Hello, I'm looking to understand the concept of the constancy of the speed of light. In particular, I think I need to understand what is meant by the speed being constant for all observers. Even more particular, I'm looking for experimental evidence that I can understand that will show me that light does/does not change speed by the action of being observed. Suppose I have a system for measuring the speed of light involving beam splitters, mirrors, gears etc. I become satisfied that the speed of light within that system is constant throughout. I realize that all of the devices in the system are moving at the same relative speed as each other, so it doesn't tell me much about the speed of light relative to a moving object. I consider letting light from a nearby planet enter my system for measurement. I realize that since my device is on the moon, say, the light will be traveling in a vacuum all the way up until it interacts with the first atoms of the measuring device. I become disappointed, because I realize that the device will have tampered with the light that I wished to have measured. For all I know, from the perspective of this device, the speed of light changed from c+v to c upon physical interaction at the boundary of my device--a sort of recalibration. I seem to only be able to measure light that I've interfered with. What experiments have been done regarding the speed of light where light is closing in on a moving observer and where the speed measurement is done in such a way as to clearly not interfere with the speed? I'm thinking that such a measuring technique must enclose the starting and ending points, but I know there are some pretty clever scientists too.