Measurements on Earth are based on physical distances, while in space, distances are vast, leading to the use of units that incorporate time.

The most common space measurement is a light-year, which is the distance light, the fastest known entity in the Universe, travels in one year, around 10 trillion kilometers in distance. For instance, Alpha Centauri, the second closest star to Earth, is situated at a distance of 4.4 light-years, approximately 44 trillion kilometers away.