The units people use on Earth aren't very useful for measuring distances in outer space. For example, it took Voyager 1, which is moving at the staggering speed of 62,000 kilometers per hour (38,525 miles per hour), 35 years to leave the solar system, which is a comparatively tiny part of the universe. To avoid using incomprehensibly large numbers, astronomers have developed measurement units for the solar system and for intergalactic space.
The Astronomical Unit
Although the ancient Greeks had an idea of the average distance between the Earth and the sun, the first accurate measurement was made in 1659 by astronomer Christiaan Huygens, who used the phases of Venus as a reference. This distance is the basic unit astronomers use to measure the separation between bodies in the solar system. It is called the astronomical unit, and it's equal to 149,597,871 kilometers (92,955 miles). By definition, the Earth is 1 AU from the sun, while Mercury is, on average, 0.39 AU distant and the dwarf planet Pluto is, on average, 39.5 AU away.
The Light Year
French physicists Louis Fizeau and Leon Foucault obtained the first accurate measurements of the speed of light in the 1800s, using rotating toothed wheels and mirrors, although a 1,400-year-old statement in the Koran comparing it to the revolutions of the moon around the Earth is accurate. The value accepted by the U.S. National Bureau of Standards is 299,792 kilometers per second (186,282 miles per second). The distance light travels in a year is a popular measure of intergalactic distances, although it isn't the one preferred by astronomers. Its exact numerical value is 9,460,730,472,581 kilometers (approximately 5,878,625,400,000 miles).
Astronomers calculate stellar distances by measuring parallax, which is the angle of apparent movement a star makes against the backdrop of the universe when Earth is on opposite sides of its orbit. This gives rise to the parsec, a unit derived by scribing an imaginary right triangle in the sky. The base of the triangle is an imaginary line between the Earth and the sun -- its length is 1 AU. The other leg is the distance from the sun to an imaginary point from which, if you extend the hypotenuse to the Earth, the angle will be 1 arc second. An object at that distance from the sun is, by definition, one parsec away.
Distances from the Earth to nearby stars can conveniently be expressed in parsecs; for example, the nearest star, Proxima Centauri, is 1.295 parsecs distant. Because a parsec equals 3.27 light years, that's 4.225 light years. Even parsecs are inadequate for measuring distances within the galaxy or intergalactic distances, however. Astrophysicists frequently express these in kiloparsecs and megaparsecs, which are 1,000 and 1 million parsecs, respectively. For example, the center of the galaxy is about 8 kiloparsecs distant, which equals 8,000 parsecs, or 26,160 light years. Expressed in kilometers or miles, that's a number with 16 digits.