Who first measured the speed of light? (Intermediate)

Who discovered the speed of light? When was it discovered? How was it calculated or derived?

Scientists have been trying to study the speed of light since the ancient Greeks. Most ancient Greek astronomers believed, amongst other things, that the speed of light was effectively infinite. They had no way to test this educated guess, however. Nevertheless, it was generally taken for granted that light-speed was infinite until the astronomer Galileo in the early 1600's. Galileo supposedly attempted to quantify the speed of light, by using distant lanterns with shutters, which an assistant opened at specified times. Galileo would try to record how long it took light to get to him from across the field on which the experiment was done. His only conclusion was that light-speed was too fast to be measured by that experiment. (In fact, with what we now know about the speed of light, we can say that if Galileo and his assistant were standing about a mile apart, it would only take light about five microseconds - five millionths of a second - to travel from Galileo to his assistant. This was much too short to be measured with the technology of that time.)

The first true measurement of the speed of light came in 1676 by a fellow named Ole Roemer (Rømer). Roemer was observing Jupiter's moon Io, the innermost of the Galilean satellites. As seen by an observer on Earth, Io suddenly disappears when it moves into Jupiter's shadow, and it suddenly reappears when it moves out of Jupiter's shadow (back into the sunlight). Roemer was interested in predicting the times at which Io would be observed to emerge from Jupiter's shadow. His goal was to use those observations to determine Io's orbital period more accurately; he was not initially trying to determine the speed of light.

Roemer noticed that the time elapsed between eclipses of Io became shorter as the Earth moved closer to Jupiter and became longer as the Earth and Jupiter moved farther apart. He realized that the discrepancies between the observed and calculated Io emergence times could be explained by a finite speed of light. Since the Earth was moving away from Jupiter over the course of Roemer's observations, it would take the reflected light from Io slightly longer to reach Earth, and this would affect the exact time at which Io was observed to emerge from Jupiter's shadow.

Based on these observations, Roemer calculated that it would take light about 22 minutes to cross the diameter of Earth's orbit. Combining that value with earlier measurements of the Earth's semimajor axis (orbital radius) (described here and here) gives a speed of light of about 210,000 kilometers per second. This is about 30% lower than the modern value for the speed of light, but considering its antiquity, method of measurement, and 17th century uncertainty in the exact sizes of the planetary orbits, this value is remarkably close to the modern value of 299,792.458 kilometers per second.

Here are some pages with more information on Roemer's calculation, including some illustrations of the observing geometry:

This page was last updated by Sean Marshall on January 17, 2016.

About the Author

Dave Kornreich

Dave was the founder of Ask an Astronomer. He got his PhD from Cornell in 2001 and is now an assistant professor in the Department of Physics and Physical Science at Humboldt State University in California. There he runs his own version of Ask the Astronomer. He also helps us out with the odd cosmology question.

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