1) We say the speed of light (in a vacuum of course) is constant, because it is observed to be so. In all our observations of the speed of light, we have found no variation in it. Our measurements are accurate to small fractions of a m/s, so any variation would be easily detectable.<br /><br />2) As for distances:<br /><br />a) Gravitational lensing doesn't do much<br /><br />b) There isn't much out there to slow light<br /><br />c) There isn't much to cause significant zig-zagging<br /><br />d) Current long distance measurement techniques are inaccurate enough in their own right, that A - C are overshadowed. You don't worry about microscopic variations in length when you use a ruler do you? Since we cannot measure accurately enough, these factors are ignored (or rather unimportant, as they don't apply).<br /><br />Now, astronomers are doing studies to see if the speed of light has changed, over billions of years. So the idea isn't ignored. It is, however, firmly confirmed that light is constant now, and has been for quite some time, and is likely to be so for quite some time. <div class="Discussion_UserSignature"> <p align="center"><font color="#c0c0c0"><br /></font></p><p align="center"><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">----</font></em></font><font color="#666699">SaiphMOD@gmail.com </font><font color="#999999"><em><font size="1">-------------------</font></em></font></p><p><font color="#999999"><em><font size="1">"This is my Timey Wimey Detector. Goes "bing" when there's stuff. It also fries eggs at 30 paces, wether you want it to or not actually. I've learned to stay away from hens: It's not pretty when they blow" -- </font></em></font><font size="1" color="#999999">The Tenth Doctor, "Blink"</font></p> </div>