What is a micrometer?
A micrometer (µm; micrometre outside the US) is 0.000001 meters. The µ is the Greek letter mu, the SI symbol for the prefix micro.
A micrometer is a millionth of a meter, the scale of things you need a microscope for. A red blood cell is about 7 micrometers across and a human hair about 70.
The unit shares its name with the micrometer screw gauge, the hand tool for measuring small parts. That usually reads to 0.01mm, which is 10 micrometers, so the micrometer mostly doesn't measure in micrometers.
What is a UK Nautical Mile?
The UK nautical mile was based on one minute of latitude, ie 1/60th of a degree. The Earth is not a perfect sphere, so a minute of latitude varies slightly with latitude, which is why a fixed length had to be chosen.
The UK nautical mile, also called the Admiralty mile, was fixed at 6,080 feet, which is 1,853.184 meters. That is 1.184 meters longer than the international nautical mile of 1,852 meters (6,076.11549 feet), which the UK adopted in 1970.