To the human eye, light appears to travel at an infinite speed. For two thousand years, from the time of Aristotle, this was the prevailing wisdom on the subject. In the seventeenth century, however, Italian astronomer Galileo became convinced that light, like sound, had a measurable speed. He tried to measure this speed by clocking how long it took for flashing lantern signals to travel back and forth between two hilltops. This method failed, however, largely because light travels so fast and human reaction time over such a short distance is unreliable.

Not long after, Danish astronomer Ole Roemer studied the time it took for one of Jupiter's moons, Io, to complete its orbit around that planet. He observed that the amount of time it took Io to reappear from behind Jupiter -- that is, for the Sun's light to reflect off Io's surface and be seen by Roemer -- varied, depending on the distance between Earth and Jupiter. If the speed of light was infinite, he judged, there should have been no such delay. Using the diameter of Earth's orbit and this time lag, Roemer calculated the speed of light as 227,000 kilometers per second, or 140,000 miles per second -- about one-quarter below the modern value of 300,000 km/sec, or 186,000 mi/sec. Imprecise though it was, this was the first physical evidence that light had a finite speed.

So how fast is 300,000 km/sec anyway? Consider the 1.3 seconds that we know it takes for light to travel between Earth and the Moon. It would take nine years to walk this same distance. Even sound would take its time -- 13 days of it -- to make this trip!