The concept of the speed of light is very important because it represents the only way to provide a "limiting speed" for the universe.
But where does the determination of the speed of light come from and to whom?
As always happens with all great discoveries, there are many scientists who have tried their hand at studies and research but it is the Danish astronomer Ole Rømer who is specifically responsible for this important discovery.
Daily experience tells us that as soon as we turn on a lamp, the light instantly floods the space, which is why it has long been thought that light has an infinite speed.
It was precisely Ole Rømer, in the seventeenth century, who hypothesized that light had a speed so enormous, but not infinite and in 1676 he was able to determine this speed while working at the royal observatory in Paris.
Rømer's thesis was simple but brilliant and arose from the observation of the motion of Io, one of Jupiter's moons. The astronomer found that Io's eclipses seemed to anticipate when Earth was closer to Jupiter than when it was further away.
The difference was due precisely to the speed of light: if this is not infinite, then it must take some time to reach the Earth from Jupiter; when the Earth is further away, on the contrary, it takes longer.
Rømer's hypothesis was confirmed by various tests and the discovery was communicated to the Academy of Sciences which published the news on 7 December 1676 , a date that is now commonly remembered as that of the first determination of the speed of light.
In 1790 the Dutch mathematician Huygens used Rømer's idea to more precisely calculate the speed of light and was able to derive a numerical value very close to that accepted today.
Later it was measured by physicists with absolute precision and today we can say that a light beam travels in a vacuum at 299,792,458 meters per second and that, in one second, it would be able to make seven and a half turns of the Earth following the line of 'equator.
The light years
The possibility of measuring the speed of light has also made it possible to give life to the concept of light years that arises from the desire to simplify calculations when enormous astronomical distances are at stake. That's why those distances are expressed in light years, rather than kilometers.
In fact, a light year indicates, in kilometers, the distance covered in one year by light and it is such a large number that it is used as a unit of measurement for spatial distances.
