What is a micron?
A micron is one millionth of a meter, or 0.000001 meters. Its symbol is µm, the Greek letter mu followed by m.
'Micron' is the older name for the micrometer, and the SI dropped it in 1967. It survives on filter ratings anyway – a 5 micron filter catches particles down to about 5 micrometers.
What is a UK nautical league?
A UK nautical league is three UK nautical miles, ie 18,240 feet or 5,559.552 meters, which rounds to 5.56 kilometers. The international nautical league is slightly shorter at 5,556 meters.
Don't confuse it with the land league, which is three statute miles. The nautical league is three nautical miles, about 3.45 statute miles, so it's roughly 15% longer.
Britain's territorial waters once reached one marine league – three nautical miles – from the coast. The limit has been 12 nautical miles, or four leagues, since 1987.