I assume this has to do with calculating the mass of the earth from orbital paramaters of the moon or satellite in orbit, right? If so, then consider that from basic orbital mechanics:
GMm/R^2 = m w^2 R, where w = rotational velocity, and the orbital period T= 2 pi/w.
Do some rearranging:
GM = 4 pi^2 R^3/T^2
Note that the powers that R and T are raised to are different. From this  would an error of, say 2% in R have a bigger or smaller effect on the calculated quantity GM than a 2% error in T?
