November 18, 2012

Just call it 2,000 yards

We all know (don’t we?) that it’s wrong to talk about knots per hour. The words “per hour” are redundant because a knot is already a speed of one nautical mile per hour. If you talk about knots per hour, therefore, you’re really talking about nautical miles per hour per hour, which drives other people mad.

That’s a pity, because the nautical mile is a pleasingly natural measurement, unlike the contrived kilometer and its affected minor offspring, the centis and the millis. The mariner’s mile is simply one-sixtieth of one degree of latitude.

Now, man’s ability to translate degrees of latitude into humble feet and inches that we can all understand has always varied with man’s ability to measure accurately the surface of the earth. But one degree of latitude has always equaled 60 nautical miles.

For many years, certainly as far back as I can remember, one nautical mile was held to be 6,080 feet. But we now know, thanks to brainier scientists and improved satellite equipment, that it’s actually 6,076.1 feet or 2,025.4 yards.

For all practical purposes, the harried navigator on a heaving yacht in bad visibility can take the nautical mile to be 2,000 yards. After all, the extra 25 yards is merely the length of one large yacht, and if your navigation is that good, your skipper will have nothing to complain about.

The land mile, used on the Great Lakes, inland rivers, and the Intracoastal Waterway, is still 1,760 yards, as always.

Another nice thing about the nautical mile is that one-tenth of one is a cable — say 200 yards, or two football fields. Since your GPS gives readouts in tenths, you have a ready visual reference for nautical distances in terms of familiar football fields. That’s something the metric system can’t hope to emulate. Who among us can visualize one tenth of a kilometer? Or ever want to?

Today’s Thought
New times demand new measures and new men;
The world advances, and in time outgrows
The laws that in our fathers’ day were best;
And, doubtless, after us, some purer scheme
Will be shaped out by wiser men than we.
— J. R. Lowell, A Glance Behind the Curtain.

“Nurse, get the patient’s name so we can inform his parents.”
“What for, doctor? His parents already know his name.”

(Drop by every Monday, Wednesday, Friday for a new Mainly about Boats column.)


David said...

Actually, one tenth of a kilometer is just about equal to *one* football field -- at least if you are playing in Canada, or if you are playing football outside North America (ie soccer).

Jack said...

Don't get me started on 1 litre per 100 kilometres, instead of good old M.P.G. ! John, be glad you are many miles (not Klicks), from the faceless law makers foe the E.U.

Travis C. said...

While not as useful to the average sailor, a knot per hour is a measurement of acceleration. A better unit would be a knot per second, but who's counting, right?

Tillerman said...

I have never played (or watched much) American football. But I have, at times, run on a track. So I find it very easy to visualize the 100 meter straight on a running track - which is of course one tenth of a kilometer.

Aaron Headly said...

Funny how the Earth ended up measuring 40,007.86 of the "contrived" kilometers in circumference (when measured around the poles).

Tillerman said...

Funny how the speed of light in a vacuum ended up such that distance it travels during a time interval of 1/299,792,458 of a second is exactly one metre.