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 league?
A UK league was three miles, or about 4.828 kilometers. The length of a league varied between countries and centuries – three miles is the English figure.
The league has long since dropped out of use in Britain and survives mainly in fiction. Seven-league boots, at three miles a league, would cover 21 miles a step.