The speed of light is one of the most fundamental measurements in astronomy. Measured in miles per second or kilometers per second, the speed of light determines distance as well as time. The term light-year refers to the distance light travels in one year. If you look at something that is one light-year away, you are looking into the past; the light entering your eye at that moment actually left the object one year ago, so what you see the object as it existed one year ago.

While defining the speed of light was easy, determining its actual velocity was not, because the true speed of light was vastly greater than could be precisely measured with equipment of the day. With characteristic ambition, Galileo tried to determine the actual velocity of light by stationing an assistant on one hilltop with himself on another. They...