## Useful Measures of Distance

A kilometer is a thousand meters (see the table "International System Prefixes"), and a light-year is the distance light travels in a vacuum during one year (exactly 299,792,458 m/sec, but commonly rounded to 300,000,000 m/sec). A light-year, therefore, is the distance that light can travel in one year, or:

299,792,458 m/sec x 60 sec/min x 60 min/hr x 24 hr/day x 365 days/yr = 9.4543 3 1015 m/yr.

For shorter distances, some astronomers use light minutes and even light seconds. A light minute is 17,998,775 km, and a light second is 299,812.59 km.The nearest star to Earth, Proxima Centauri, is 4.2 light-years away from the Sun.The next, Rigil Centaurs, is 4.3 light-years away.

An angstrom (10-10m) is a unit of length most commonly used in nuclear or particle physics. Its symbol is A.The diameter of an atom is about one angstrom (though each element and isotope is slightly different).

An astronomical unit (AU) is a unit of distance used by astronomers to measure distances in the solar system. One astronomical unit equals the average distance from the center of the Earth to the center of the Sun.The currently accepted value, made standard in 1996, is 149,597,870,691 meters, plus or minus 30 meters.

One kilometer equals 0.62 miles, and one mile equals 1.61 kilometers.

The following table gives the most commonly used of the units derived from the fundamental units above (there are many more derived units not listed here because they have been developed for specific situations and are little-used elsewhere; for example, in the metric world, the curvature of a railroad track is measured with a unit called "degree of curvature," defined as the angle between two points in a curving track that are separated by a chord of 20 meters).

Though the units are given in alphabetical order for ease of reference, many can fit into one of several broad categories: dimensional units (angle, area, volume), material properties (density, viscosity,