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

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Venus with Earth Oblateness
From: Michael Dorl
Date: 2004 Oct 13, 14:24 -0500
Don't know what I might have done but George says I sent some html.  So here's another try.

Here are the bloody details on the calculation of TPC for Geocentric coordinates from Mosier's
routines for Venus and the Moon

11/9/2004
11:13:05
dt = 64.4 seconds
temp = 5Cpressure 1010 MB
elevation 294M

Venus                            Moon

geocentric RA                       10h34m45.1022s           9h37m13.8949s
diurnal aberration                 0.0076s                           0.0113s
dirunal parallax                      0.3498s                          2m7.3158s
refraction                               -5.6126s                         -3.2805s
tpc RA                                   10h34m39.8470s           9h39m17.9415s

geocentric DECL                  9d40'04.3470'"               19d29'00.7421"
diurnal aberration                -0.0346"                          -0.0573"
diurnal parallax                    -4.8748"                          -26'23.5639"
refraction                               1'18.7829s                      41.3482"
tpc DECL                               9d41'18.2205"                19d3'18.4692"

I can supply the details on the J2000 to Geocentric transformation if that would help but  I think we all three agreed on the Geocentric position.

Calculating Venus/Moon Ctr/Ctr distance from TPC coordinates, one gets 16d20'22.16".  Assuming Moon semi-diameter of 15'3.876" gives distance from venus to Moon limb of 16d05'18.284" = 16d05.3"; half way between your two estimates.

As far as I can tell there is nothing in the routines to allow for oblateness of the earth. One can specify a elevation. Looks like the earth is assumed to have a radius of 6378137 Meters, same as equatorial radius in my 1988 AA page K6.

If I include an elevation of -9600 meters, I get a ctr/ctr distance of 16d20'25.88". Taking off a  moon semi-diameter of 15'3.88" gives a ctr to limb distance of 16d5'22" or 16d5.37' which is close to Frank's 16d5.5'. I got the -9600 meters by considering the earth to be an oblate spheroid. I think Mosier considers the earth to have an equatorial radius so setting the elevation negative should compensate for the oblateness.

Michael

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