# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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Re: At the centre of time
From: Peter Hakel
Date: 2009 Oct 25, 08:10 -0700
After a few days without an Internet connection I am catching up with NavList.  Gotta be quick, my plane boards in a couple of kiloseconds.

Peter Hakel

From: "frankreed@HistoricalAtlas.com" <frankreed@HistoricalAtlas.com>
To: NavList@fer3.com
Sent: Sat, October 24, 2009 7:06:29 PM
Subject: [NavList 10270] Re: At the centre of time

Gary, you wrote:
"Get in a French car and you will see a speedometer marked in kilometers per
hour and a tach marked in RPM. Trouble is neither is an SI unit. The proper
units should be meters per second and radians per second (there are no hours
or minutes in the SI system)."

Good point.

Speaking of meters per second, has anyone else noticed an annoying tendency among oceanographers to express ocean current speeds in meters per second? Why can't they just use knots like everybody else? :-) And when converting in my head to knots I always find myself stumped for a moment or two trying to remember whether I should divide by two or multiply by two (two knots is very close to one meter per second).

For angular frequencies, the proper SI unit for a rotation frequency is presumably just "per second" (inverse seconds) rather than "radians per second" since radians (and likewise degrees, minutes, and seconds) are not units at all. They're just number labels like the words "percent" and "a dozen" and "a gross".  I keep myself entertained by thinking about this radians versus percent thing every time I come down the steep slope of the Poconos when driving east on I-80. There are signs for steep highway slopes given as percentages, e.g. "6% grade". How different would it be if it said "0.06 radians grade"? Yes, I do think about these things... Pennsylvania is a boring drive.

You concluded:
"If even the French can use non SI units when it makes [sense] to do so then it is OK for the English speaking world to do the same."

Yep, and if French astronomers, who are "vraiment serieux", use parsecs and mega-parsecs for distances when petameters are zettameters are readily available for those scale ranges, then I think we can safely admit that sometimes convenience and convention wins out over purism.

-FER
PS: Another word that's just a label for a count, much like our "dozen", is the Mayan word "baktun" which is the same as saying "a dozen dozen thousands" with fewer syllables... We're nearing the end of the 12th numbered "baktun" of the Maya calendar. Unfortunately, this one only goes up to 12. The system tops out at 12. So they either need to modify the calendar, or maybe it's the end of the world ;-). And hey, that's another movie connection, but a silly one, so I'll send you to a nice Wikipedian summary of the calendrical issue: http://en.wikipedia.org/wiki/2012_phenomenon

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