# How do astronomers define a light year?

The problem with the definition of light year (ly) is the duration of year. IAU year is Julian 365.25 days of 86400 s/day. An improved ly is for Gregorian 365.2425 days

Light second (ls) is 299792458 meters, nearly, The ly is the distance traveled by light, in a sidereal year.

Older approximation to sidereal year is 365.25 days. Later approximation was 365.2425 days. For more significant digits (sd), it is 365.256363 days.

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Order Paper NowAnyway, it is safe to use the common (tallying) 5-sd approximation 1 ly = 1.0950 E+11 meters.

It is better to define day = 86400 s and express year in days.