 The moon is non-luminous. But you see it because sunlight is reflected from its surface to the Earth's surface. So light has a path. Is this an instantaneous path, or does the light travel along this path with a finite speed? Say a slow speed like one meter per second, or a fast speed, say a million meters per second. The way to determine the speed of anything, say a car, is to measure the time it takes the car to travel a given distance. That is, speed is the distance traveled divided by the time that it takes to travel that distance. The units of speed or distance, say kilometers over time, say hours, kilometers per hour. Indeed, Galileo in 1638 tried to find the speed of light in exactly this way. He arranged two observers, each having lanterns equipped with shutters, observe each other's lanterns at some distance. The first observer opens the shutter of his lamp, and the second, upon seeing the light, immediately opens the shutter of his own lantern. The time between the first observer's opening his shutter and seeing the light from the second observer's lamp indicates the time it takes light to travel back and forth between the two observers. So the speed of light would be the distance traveled from first lantern to second, and back to the first, divided by the time. Galileo reported that when he tried this at a distance of less than a mile, he was unable to determine if the light path was instantaneous or if it was finite. Clearly, the light traveled so fast that he could not resolve the time on his timepiece. So for example, let's estimate the speed of light to be one million miles per second. For Galileo to detect the speed of that light over one mile, he would need to measure a millionth of a second. Not an easy task, particularly given that these were human shutter openers, and the human response time is roughly a quarter of a second. In order to use this method to find the speed of light, Galileo needed to track the path of light over a much larger distance. He didn't get around to doing it. In 1638, the 73-year-old Galileo completely lost his sight, and he was suffering from insomnia and a painful hernia. And he was also effectively under house arrest, having been found to be a heretic by the Inquisition. Not an easy life to be a scientist in those days. However, in his earlier years, Galileo was one of the first to produce telescopes. He made a bit of money selling them to seafarers, and he made several important telescopic astronomical observations, including the moons of Jupiter and their orbit about Jupiter.