They use a distance called the parsec (a contraction of parallax and second), based on a universal measure called the stellar parallax and equivalent to 3.26 light-years.
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Now, through the early 20th century, typesetters were paid not by the hour but by how much type they set, measured not by the letter but by the width of type using the capital letter M as the gauge.
Motion is the ultimate object of science, and measures of motion are many; but with thought as with matter, the true measure is mass in its astronomic sense — the sum or difference of attractive forces.
The most common explanation is that it comes from English or British bars, what they call pubs, where " P" meant pint, which is a measurement of liquid.