What is a Light Year? Although it sounds like a unit of time, a light year is actually a unit of distance. The distances in space are so huge, they require special units to measure them in. One light year is 9.46 trillion km, which is the distance that light travels in one year.  (It might feel strange to think of light as 'travelling' anywhere, because the speed of light is so fast we do not observe this in everyday life.) Pictured above is Proxima Centauri - the closest star to Earth after the Sun. It is 4.25 light years, [...]