ... o/Planet.c. Objective-Chipmunk doesn't fully wrap integration functions, but I have an example of a (position) integration function from Objective-Chipmunk here:
https://gist.github.com/3033927.
Can't sleep... Chipmunks will eat me...
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