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(Here goes Jim Thompson again, nattering on about celestial datums...)

In Bill Murdoch's post, he explained:

"The next task is to change from ecliptic to equatorial coordinates.  If we
assume that the sun's celestial latitude is zero, the formulas for right
ascension and declination are short, and we need only know the obliquity of
the ecliptic.
Ob = 23.43929-(0.01300*Te)+(0.00256*cos(N))+(0.00016*cos(L))
RA = tan-1(tan(EL)*cos(Ob))
?if 90 Dec, and LHA = 000?.

The observed altitude (Ho) is the angle of the sun from the celestial
horizon, but measured by sextant along the local vertical, or direction of
gravity. Declination is the angle of the sun north of the celestial equator,
indepenant of the geoid. So the resulting Latitude is an angle north of the
celestial equator. But since Ho was measured by a sextant along the local
vertical, then it automatically represents the geographic parallel of
latitude that a chartmaker uses. The sextant sight itself is the conversion
between celestial and geographic latitudes.

OK now?

Jim Thompson
jim2@jimthompson.net
www.jimthompson.net
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