Did you know that in 1676 the Danish astronomer, Ole Rømer proposed that there was a measurable speed of light? Before this, people thought that light traveled instantly or had infinite speed. Ole Rømer noticed that when the distance between Earth and the moon Io was greater, the amount of time it took for Io to ‘appear’ from Jupiter’s shadow after an eclipse was slightly longer. This led Rømer to the idea that light did not travel instantaneously; if light traveled instantly, Io would reappear in the same amount of time, even with a greater distance to travel. Ole Rømer calculated that the speed of light was 220,000 km/s.
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Ole Rømer’s papers were destroyed during the Copenhagen Fire of 1728, but some of his observational data and ideas survived in a letter to Christiaan Huygens.
Ole Rømer’s idea about the finite speed of light helped to explain why the calculated predictions related to Io’s eclipses did not match up with the observations. In some cases, Io emergence from Jupiter’s shadow was fifteen minutes off from calculated times.
Many natural philosophers, such as Christiaan Huygens and Isaac Newton, at the time supported Ole Rømer’s ideas. Others, such as Cassini did not support the idea, since there were other explanations for the irregularities such as imperfect orbits for the Earth, Jupiter, or Io.
Currently, the speed of light is calculated as 299,792.458 km/s.