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Wednesday, April 24, 2024

What do the measures knot, fathom, and league mean?

A STAFF REPORT FROM THE STRAIGHT DOPE SCIENCE ADVISORY BOARD

By  Straight Dope Staff
   
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Dear Straight Dope: I was in the Navy some years back and really enjoy naval action movies and books. But for the life of me, I cannot remember what distances were in nautical terms. How do knot, league, and fathom translate into the standard miles, feet, yards, etc.? How did these terms come about? Cam Musquiz, Carrollton, TX

SDStaff Monty replies:

What, they didn’t have dictionaries when you were in the Navy? Luckily a few copies had filtered down to the ranks by the time I enlisted. For those unitiated to said tome, here are the definitions:

Knot — one nautical mile an hour. This came about because the divisions on the log line used to measure the speed of the ship were marked by knots in the line. And people wonder why I love the Navy. In case you’re wondering, a log line is the rope (landlubbers call it that, we sailors call the stuff line) used to tow the log (yep, piece of wood) behind the ship back in the old days.

League — three statute miles or approximately 4,828 meters. Came from medieval Latin leaga, a measure of distance. As readers of Ivanhoe and other romantic tales know, the league was once used to measure distances on land and is not strictly a nautical measure.

Fathom — six feet or approximately 183 centimeters. Comes from Middle English fathme which itself comes from Old English faedm [the a and e are one letter and the d has that cross over it] meaning “outstretched arms.” Kind of reminds you of how a yard is about the distance from the nose to the tip of the arm, huh?

You may ask — actually, fellow Straight Dope Science Advisory Board member Karen Lingel asked — what’s the difference between a nautical mile and a regular old statute mile? The nautical mile is 1.852 km and the regular mile is 1.6093 km, a difference of about 15%. (Fortunately for all concerned, a kilometer is a kilometer no matter where you measure it.)

The nautical mile is the length of one minute of arc of a great circle. A great circle is the intersection of the surface of a sphere with a (geometric) plane passing through the center of said sphere. The Equator is one great circle, so essentially a nautical mile is the the length of one minute of arc on the Equator. Presumably they felt the need to define it this way because of the impossibility of measuring 5,280 feet on the trackless sea surface. It’s just happy coincidence that this distance is approximately the same as a “regular” mile (hey, 15% isn’t too far off when you’re talking about the ocean).

So you say you were in the Navy? Dubious, but I bet you do enjoy the really great movies. My personal favorite is “The Enemy Below.” Just ask my old schedules officer, LT Byrne, about that.

SDStaff Monty, Straight Dope Science Advisory Board

Send questions to Cecil via cecil@straightdope.com.

STAFF REPORTS ARE WRITTEN BY THE STRAIGHT DOPE SCIENCE ADVISORY BOARD, CECIL’S ONLINE AUXILIARY. THOUGH THE SDSAB DOES ITS BEST, THESE COLUMNS ARE EDITED BY ED ZOTTI, NOT CECIL, SO ACCURACYWISE YOU’D BETTER KEEP YOUR FINGERS CROSSED.