Question

How do astronomers define a light year?

Answer 1

A astronomical unit of distance

Explanation:

Light year is defined as the Astronomical unit of distance. It is the distance travelled by light in `1` year

Remember that, Light travells approximately `3.5*10^8` `m/s`
Which is that light goes round the Earth `3` times in a fraction of second. WHOOSH!!!!!!!!!!!!!!!!

`1` `Light` `year~~9.46*10^12` `km`

Answer 2

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

Explanation:

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.

Anyway, 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.