It was the Danish astronomer, Olaus Roemer, who, in 1676, first successfully measured the speed of light. His method was based on observations of the eclipses of the moons of Jupiter (by Jupiter).
Google is commemorating the 340th anniversary of the Determination of the Speed of Light. The discovery was reported on 7 December 1976 by the first scientific journal in Europe, the Journal des sçavans. The Journal is still operating, in a modern form known as Journal des Savants.
Astronomer Danish Olaus Roemer was observing the eclipses of Jupiter’s moon Io. Roemer found a discrepancy in the predicted and actual appearances of the eclipses.
In an announcement to the Royal Academy of Sciences in Paris, Roemer predicted that the Io would come out of the shadow of Jupiter ten minutes after it was anticipated, on 16 November 1676. Instead of the speed of light, Roemer was more interested in explaining the discrepancies of the emergence or immersion of IO during its eclipses.
Roemer calculated that it would take roughly ten to eleven minutes for light to travel a distance equal to half the diametre of a terrestrial orbit. This estimation works out to 220,000 kilometres per second, a value that is twenty six percent below the actual value. Roemer rightly predicted that light had a finite speed, which was itself not yet established in the scientific community.
While Google is commemorating Danish Olaus Roemer determining of the speed of light, it was only an indirect inference of his findings. There is some debate over who exactly should be credited for the discovery, with some scientists preferring to credit Christiaan Huygens who calculated the approximate speed of light in Earth based units.