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 AU—the mean distance between Earth and the Sun—is 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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