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Posted by Carl Floyd/">Carl Floyd on May 7, 1998 13:01:11 UTC

Olaf Romer (1644-1710) made the first successful measurement of the velocity of light by looking at variations in the time of emergence of Jupiter's satellites. Many observations are required over many months. Romer's measurements gave a time of 11 minutes for light to go a distance equal to the earth's orbit. A more modern is 8 min 18 sec, so his estimate of the velocity of light was off a bit. Details on how he made the measurements should be available in a good encyclopedia or other astromomy books. James Bradley (1693-1762) estimated the velocity by examining an apparant motion of the stars (called aberration and distinct from parallax) due to the earth's velocity and got a better answer. Again, these measurements require great accuracy and must be performed over a long period of time.

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