What is a light year?

A light year measures distance rather than duration, which is the most common misunderstanding of the term. Light travels at roughly 186,282 miles per second in a vacuum, and over a year that adds up to about 5.88 trillion miles, or 9.46 trillion kilometers. Astronomers use it because the numbers involved otherwise become unmanageable: expressing the distance to a nearby star in miles produces a figure with thirteen digits that conveys nothing to human intuition.

The consequence is that looking outward is looking backward. Light from the sun takes about 8 minutes and 20 seconds to reach Earth, so you always see the sun as it was eight minutes ago. Proxima Centauri, the nearest star beyond our own, sits 4.2 light years away, so its light left more than four years before it reaches your eye. The Andromeda galaxy is roughly 2.5 million light years distant, meaning the smudge visible to the naked eye on a dark night is an image of a galaxy as it appeared before modern humans existed. Some of the most distant objects observed are seen as they were within the first few hundred million years after the Big Bang.

Two related units are worth knowing. The astronomical unit is the average Earth-sun distance, about 93 million miles, and is used for measurements inside a solar system where a light year would be absurdly large. The parsec, equal to about 3.26 light years, is preferred in professional astronomy because it derives directly from the parallax measurement technique used to determine stellar distances. And the light year sets a hard limit on communication: a message to a civilization ten light years away would take a decade to arrive and another decade for any reply, which is why interstellar conversation is a physics problem rather than an engineering one.