英语时差：如何计算光速

How fast does light travel? What could you do to find out? One natural thing to try is to have someone send some light your way and see how long it takes to reach you. How about if you got a friend to shine a flashlight at you from a distant hill, exactly at midnight? With your stopwatch you start counting theseconds until you see the light. Galileo actually tried this kind of experiment, but without much success. Light goes much too fast to be timed in that way. Your tests would give you the same impression he got — that light moves infinitely fast, reaching your eye the same instant the flashlight is clicked on. This is, in fact, what people believed for a long time: that light moved infinitely fast.