What Is A Light Year?



Friedrich Bessel successfully measured the distance to a star other than our Sun in 1838, and the light year measurement followed soon after. The star in question was 61 Cygni and Bessel said it took 10.3 years for the light to cover the distance between Cygni and Earth, and from this the term light-year was coined.

Bessel may have held back from using the light-year unit because he felt his calculations would not be as accurate if he did so. This was because the actual speed of light was not certain in 1838 and did not become more accurately calculated until later in the 19th century. Today’s astronomers would often rather use a measurement called parsec, but popular communication of the expanses across the universe continues to reference light-year. A parsec is 3.26 light-years.


A light-year is calculated by the distance that light can travel in the vacuum of space over one Julian year. A Julian year is 365.25 days long.

To calculate a light year you do the following:

Calculate the 31,557,600 seconds in a Julian year and then multiply this by the 300,000 kilometres that light can cover in one second.

The total distance of this sum is 9.5 trillion kilometers (5.88 trillion miles).


To offer a further perspective on the distance:

  • Traveling at the speed of light you could circumnavigate the Earth 7.5 times in one second.
  • It takes approximately 1.2–1.3 seconds for reflected light from the moon to reach Earth over the 350,000 to 400,000 kilometres.