# NavList:

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

**The length of a meter, was:Timing Lunars with a Rock**

**From:**Robert Eno

**Date:**2005 Jul 19, 14:57 -0400

This reminds me of an unanswered question that I posed to our local geographical experts in my part of the world and about which I did not receive a satisfactory answer. The UTM Grid reference system for recoking one's position on earth is expressed in terms of eastings and northings of meters. The system for the northern hemisphere considers the northing reference at the equator to be 0000000 N. Easting is based on meridianal sectors and I will leave that one aside for now. Essentially, the Grid reference system for northing tells you how many meters one is from the equator. Theoretically, if one wishes to find one's latitude in degrees and minutes, all one has to do is divide the northing reference by 1852, then by 60, to come up with the right answer, but this is not the case. The further one gets from the equator, the more of an error one gets. In my case, the math puts me 9 miles south of my actual position. As I said at the beginning, I posed this question to one of our local GIS experts but he didn't have the foggiest notion of what I was going on about. I think I know the answer: the error is caused by the fact that the earth is not a perfect sphere but an oblate spheroid and therefore 1 minute of arc (or one nautical mile) at the equator is not the same length as one minute of arc at higher latitudes. I am pretty certain that this is the reason, however I would like to pose the question to my fellow list members for confirmation (or refutation). I'm having serious engine problems with my boat these days so I find myself with more time to ponder such questions as I soil the keys of my computer with diesel-soaked fingers...... cheers, Robert ----- Original Message ----- From: "Gary J. LaPook"To: Sent: Tuesday, July 19, 2005 11:22 AM Subject: Re: Timing Lunars with a Rock > At one time, about 1790, many, including Thomas Jefferson, wanted to > the use the length of a one second pendulum as the standard meter, or > standard unit of length, instead of the one proposed by the French > Academy of one ten millionth of the meridian from the equator to the > pole. The discussion broke down because the americans wanted to use a > pendulum at 38? north (near the Monticello) and the french wanted to use > one at 45? near Paris. The length of such a pendulum is 39.2 inches. The > french eventually surveyed the meridian form Dunqerque to Barcelona and > extrapolated it to the pole and defined the meter from it. An > interesting book about the travails in accomplishing this survey, which > took seven years and was conducted during the french revolution, is "The > Measure Of All Things" by Ken Adler, $4.95 at Barnes and Noble. They > came close. There are actually 10,002,290 meters in the arc. > > Gary LaPook > > george huxtable wrote: > >> Come on, Bruce, your memory is as bad as Jared's, when you write- >> >>> The old navigation manuals suggested checking the log line and half >>> minute >>> glass occasionally. One way to check the glass was by pendulum. As I >>> recall, the length of the pendulum, to the center of the musket ball >>> that >>> formed the weight, was sometimes given as 29 and 1/4 inches, and >>> sometimes >>> as 29 and 1/8. Count a second each time the pendulum passed the >>> bottom. I >>> suppose you had to give the pendulum a few moments to settle the >>> length of >>> its swing. >>> >>> Bruce >> >> >> It's not 29 and-a-bit, but 39 and-a-bit inches, which I have just >> confirmed >> by working it out from the expression for a period of 2 seconds as 2 x >> pi x >> sguare-root-of( length / gravity acceleration ). And to be doubly sure, >> I've just checked it against the pendulum of my old grandfather clock in >> the hall. 39 and-a-bit it is. >> >> What a mess listmembers would make of estimating time at an African lake, >> if that's the best they can do between them! >> >> George. >> =============================================================== >> Contact George at george@huxtable.u-net.com ,or by phone +44 1865 820222, >> or from within UK 01865 820222. >> Or by post- George Huxtable, 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 >> 5HX, UK. >> >>