The units of measurement people use on Earth aren't very useful for gauging distances in outer space. For example, it took Voyager 1, moving at the staggering speed of 62,000 kilometers per hour (38,525 miles per hour), 35 years to leave the solar system, 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.
TL;DR (Too Long; Didn't Read)
Miles, kilometers, and other units we use to measure distances on Earth aren't up to task of handling the much vaster ones between celestial bodies and galaxies. Common measurement units for outer space include the astronomical unit, the parsec and the light-year.
The Astronomical Unit
Although the Ancient Greeks had an idea of the average distance between the Earth and the sun, astronomer Christiaan Huygens made the first accurate measurement in 1659, using the phases of Venus as a reference. Astronomers call this distance – equal to 149,597,871 kilometers (92,955 miles) – the astronomical unit and use it as the basic unit for measuring the separation between bodies in the solar system. 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.
By using rotating toothed wheels and mirrors, French physicists Louis Fizeau and Leon Foucault obtained the first accurate measurements of the speed of light in the 1800s, 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, or light-year – 9,460,730,472,581 kilometers (approximately 5,878,625,400,000 miles) – makes a popular measure of intergalactic distances, although astronomers prefer another unit: the parsec.
Astronomers calculate stellar distances by measuring parallax: 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 being 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 lies, 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, however, prove inadequate for measuring distances within the galaxy or intergalactic distances. Astrophysicists frequently express these in kiloparsecs and megaparsecs, which equal 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. You’d need 16 digits to express that number with kilometers or miles.