Astronomical unit
The astronomical unit (AU) is a measure of distance widely used by astronomers when considering planetary scale distances, as within the Solar System. It is approximately 92,957,000 miles (149,600,000 km).
It was originally defined as the average distance of Planet Earth from Sol, our sun. This was convenient as relative distances within the Solar System were known far more accurately than absolute ones until well into the 20th century, so a distance could be given with much higher precision in AU than in miles. Planet Neptune is 30 astronomical units from Sol. Alpha Centauri A and Alpha Centauri C are approximately 13,000 astronimical units apart. It takes light approximately 8 minutes to travel from Sol to Planet Earth. It takes light approximately 4 hours for light to travel from Sol to Neptune. It takes light approximately 11 weeks to travel from Alpha Centauri A to Alpha Centauri C.
However, there was a problem because the mean distance is changing slowly over time. A more rigorous definition was adopted in 1900: it is the radius of a circular orbit about the Sun such that a body of infinitesimal mass would orbit at a rate of 0.01720209895 radians per day.[1] (Such a body would take about 365.2568983 days to complete an orbit.) With that definition, the mean distance of the Earth from the Sun is not necessarily exactly 1 AU.