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Light-Year

Light-year

The speed of light is one of the most fundamental measurements in astronomy. Measured in miles or kilometers per second, the speed of light determines distance. The term light-year refers to the distance light travels in a vacuum in one year. Since light travels at slightly more than 186,000 miles (300,000 kilometers) per second, one light-year is roughly equal to 5.9 trillion miles (9.5 trillion kilometers).

The light-year is a convenient unit of measurement to use when discussing distances to the stars in the Milky Way galaxy and throughout the observable universe. When considering distances within our solar system, the astronomical unit (AU) is commonly used. One AUthe mean distance between Earth and the Sunis roughly equal to 93 million miles (150 million kilometers). One light-year equals about 63,500 astronomical units.

The sky is a map of celestial history. The light from the Sun takes just over eight minutes to reach Earth. When we look at the Sun, we don't see how the Sun appears, but how it appeared eight minutes ago. If you look at something in the sky that is eight light-years away, you are seeing light that left that object eight years ago. You are therefore looking backward in time, seeing that object in the condition it was eight years ago. In this sense, a light-year can also be thought of as a measurement of time.

Alpha Centauri, the closest star to Earth, is 4.35 light-years distant. The center of the Milky Way galaxy is 27,000 light-years away, while the most distant clusters of galaxies are roughly estimated to be one million light-years away.

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light year

light year • n. Astron. a unit of astronomical distance equivalent to the distance that light travels in one year, which is 9.4607 × 1012 km (nearly 6 trillion miles). ∎  (light years) inf. a long distance or great amount: the new range puts them light years ahead of the competition.

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light-year

light-year, in astronomy, unit of length equal to the distance light travels in one sidereal year. It is 9.461 × 1012 km (about 6 million million mi). Alpha Centauri and Proxima Centauri, the stars nearest our solar system, are about 4.3 light-years distant. See also parsec.

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light-year

light-year Unit of astronomical distance equal to the distance travelled in free space or a vacuum by light in one tropical year. One light-year is equal to 9.4607 × 1012 km (5.88 × 1012 mi).

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