|
|
|
|
|||||
|
||||||
Subject: Re: Sextant Positions versus Map Datums?
From: Hal Mueller (hal@XXX.XXX)
Date: Thu Jan 17 2002 - 16:22:32 EST
According to Jean Meeus, Astronomical Algorithms, 2nd edition,
astronomical observations (which are the basis for the Nautical
Almanac) use the IAU 1976 ellipsoid (International Astronomical Union
1976). Those values are
a: 6378.14 km (the radius of the earth at the equator)
f: 1/298.257 (flattening of the ellipsoid)
b/a: 0.99664719 (ratio of polar radius to equatorial radius)
b/a = 1-f
e = sqrt(2f -f*f) = 0.08181922 (eccentricity of the meridian)
I just took a look at the US Naval Observatory's NOVAS package source
code, and they're using the same values (and to the same number of
significant digits, to boot!)
Sextant positions are referenced to the surface of this ellipsoid,
not to a sphere.
See also http://www.colorado.edu/geography/craft/notes/datum/datum.html
Hal
---- Hal Mueller hal@XXX.XXX Mobile Geographics LLC http://www.mobilegeographics.com/ 8015 28th Ave NW Seattle, Washington (206) 297-9575 98117 fax (206) 297-9576
|