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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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Re: venus
From: Michael Dorl
Date: 2004 Oct 13, 12:35 -0500
At 03:00 AM 10/13/04 -0400, you wrote:

NOTE CHANGE  BELOW TO INCLUDE OBLATENESS

George H wrote:
"I get 16d 05.1'; Frank
calculates 16d 05.5'. So there's a discrepancy of 0.4' between us,
somewhere, in the process of allowing for semidiameter(s) and clearing the
lunar distance. Not an enormous divergence, and certainly much smaller than
the difference Michael Dorl observes with his measured distances. But
clearing the distance and allowing for semidiameter should be a rather
exact science, so I wonder why we disagree by that amount. It might be
enlightening to track it down."

Thanks for questioning this. It could easily be that I've got a sign wrong somewhere or something like that. I'll see what I can find. And thanks also to Fred H, Herbert P, and Michael D for posting their calculations which all seem to differ from mine. By the way, does anyone do oblateness the way Chauvenet does? That's how my calculator does it.

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 = 5C
pressure 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 an 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 my 1988 AA page K6.

NEW PARAGRAPH BELOW

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.

I remember someone saying "It's a good thing there are so many standards, you get to choose
one you like." :-)

Michael
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