Earliest Measurement of the Speed of light

It is interesting to think about how fast light travels. It seems to travel very fast! When I push the button on my flashlight it seems to take no time at all for the spot of light to appear on the wall on the other side of the room. If I wave my hand in front of a slide projector, the shadow seems to immediately follow my movements instead of lagging behind as the result of some delay for the travel time of  the light. It is well known these days that light travels about 186,000 miles/second or about 300,000 kilometers/second. Yet only 400 years ago, in 1606, we had almost no idea how fast light traveled. But by about 1676 or some clever humans had figured out the speed of light to within about 33% of it's accepted value today? (They got 125,000 miles/sec instead of 186,000 miles/sec) How was the speed of light first measured and what obstacles were overcome by those clever folks? That is the story related here. This story starts with Galileo...

For a quickie history take a look at this article on Measuring the Speed of Light from the Dept. of Physics at the University of Colorado, Boulder.

Well...that seems pretty straight-forward. Why wasn't it figured out before that in all the thousands of years that humans have been at least writing down their history, not to mention the hundreds of thousands of years humans must have been pondering their world before that? For one thing, it just wasn't practical to measure time accurately enough to see any effects of the speed of light. This was essentially Galileo's predicament when he tried his experiments with lamps in about 1607. He couldn't tell whether the speed of light was infinite (instantaneous travel) or not, but he figured out that it was too fast for him to measure with flashing lanterns with his buddies standing on hilltops. You see...the first practical pendulum clock was not invented until about 1656 by Christian Huygens. Galileo had the idea  of using a pendulum as the basis of more accurate clocks, but the practical embodiment was not achieved until after Galileo's death in 1642.

The practical pendulum clock is credited to Christian Huygens in 1656. by 1675 Huygens had the balance-wheel watch that could keep time to 8 minutes/day. Apparently time could be kept even better than that in the 1670's by the guys in the research labs. Astronomers had been watching the orbit of Io, a satellite (moon) of Jupiter, ever since the telescope was introduced by Galileo in about 1609. By the 1670's their clocks and observations were good enough that they had noticed that the little mood, Io, didn't exactly keep a regular schedule for when it appeared from behind the planet Jupiter. But someone had to figure out that it was the finite speed of light that was causing the observations to be off schedule. Please check out this nicely told story about Ole Roemer.

But something is missing from this story!  Roemer, got a measurement of the time it takes for light to travel across the orbit of the earth. He essentially worked out that there was about a 20 minute difference between when Io appeared when the earth was on the same side of the sun as Jupiter than when the earth was on the opposite side of the sun. Thus, to to believe Roemer's ideas light was now known to take about 22 minutes to cross the diameter of the earth's orbit. But to calculate the speed of light from this...one needs to know the size of the earth's orbit. How did they know that in 1676?

To get a feel for the answer to this question we need to go back a little bit to the Early 1500's. People had been watching the planets for a long time and had written down a lot about where their paths would go. It was useful to work out such things as when eclipses would occur (so you could scare the hell out of the king once in a while by predicting one I suppose).  Anyway Copernicus proposed that the equations were a lot easier to understand and simpler if the sun was in the middle of things rather than the earth. That got people thinking along those lines (perhaps again). Copernicus got the planets in the right order and roughly the right scaled orbit sizes, but he still had circular orbits. Tycho Brahe (1546-1601) made a large number of very accurate (pre-telescope) measurements of the positions of planets. Kepler (1571 - 1630) took this information and combined it with measurements of the periods of the planetary orbits (and a lot of smarts) and worked out the ellyptical orbits of the planets. So by the early 1670's astronomers had a very good understanding of the layout of the solar system EXCEPT for it's overall size. They could make a nearly perfect scale model and chart the paths of all the planets, but they needed one distance measurement between any two bodies in the solar system to determine it's size. Some of this story is nicely told here.

The first decent measurement of the size of the solar system deserves some elaboration. This was accomplished in 1672 by Cassini (1625-1712) and Richer. They measured the very small difference in apparent position of Mars relative to the background stars during the time when Mars was known to be about at its closest approach to earth. They did this on the same night both in Paris, France and in Cayenne, French Guyana. The shift in position of Mars relative to the stars was then used together with the "known" distance between the two cities to triangulate the distance to Mars. The angular shift was very small, but it was possible to measure small angles by comparing the separation between Mars and the stars in the field of view of the telescope. The scale of the field of view of the telescope could presumably be measured with terrestrial measurements. Apparently the first measurements of size of the solar system was off by about 25 or 30%, but converged fairly quickly to a pretty good answer. With the size known and the time of flight of light across the orbit of the earth measured to be about  22 minutes the value of the speed of light was calculated to be 125,000 miles/sec.

A very nice summary of the history of the measurement of the speed of light up through this point and beyond is given here.

Another interesting link related to this is this Astronomical Timeline