A light-year measures distance rather than time, despite the word year inside it.

The longer version

Distance in astronomy is so vast that ordinary units become awkward, so astronomers use the distance light covers in one year. Because light has a fixed speed in a vacuum, multiplying that speed by time gives a length, not a duration. Kilometers would require unwieldy strings of digits for even the nearest stars.

The unit also encodes history. Observing an object ten light-years away means seeing it as it was a decade ago, since the light spent that long in transit. Astronomers also use the parsec, a unit rooted in parallax geometry rather than in light travel. Both units measure separation, just through different reference points.

Where this entry sits

The night sky looks calm, yet almost everything up there is moving, burning, or slowly changing. These facts cover planets, stars, the Moon, gravity, black holes, comets, and the story of how people began exploring beyond Earth. The full list sits on the Space and Sky page, where every entry is listed in order.