Constraints on the gravitational constant at large distances

D. R. Long and others have speculated that the gravitational force between point masses in the Newtonian regime might not be exactly proportional to $\frac{1}{r^2}$. Distance-dependent deviations from the $\frac{1}{r^2}$ law can be represented by a distance-dependent gravitational "constant," $G\left(r\right)\mathrm{}$. Long has summarized the experimental evidence which constrains $G\left(r\right)\mathrm{}$ to be very nearly constant for $5\mathrm{cm}<r<\mathrm{}1\mathrm{}\mathrm{m}$. This paper presents observational evidence for constancy in the range ${10}^3\mathrm{km}<r<\mathrm{}{10}^8\mathrm{km}$, and points out that the value of $G\left(r\right)\mathrm{}\equiv G_c$ in this range has not been experimentally determined. Constraints on $G\left(r\right)\mathrm{}$ in the intermediate distance range 10 m $r$ $G_c$ differs greatly from the laboratory value $G_0$. Models of the earth and sun are used to argue that $G_c$ differs from $G_0$ by not more than ∼40%. Methods of improving the determination of $G_c$ are suggested.