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.

The difference in time added up to nearly 17 minutes. This could mean only that it takes that length of time for light to travel the diameter of the earth’s orbit. This distance was known to be very nearly 186,000,000 miles. Since it took light about 1,000 seconds (nearly 17 minutes) to go this distance, it meant that the speed of light is about 186,000 miles per second.

In our own time, Professor Albert Michelson spent years trying to determine the exact speed of light. Using another method, he arrived at a speed of 186,284 miles per second, and this is now considered quite accurate.

If we multiply this speed by the seconds in a year, we find that light travels 5,880,000,000,000 miles in a year—and this is called a light-year.