While we can’t fully explain light, we can measure it quite accurately. We have a pretty good idea of how fast light travels. Since a light-year is merely the distance that a beam of light will travel in a year, the real discovery had to do with measuring the speed of light.

This was done by a Danish astronomer named Olaus Roemer in 1676. He noticed that the eclipses of one of the moons of Jupiter kept coming later and later as the earth moved in its orbit to the opposite side of the sun from that occupied by Jupiter. Then, as the earth moved back into its former position, the eclipses came on schedule again.