Question

According to astronomers, what is a "light year" answer key

Answer 1

A light-year is defined as the distance the light can cover in one year.

The light travels at speed of $c=3 \cdot 10^8 m/s$, and one year converted into seconds is
$1 y = 365 \frac{d}{y} \cdot 24 \frac{h}{d} \cdot 60 \frac{m}{h} \cdot 60 \frac{s}{m}=3.15 \cdot 10^7 s$

So the distance covered by the light in one year is
$S=vt =(3 \cdot 10^8 m/s)(3.15 \cdot 10^7 s)=9.45 \cdot 10^{15} m$
And this is the distance that corresponds to one light-year.