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Gravity Probe B

Testing Einstein's Universe

Special & General Relativity Questions and Answers

How did Roemer measure the speed of light back in the 18th century?

In 1676, Roemer was studying the eclipses of Jupiter's moons by the disk of Jupiter, and he noticed that during the year his predictions ran behind the actual eclipses he would see at the telescope. Since the satellites had well- determined orbits with fixed orbit times around Jupiter, he correctly deduced that the delay between his prediction and the actual eclipse had to do with the difference in time it took for light to get to earth as the distance of Jupiter from the earth changed during the year. The amount of time was of the order of an hour or so throughout the year. Knowing the distance to jupiter from the astronomical ephimerides, and the time delay, he estimated the speed of light to be 186,000 miles/second, and his accurate estimate remained unsurpassed for over a century. For more information about the history of the speed of light, see's version

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All answers are provided by Dr. Sten Odenwald (Raytheon STX) for the NASA Astronomy Cafe, part of the NASA Education and Public Outreach program.