|MadSci Network: Astronomy|
Hello, Kevin... This was known to be the case as soon as the speed of light was shown to be finite. That was accomplished by Roemer in 1676, who observed the timing of the eclipses of the satellites of Jupiter to do it. See this Wikipedia article for more details: http://en.wikipedia.org/wiki/RÝmer's_determination_of_the_speed_of_light
Try the links in the MadSci Library for more information on Astronomy.