01/22/92 - What is a light year?

What is a light year?

(Lansing State Journal, Jan. 22, 1992)

The light-year is probably the most misused scientific term in the popular literature. If you watch old science-fiction films carefully, you may hear the characters using the light-year as a unit of time - which it is not! The light-year is actually a unit of distance commonly used in astronomy. Astronomy is the scientific study of the universe beyond the Earth: planets, stars, galaxies and solar systems. Because of the size of these objects and the distances between them, measurements and calculations often require working with very large numbers. Specialized units - such as the light-year - make these calculations much less cumbersome.

A light-year is defined as the distance light travels during one year. Light from the sun travels with a speed of about 300,000,000 meters per second. By multiplying the speed of light by the number of seconds in one year, we find that one light-year is equal to 9.5 x 1015 (that's 9,5000,000,000,000,000!) meters. To give you an idea of how large a distance this is, the average separation of the Earth and the Sun is about 1.5 x 1011 meters. This means that, in order to travel a distance of one light-year, one would have to make 32,000 round trips between the Sun and the Earth! Use of the light-year allows astronomers to write, for example, the distance between the Earth and the nearest stars beyond the Sun as 4 light-years, instead of 38,000,000,000,000,000 meters. The light-year is just one of a number of specialized units which have been defined by scientists in order to make measurements and calculations involving very large (and very small) distances more convenient.

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