what is a lightyear used to measure
A light-year (abbreviation: ly), sometimes written light year, is a unit of length used informally to express astronomical distances.It is approximately 9 trillion kilometres (or about 6 trillion miles).efinitions [edit]. As defined by the IAU, the light-year is the product of the Julian year (365.25 days as opposed to the 365.2425-day Gregorian year) and the speed of light (7008299792458000000â 299792458 m/s). Both these values are included in the IAU (1976) System of Astronomical Constants, used since 1984.