The Speed of Light
Part 1 - About the time Father Jacques Marquette was founding missions on the shores of Lake Michigan, others in Europe were realizing that light had a finite speed. Your mission is to discover who did it and how. Please use your own words and include all your sources. 10 points
First you need to know that Father Jacques Marquette (born: June 1, 1637 – died: May 18, 1675) was exploring around here around the 1670's. Your research in the speed of light should lead you to two persons - Giovanni Domenico Cassini and Ole Roemer.
Background: Galileo actually pondered the question - does light travel at a finite speed? He attempted answer the question by placing two observers at distant points. Observer #1 would shine a lantern at Observer #2. When Observer #2 first saw the light, they were to return a light signal back to Observer #1. Observer #1 would then be able to determine if there was any delay .... and that delay could be attributed to the time it took for the light to travel back and forth. Of course, light travels so fast that any human reaction time would overwhelm the answer. Galileo simply concluded that if light did have a finite speed, it was too fast for him to measure. That was in the early 1600's.
In the late 1660's, Italian astronomer Giovanni Domenico Cassini was observing the 4 bright moons of Jupiter. Around and around they would go .. each moon with its own orbital period. Cassini could use this data to predict the exact position of these moons at any time. He came to realize that the moons of Jupiter could be used as timepiece. All one had to do was observe the pattern of these moons to know the exact time in Greenwich England. This became a useful took to determine an observer's longitude. Ole Roemer was working with Cassini on this project.
During their observations, they noticed some unexpected discrepancies in their data. Over the course of years, the orbital periods of these moons seemed to speed up and slow down. Cassini came to realize that the discrepancies could be attributed to the finite speed of light ... since our distance to Jupiter was constantly changing.
Cassini may have written: "This second inequality appears to be due to light taking some time to reach us from the satellite; light seems to take about ten to eleven minutes [to cross] a distance equal to the half-diameter of the terrestrial orbit." This quote comes from an unpublished manuscript in Latin preserved in the library of the Paris Observatory. It is actually unclear if Cassini really believed that light had a finite speed or not. However, it is very clear that Roemer did believe it.
Ole Roemer continued the observations throughout the 1670's.
Illustration by Roemer - 1676
To help you see effect, please refer to the image. When the earth is at position H, we get the light signal sooner (since we are closer to Jupiter). However, when the earth is at position E, we get the light signal later (since we are further from Jupiter). Using this, you may see that as the earth moves progressively from positions E-F-G-H .... the moon's orbital periods appear to speed up. Moving from positions H-L-K-E the moon's orbital periods appear to slow down. Roemer called this "retardation of light".
Oddly enough, Roemer actually never calculated a numerical value for the speed of light, mainly because the size of the earth's orbit was mostly unknown at the time. In addition, the scientific community really never accepted Roemer's conclusion that light had a finite speed. Dutch scientist Christiaan Huygens (a contemporary and friend of Roemer) calculated the speed of light at 131,000 miles per second using Roemer's idea. The actual value is 186,282 miles per second.
Part 2 - Then in 1848-1849, another ingenious method was designed to determine the speed of light. This method was later refined to give more accurate results but your job is to briefly explain the basic idea and the person who originally developed the idea. How accurate was this initial experiment (compared to modern values)? 10 points
Hippolyte Fizeau determined the speed of light to within 5% the modern value using a system depicted below. Basically a beam of light passed between the cogs of a rapidly spinning gear. This beam was reflected back to the gear but would return to the source only if the gear had rotated just the right amount (to the adjacent valley in the cogs) as the light beam took a very short (but finite) time to travel to the mirror and back. This gave the time for the beam to travel (since the speed of the gear was known) as well as the total distance the beam traveled. Since speed is distance divided by time, you now know the speed of light.
Courtesy Wikimedia Commons