Studying the Speed of Light “All light travels through empty space at the same speed – the speed of light…which is about 300,000 kilometers per second” (Bennett, Donahue, Schneider & Voit). While this is fact, it has not always been known. Many early scientists and philosophers, including Aristotle, believed that the speed of light was infinite, or instantaneous (Fowler). It was thought that light could travel any distance in no time at all. It wasn’t until the 17th Century that scientists began to question the speed of light and explore it further. To accurately measure the speed of light, several early attempts were made. The efforts of Galileo, Roemer, Fizeau, and Foucault are some of the most notable early attempts at determining the speed of light. Galileo Galilei (1564 – 1642) is one of the most noted astronomers of all time (Hutchins and Adler). He was also one of the first researchers to question Aristotle’s view of light’s speed. Galileo’s experiment involved two participants standing at two hilltops one mile apart. Each participant held a covered lantern. One participant uncovered his lantern, allowing light to be emitted. As soon as the other participant observed the light from the lantern, he uncovered his own lantern and allowed its light to shine. The time it took for the light to “return” to the first participant was recorded. This was repeated several times at a distance of one mile, and an average time was documented. The participants then moved to hilltops that were ten miles apart and the entire process was repeated. Having documented the average times for both one mile and ten miles apart, Galileo expected to see a significant difference between the average times. However, he was unable to detect any time difference. Therefore, he concluded that either light is infinite and takes absolutely no time to travel from one point to another, or that light travels so quickly that it could not be measured with the tools and methods to which he had access. “Most likely he [Galileo] used a water clock, where the amount of water that empties from a container represents the amount of time that has passed. Galileo just deduced that light travels at least ten times faster than sound.” (History of Speed of Light: Historical Measurements) Olaus Christensen Roemer (1644 – 1710) was a Danish astronomer who made the first successful estimate of the speed of light (Hathaway). Roemer was interested in the planet Jupiter and its moons and made many observations concerning their orbits and eclipses. Based on calculations of Roemer, as well as previous astronomers, Io, one of Jupiter’s moons, makes a revolution around Jupiter every 42 hours, 28 minutes, and 34 seconds (A Case History in Astronomy and Physics: The Speed of Light). This observation led Roemer to believe that he could accurately predict the eclipses of Io. Contrarily, his predictions were not so accurate. “Actually, Roemer found, for several months the eclipses lagged more and more behind the expected time, but then they began to pick up again.” (Fowler) He soon realized that, during times when the Earth was closer to Jupiter, the eclipses were occurring at closer intervals, and while the Earth and Jupiter were farther apart, the intervals of eclipses were farther apart. Roemer presumed that the interval difference must be because of the extra time it took for light to travel when the Earth and Jupiter were father apart. “Using the commonly accepted value for the diameter of the Earth's orbit, he came to the conclusion that light must have traveled at 200,000 kilometers per second.” (History of Speed of Light: Historical Measurements) Armand Hippolyte Louis Fizeau (1819 – 1896) was a French physicist who also made a contribution to the study of the speed of light (Singer). Fizeau designed an apparatus in which a beam of light shone between the teeth of a rapidly rotating toothed wheel. Unlike a “second lantern” of Galileo,