> Be careful! You are confused, because in order to have the gravitational 
> potential be zero at "spatial infinity", it must be negative near or 
> inside a mass (e.g. the earth).

Gravitational potential is always positive.  It is

**  U = G M / r, outside of earth
**  U = G M r / R^2, inside earth

Where

**  G = gravitational constant
**  M = mass of earth
**  R = radius of earth
**  r = distance from center of earth

The gradient of the gravitational potential is the gravitational 
acceleration according to Newtonian physics.  It is

F = dU/dr = - G M / r^2, inside of earth

Please think about it and re-address my concerns if you like.