In 1676, Danish astronomer Ole Roemer made one of the most important breakthroughs in Physics, when determined the speed of light - 299,792,458 metres per second.
Roemor had begun his experiments in 1673.
Sten Odenwald from the NASA Education and Public Outreach programme explains that Roemer was studying the eclipses of Jupiter's moons by the disk of Jupiter, and he noticed that during the year, his predictions ran behind the actual eclipses he would see at the telescope.
Since the moons of Jupiter had well-determined orbits with fixed orbit times around the planet, he correctly inferred that the delay between his prediction and the actual eclipse was due to the time difference for light to get to Earth, as the distance of Jupiter from the Earth changed during the year.
Knowing the distance to Jupiter, and calculating the time delay, he estimated that the speed of light was 299,792,458 metres per second.
According to The Indian Express, Roemer had been working at the Royal Observatory in Paris, where Giovanni Domenico Cassini, who was his boss, was not convinced as his methods were controversial. However, many others including Sir Issac Newton later validated the experiment.
Roemer was also not the first to try and calculate the speed of light.
Galileo, one of the earliest astronomers, was reported to have tried to measure light with an experiment where two people stood on a hill top several miles apart with a lamp, and each person would flash their lamp, when they saw a lamp being flashed from the other side.
Since the distance was too small, all Galileo could deduce was that light traveled 10 times faster than sound, The Telegraph reported.
Ole Roemer's discovery was crucial to science, as the distance between two objects in the universe, is still measured in 'light years', ie. the distance that light travels in a year.
In fact, many centuries later, Albert Einstein would build his theory of relativity on the premise that the speed of light in a vacuum is always the same.
Google Doodle also celebrated the occasion with a Doodle on Wednesday.