World Facts

How Far Is A Light Year?

A light year is an astronomical unit used for measuring astronomical distances in space.

Definition and Overview

A light year is the distance traveled by a light in a vacuum within a Julian year (365.25 days.) A light year can also be described as an astronomical unit of measuring astronomical distances in space. Since light travels at a speed of 186,000 miles per second, a light year covers a total of 5.878625 trillion miles in one year. Different abbreviations are used to represent factors of a light year. One light year is represented as “Ly,” 1000 light years is represented as “Kly” for kilolight years, 1,000,000 light years is represented as “Mly” for megalight years, and 1,000,000,000 light years is represented as “Gly” for gigalight years.

Scientists prefer using light years to represent intergalactic distances because a small number represents a large distance. For example, the distance to the Andromeda galaxy from earth is 2.5 million light years. The consistency of the speed of light has made the light year a reliable method for measuring astronomical distances. Scientists have also used the concept of light travel to estimate the age of stars by the time they take to be visible on earth. For example, since it takes a light beam eight minutes from the sun to reach the Earth, then the sun is seen on the earth eight minutes later. Similarly, stars within the Milky Way are visible on earth after traveling for 100,000 light years, about 20 times the age of the earth.


Since the time of the ancient Greeks, scientists have had several debates on the speed of light. The Danish physicist Ole Christensen Rømer proved that the speed of light is constant. The German astronomer Friedrich Wilhelm Bessel was the first scientist to express distance in the light year and the first to measure the distance between a star and the earth using parallax. He found the distance to the star, 61 Cygni, to be 660,000 astronomical units which is equivalent to 6.1 x 1013 miles and compared it to the time taken by light to cover the same distance which is 10.3 light years. In the 19th century, scientists considered using the term “light years” but astronomers instead preferred using the Parsec (equivalent to 3.26 light years) for measuring astronomical distances. Popular scientific and non-specialist publications mainly use light years for describing cosmic distances.

Distances in Light Years

The distance of objects in outer space is easily measured by the calculating time taken by a beam of light to the object, against the speed of light. Since the speed of light is known, it becomes easy to calculate the distance covered in light years. Light years also express the distance from the bodies within the solar system and the galaxies and star systems outside of the solar system. By using light years, the distance to Proxima Centauri is about 4.24 light years, 4000 light years to the Crab Supernova, and 2.3 million light years to the Andromeda Galaxy. Planets are at distances of less than one light year away.

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