Fans of science fiction may be familiar with the term light years by watching any incarnation of the Star Trek franchise. They commonly talk about planets and star systems being so many light years away. However, the word may have had little meaning at the time. Light years, though, are more than just a term coined by science fiction writers. There is a true scientific meaning behind the term. Before why the light year is used to measure distances in the universe, let’s define the term and examine it in terms of other methods of measurement.

A light year (ly) is a term used by astronomers to measure the distance light travels in a single year, specifically the distances to stars. According to scientific estimates, light moves at a speed of 300,000 kilometers per second; that calculates to about 10 trillion kilometers per year. Thus, one light year equals 9,500,000,000,000 kilometers.

The calculations for the light year are based on the Julian calendar, the solar calendar introduced by Julius Caesar that equals 365.25 days. Each day is 86,400 SI seconds and totals 31,557,600 seconds. This calendar year is different from the Gregorian year which is used in the majority of the world today.

The light year is necessary to measure distances in the universe because the kilometer, the common way to measure distance in most countries, is too small a unit. The universe is too vast an expanse to use something so small. If you measure the distance in kilometers between our Milky Way Galaxy to the Andromeda Galaxy, the nearest large galaxy, it would be 21 quintillion kilometers, or 21,000,000,000,000,000,000 km. An easier unit of measure is essential to interpret such great distances.

The way to measure distances is larger than the kilometer on Earth, but it still is not large compared to the light year. That unit used to measure distances in space is the Astronomical Unit (AU). This unit is the average distance between the Earth and the Sun, nearly 150 million kilometers or 90 million miles. The AU can measure distance within the solar system but is not large enough to measure anything beyond its borders.

Astronomers like to measure star distances with a unit of the light year called the parsecs, the distance an object appears to move “one arcsecond of parallax when the observer moves one astronomical unit perpendicular to the line of sight to the observer.” The measurement equals roughly 3.3 light years.

When the light year is compared to other units of measurements, it is clear that this unit is necessary to easily decipher data. Any other unit of measurement would be time consuming and likely waste a lot of paper.