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On 3/21/2014 6:06 PM, Peter Hakel wrote:
> Declination is measured against the celestial equator - are you
> referring to precession and nutation?  Can you point me to H. Prinz's
> original postings?

I can do better than point you. Below is the text. The text is also
attached as a ms word.doc, and the diagram as a jpeg. Dug them up this
AM from and old machine. Honestly, there was a term I had to Google at
the time in an attempt to grasp Herbert's scholarly explanation.

All said and done, it fell into my "things I didn't know I didn't know"
sphere of ignorance. :-)

==========================================================

http://fer3.com/arc/m2.aspx/Welcome-Spring-or-Fall-HerbertPrinz-mar-2006-w27802

Oops! You are some 5 minutes early. The correct value is 18:25:33. If
you compute this from the N.A. you should be able to get the time within
  an accuracy of better than 15 seconds. I don't have a N.A. handy to
check it out.

In sync with the equinox related traffic that occurs with a period of
0.5 tropical years on this list, I posted a diagram showing the apparent
place of the sun around today's equinox with respect to the two
fundamental reference frames:

http://www.hprinz.us/equinox.jpg

In the diagram, the ecliptic is blue. The true equator and equinoctial
colure of date are green. The Sun (painted red) moves currently on a
parallel of ecliptical latitude of -0.7".

At 18:25:26 UT the Sun hits the equinoctial colure (i.e. R.A. = 0) at a
declination of -0.73". At 18:25:33 UT it reaches ecliptical longitude 0.
This is equinox.. At this point the decl. is 0.62" At 18:26:10 UT the
sun reaches declination 0, at a longitude of 1.5" and a R.A. of 0.1s.

All data computed with MICA Vers. 2, U.S.N.O.

Conceptually, equinox implies that the declination of the sun equals
zero. Hipparchus supposed this to happen in the equinoctial point, i.e.
at zero longitude. When it was realized after Newton that the apparent
Sun does not necessarily have to move through the equinoctial point each
year, one had to make a decision whether to choose zero longitude or
zero declination for the definition of equinox. Longitude was more
practical.

How come the Sun has latitude? Hint: The latitude of the Sun is always
very roughly (give or take 0.2") that of the Moon, divided by 32000.

Have fun.

Herbert Prinz

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