Distances · Learning guide

Looking far away means looking back

Light has a finite speed. Every astronomical view is a view of the past.

A unit of distance

A light-year is the distance light travels through vacuum in one Julian year, approximately 9.46 trillion kilometres. It is a distance, despite the word year.

Nearby examples

Sunlight takes about 8 minutes 20 seconds to reach Earth at a distance of 1 AU. The exact travel time varies as Earth moves along its elliptical orbit.

A larger scale

For cosmological distances, expansion complicates the relationship between travel time, present distance and redshift. A simple light-speed conversion cannot replace a cosmological model.

Further reading: Reference source ↗. Written for this starter collection. Check primary references for specialised use.