How Big is a Light Year ? First of all you have to understand:- A light-year is a unit of distance, not time. A light-year is how astronomers measure distance in space. It’s defined by how far a beam of light travels in one year – a distance of six trillion miles. Think of it as the bigger, badder cousin of the inch, the mile, the kilometer, and the furlong. If you like to keep up with what’s going on in astronomy, it’s worth spending a little bit of time understanding what the deal is with this