A lightyear is the distance that light travels in one year. Since there are various definitions for the length of a year, there are correspondingly slightly different values for a lightyear. One lightyear corresponds to about 9.461e15 m, 5.879e12 mi, or 63239.7 AU, or 0.3066 pc.
A nautical league was 6,080 yards - or three nautical miles (each league being 6,080 feet). Note that there also US Leagues, UK Leagues and UK Nautical Leagues which all differ.