Distance is the space between two points. It can be measured in miles, rods, feet, inches, meters, kilometers, centimeters, and many other units of measurement. The vast spaces between the stars and planets, or astronomical distances, are measured by the speed of light. Astronomers say, for example, that a star is six light-years away, which means that light reaches the earth six years after it leaves the star. Light travels at a speed of about 186,282 miles (299,792 kilometers) per second. In one year, light travels 5.88 trillion miles (9.46 trillion kilometers). If a star is 10 light-years away, it is about 59 trillion miles (95 trillion kilometers) away.
Ordinary distances, such as a few miles, are too small to have meaning in astronomy. But these same distances are extremely large in other sciences. In biology and physics, scientists can measure the distance between two cells, or between atoms in a crystal. Scientists measure such distances in micrometers (millionths of a meter), or in nanometers (billionths of a meter). Micrometers are sometimes called microns. The micrometer and nanometer are standard metric units. Scientists also use the angstrom, a nonstandard metric unit equal to 1/10 of a nanometer.
See also Astronomy (Units of distance); Parallax; Telemetry; Weights and measures (table: Inch-pound length and distance).
