This is related to time dilation effect due to strength of gravity, basically GR effect on calculated speed of light. Two observers, observer#1 on a very large non-rotating sphere, experiencing 1.0g of gravity, observer#2 is a large distance from the sphere, experiencing 0.1g of gravity. Both observers have identical clocks, and it is known that #1's clock's rate is slower than #2's clock's rate. There are two distant spheres sphere#2, and sphere#3, a large and known distance apart (as measured at 0g while between the two spheres). A beam of light travels from sphere#2 to sphere#3. If the two observers caclculate the velocity of light based on the known distance versus their local time, they will calculate different velocities. It seems the only solution to this dilema is if the observed distance between sphere#2 and sphere#3 differs depending on the amount of gravity experienced by an observer, so that observer#1, experiencing 1.0g of gravity, observes a shorter distance between the two distant spheres than observer #2, experincing 0.1g of gravity.