# NavList:

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

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

**From:**Nicol�s de Hilster

**Date:**2009 Jan 15, 21:30 +0100

While were busy doing some math: on Clarke 1866 one arc minute in latitude equals at 0 degrees 1842.786m or 6045.886ft at 30 degrees 1847.472m or 6061.260ft at 60 degrees 1856.910m or 6092.224ft at 90 degrees 1861.656m or 6107.795ft in longitude this is at 0 degrees 1855.344m or 6087.087ft at 30 degrees 1608.138m or 5276.043ft at 60 degrees 930.036m or 3051.299ft and zero at the pole. Nicol�s glapook{at}PACBELL.NET wrote: > 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. >>> >>> ------------------------------------------------------------------------ >>> >> > > > > > ------------------------------------------------------------------------ > > > No virus found in this incoming message. > Checked by AVG - http://www.avg.com > Version: 8.0.176 / Virus Database: 270.10.7/1894 - Release Date: 1/14/2009 7:27 PM > > --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To unsubscribe, email NavList-unsubscribe@fer3.com -~----------~----~----~----~------~----~------~--~---