Light-Year

That light travels at all was settled by Ole Rømer in 1676. Timing the eclipses of Jupiter's moon Io, he found them running late when Earth was on the far side of its orbit and early when it was near — the difference being the time light needed to cross the diameter of Earth's orbit. Rømer's colleagues largely disbelieved him; Huygens took the number seriously and combined it with the crude au of the day to get a speed within 30 per cent of the modern value.

The light-year is exact for a bookkeeping reason rather than a profound one: the speed of light has been a defined constant since 1983, and the IAU's Julian year is exactly 365.25 × 86 400 seconds, so their product is an exact integer of metres. Astronomers themselves rarely use it — parsecs come straight out of parallax measurements — but it is the unit that carries the physics to a general audience, because it also states how far into the past you are looking.