The astronomer’s unit for measuring distance
In astronomy, distances between celestial objects – like stars and galaxies – are very large. For example, the distance that separates us from out closest neighbour, the Moon, is on average 385,000 kilometres. The Sun, on the otherhand, is on average 150 million kilometres away, and the next closest star is Proxima Centauri at 40,000 billion kilometres.
To avoid manipulating such long numbers on a regular basis, a unit of distance better adapted to the scale of the Universe quickly became a necessity. One unit of distance commonly used in astronomy is the light-year. A light-year is the distance travelled by light in one year, which is 9 trillion, 460 billion, 730 million, 472 thousand and 581 kilometres. In other words, almost 10,000 billion kilometres.
The creation of this unit of distance was not accomplished overnight. It was first necessary to determine the speed of light in order to know the distance that it will travel in one year.
Before 1676, most scientists thought that light moved instantaneously from one point to another. In that year, however, the Danish physicist Ole Christensen Rømer was the first to demonstrate that light had a finite speed.
Rømer, on the other hand, continued to pursue the idea and estimated that light takes 22 minutes to travel a distance equal to the diameter of the Earth’s orbit (we know today that the true value is closer to 16 minutes and 40 seconds). If Rømer had calculated the speed of light using his data, he would have come up with an answer of 135,000 kilometres per second, which is about half the accepted value today of 299,792 kilometres per second. The main source of error would have been the diameter of Earth’s orbit, which was not well known at the time. If the orbital diameter of the Earth as we know it today is combined with Rømer’s data, the answer would be 214,000 kilometres per second, much closer to the currently accepted speed.
It was only in 1729 that the majority of the astronomical community were finally convinced that light had finite speed. The British astronomer James Bradley published a study demonstrating that the annual variation observed in the position of the stars is related to the speed of light. He estimated that light travels at 301,000 kilometres per second, which is very close to the accepted value.
Gilles Fontaine explains what a light-year is.