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A light-year is the distance that light (or any other form of electromagnetic radiation, such as radio waves) travels in a vacuum in one year. Since light travels at a velocity of 186,171.1 mi/s (299,792.5 km/s) in a vacuum, one light-year equals 5,878,489,000,000 miles (9,460,530,000,000 km). The light-year is a convenient unit of measurement to use when discussing astronomical distances. When discussing distances within our solar system, the astronomical unit (the mean distance between Earth and the sun) is commonly used. One light-year equals 63,239.7 astronomical units.

The distances between Earth and even the nearest stars are enormous; Sirius, for example, is 8.57 light-years away. This means that when an observer on Earth looks at Sirius, they see light that left Sirius 8.57 years ago. The observer is therefore looking backward in time when they observe a distant star, seeing the star in the condition it was in more than eight years ago. Indeed, all vision has the same property, except that the time differences between observed and observer are very small for everyday, local objects.

Alpha Centauri, the closest star to Earth, is 4.35 light-years away. Among other stars, Barnards star is 5.98 light-years away, 61 Cygni is 11.3 light-years away, and Antares is 400 light-years away. The center of the Milky way galaxy is 27,000 light-years away, while the most distant galaxy yet observed, as of 2004, was approximately 13 billion light years away.

Frederick R. West