Until the 17th century most scientists believed that the speed of light was infinite.
It was only in 1676 that Ole Rømer and Giovanni Cassini made the first measurement of the velocity of light (just 27% off the real value) by observing the orbits of the moons of Jupiter.
By timing the eclipses of the Jupiter moon Io, Rømer estimated that light would take about 22 minutes to travel a distance equal to the diameter of Earth's orbit around the Sun. This would give light a velocity of about 220,000 kilometres per second in SI units, about 26% lower than the true value of 299,792 km/s.
source: wikipedia
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