# NavList:

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

**Re: FW: Re: Chronometer Suggestions**

**From:**Gary LaPook

**Date:**2009 Jan 15, 12:09 -0800

The nautical mile used to be defined, in the U.S., as one minute of arc on a sphere having the same area as the earth as defined on the Clarke spheroid of 1866 and was 6,080.2 feet. The length of one minute of latitude varies from 6,046 feet at the equator to 6,108 feet at the poles on this spheroid (I don't know what it is on WGS84.) The length of the geographical mile, one minute of longitude on the equator, is 6,087 feet. (Bowditch, 1977) Also see: http://www.i-DEADLINK-com/bowditch/pdf/chapt02.pdf gl On Jan 15, 10:55�am, Lu Abelwrote: > Curiosity question: > > It's well known that the diameter of the earth across the equator is > about 1/300th greater than the diameter across the poles. > > I would intuitively expect, therefore, that the size of a minute of > latitude to change by a like amount. �But looking at this graph, there > seems to be a 1/60 difference in the size of a minute at the poles vs at > the equator. �Is there an explanation that this technically competent, > but ignorant of the math of the oblate spheroid, person could understand? > > Also, I assume this graph is for geodetic latitude and not geocentric or > parametric latitude? > (For people curious about these terms, geodetic latitude is what you get > by drawing a line perpendicular to the surface of the earth down to its > axis. �Due to the flattening of the earth, this line will intersect the > earth's axis on the other side of the equator from the observer's > position. �The other two latitudes are what you get when you draw a line > out from the earth's center. This line is not perpendicular to the > earth's surface except that the poles and equator) > > Lu Abel > > Nicol�s de Hilster wrote: > > On NavList 7052 Irv Haworth wrote: > > >> "I think it's well known that 1' of arc varies in length as a function > >> (cos) > >> of the latitude." > > > On which Gary LaPook replied in NavList 7053: > > >> That is true for one minute of longitude because parallels of latitude are small circles. > >> This is not true for one minute latitude of for any > >> other great circle. (Technically these also vary slightly due to the > >> oblateness of the earth but these small variations are ignored for > >> �celestial navigation purposes.) > > > For those who want to know how much exactly that variation is I posted > > attached graph of it in NavList 4750 on 24/03/2008. > > > ------------------------------------------------------------------------ > > --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To unsubscribe, email NavList-unsubscribe@fer3.com -~----------~----~----~----~------~----~------~--~---