 4. Is there some principled way to resolve the dispute between the Copernican and Tychonic systems and thereby settle the question of the proper center to which all the motions in our planetary system should be referred? Newton's answer — the center of gravity of the system, about which the Sun circulates at comparatively small distances — depended on the assumed applicability of the third law of motion in claiming that the Sun is in motion, and the precise location of the center of gravity remained open in the absence of values for the relative masses of Mercury, Venus, and Mars.