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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...