light-year
Lengthexact by definition
The light-year is the distance light travels in vacuum in one Julian year, which is exactly 9,460,730,472,580,800 metres. Because the Julian year is defined as exactly 365.25 days of 86,400 seconds and the speed of light is a defined constant, the light-year is exact by definition and not a measured quantity. It is about 63,241 astronomical units, or 9.46 trillion kilometres.
Watch out: A light-year is a distance, not a time. It is also not the unit professional astronomers publish in: research papers use parsecs, and the light-year belongs to popular writing and to the intuition that distance in space is also a delay in time.
| 1 ly | 9.4607305e+15 m |
About the light-year
The light-year's appeal is that it converts distance into history. Proxima Centauri is 4.24 light-years away, so its light left in the year you were four years and three months younger. The Andromeda Galaxy is about 2.5 million light-years away, which means the photons arriving at your eye tonight departed before our species existed. Look far enough and you are not observing the universe as it is but as it was, and no telescope will ever show you anything else.
Its exactness comes from two definitions stacking. The speed of light has been a defined constant since 1983 at exactly 299,792,458 m/s, and the Julian year used in astronomy is exactly 365.25 days of exactly 86,400 seconds, or 31,557,600 s. Multiply and you get 9,460,730,472,580,800 metres with no rounding anywhere. Note the choice of year matters: the tropical year and the Gregorian calendar year are both slightly shorter than the Julian year, and using either would move the answer in the ninth significant figure. Astronomy standardised on the Julian year precisely to stop that ambiguity.