Archeo-Measuring the speed of light Olaus Roemer (1644-1710) Olaus Roemer - speed of light measured using the moons of Jupiter In 1676, Ole Roemer made the first accurate measurement of the speed of light using Jupiter's moons. Roemer made a detailed study of the frequent eclipses of Io by Jupiter. This enabled him to accurately predict when the next eclipses will occur. Roemer realized that this time difference was caused by the difference between the distance between the earth and Jupiter. When Jupiter was closest to earth, the eclipses happened on time. The further Jupiter was away from the earth, the later the eclipses became - light had a longer distance to travel to earth and this took longer. The size of the earth's orbit and Jupiter's orbit around the sun was known at that time. Roemer used these figures to determine the distance between the earth and Jupiter for all his observations. Roemer used these somewhat inaccurate distances (as it was known in 1675) to calculate the speed of light to be around 186,000 mi/s. http://www.what-is-the-speed-of-light.com/roemer-speed-of-light.html

However, over a period of months, Roemer's predictions were steadily off by longer and longer intervals of time. His predictions were eventually off by eight minutes. Even stranger was the fact that these predictions then became more accurate, till they were correct again. This strange cycle repeated itself again and again with great regularity.