How this calculator works
A light year is a distance: vacuum light speed multiplied by a Julian year of exactly 365.25 days. Available distance units include the exact astronomical unit and the IAU parsec convention. Default propagation uses vacuum c. A smaller positive speed can model travel in a medium, but the light-year unit itself always uses vacuum c. This straight-line calculation ignores expansion and gravitational delay.
Formula
d = vt; 1 Julian year = 31557600 s; 1 ly = 299792458 * 31557600 = 9460730472580800 m.
Worked example
Vacuum light crosses 1 au in about 499.0048 s, or 8.3167 minutes. In one Julian year it covers exactly one light year.
Advertisement
Frequently asked questions
Is a light year a unit of time?
No. It measures the distance travelled by light in one Julian year.
Why use 365.25 days?
That is the Julian-year convention used in this astronomical length unit, rather than a particular calendar year.
Does slower propagation change the unit?
No. Only the distance travelled changes; the light-year conversion constant stays fixed.