The light-year (ly) is a unit of distance used by astronomers in measuring the space between stars. Specifically, it refers to the distance travelled by light through a vacuum in one Julian year (365.25 days). Light travels at approximately 300 million meters per second, or 186 thousand miles per second.

By multiplying the light-second distance by the number of seconds in a year, we get the light-year distance – approximately 9,500,000,000,000 kilometers (9.5 trillion). As the numbers get bigger, it becomes more difficult to write and understand them. Therefore, units such as the light-year make life a little easier for astronomers. Light-years are also an important consideration when viewing stars.

A star fifty light-years from Earth can only be viewed as it appeared fifty years ago (this is an approximation as other factors, such as relativistic effects may come into play). If the aforementioned star exploded today, this would be undetectable, if not unpredictable, for fifty years (again, an approximation). This doesn’t imply that light-years are a unit of time, though they are sometimes mistaken as such.

Expressions such as, “they’re light-years ahead of us” suggest a unit of time is in play. However, people also say “miles ahead” and “ten minutes away” colloquially, while hopefully recognizing that these refer to units of distance and time respectively. This is usually considered acceptable in general conversation, but is confusing in an astronomy context.

Having an international system of units is also useful because is allows scientists all over the world to share astronomy findings without confusion or inaccuracy. The International Astronomical Union are responsible for the implementation of such units, including the astronomical unit (AU) and the parallax second (parsec/pc).

The astronomical unit is approximately 150,000,000 kilometers, or 8 light-minutes, and is based on the distance from the Earth to the Sun. The parsec is approximately 31,000,000,000,000 kilometers, or 3.26 light-years, and has a more complicated derivation (see the recommended reading below).

For more information on these units, read Distances – Trigonometric Parallax on Nick Strobel’s Astronomy Notes website. This detailed page explains the parallax method for measuring distance, which is important in understanding the parsec.

Sources

Prologue and Brief Tour of the Universe, Abell’s Exploration of the Universe (7th edition), pp xvi – 1, published: 1995 by Saunders College publishing, authors: Morrison, D et al.

The IAU and astronomical units, International Astronomical Union website, accessed: 28th September 2008, no author specified.

What is a light-year and how is it used?, NASA StarChild website, accessed: 28th September 2008, authors: StarChild Team.