Is there any way in relativity to mathematically compute what the rate difference would be between two identical clocks, one on the surface of the Earth and one on the surface of the Moon? The points on the surfaces to be considered are on the line joining the individual centers of gravity (i.e. closest points between the two bodies), and ignoring the Earth's small rotational velocity. (Assuming both bodies to be perfect solid spheres of uniform density, and in an isolated two-body gravitational system where external gravitational influences are negligible, e.g. as done for the computation of GPS time dilation.) Q1) How much is the relative velocity time dilation? (This is the key question, as there seems to be no clear answer in the relativity equations) It will be very small compared to the gravitational time dilation, but it must still exist. How much is it for either clock? Q2) The gravitational time dilation from the individual bodies can be worked out at each point under consideration, but - Do we add or subtract these individual time dilations to get the net effect on an actual clock at either point? Add probably?