NavList:

Dear Dave,

In further reference to your post :

[NavList 12638] Re: AW: Sun Moon Lunars to 155 degrees [NavList] Re: AW: Sun Moon Lunars to 155 degrees
From: waldendand---com
Date: 31 Mar 2010 16:39

and to my first reply to it yesterday :

[NavList 12651] Re: Sun Moon Lunars to 155 degrees [NavList] Re: Sun Moon Lunars to 155 degrees
From: antoine.m.couette---fr
Date: 1 Apr 2010 14:33

I just had the opportunity to run results through Frank's Computer.
(hopefully no typos ...)

*******

For this Lunar Bayly had published the following results : S 20°49' and E 227°40'30".

Here are the Lunar observation data :

04 Aug 1773, Moon-Sun Dist = 155°13'07" (entered as 155°13'1 into FER's Computer) , SUN UL = 5°41'45" (FER : 5°41'8) , Moon UL = 10°31'52"5 (FER : 10°31'9 ) and T = 76° F.

With delta T = + 16.4 s (half a second difference with the + 15.9 s you used), I earlier published the following results :

Time of the Lunar Distance UT = 15h47m58s1,
with a position at S 20°49'29" (FER : 20°49'5 ) W 132°20'09" / E 227°39'51" (FER : W 132°20'2).

I just processed this example with Frank's Computer with the 3 sextant data and the resulting position and time I derived.

Here are the results of Frank's Computer :

1 - With both Oblateness and Flattening : Error in Lunar -0.1' Approximate Longitude Error : 3'7 , ( I almost always get a " 0.0' Error in Lunar " grade in such cases, but here we have a number of standing round-off errors which I am totally unable to eliminate through the data fine tuning procedure recently indicated by Frank ) , and

2 - With only Oblateness and no Flattening : Error in Lunar +0.1' Approximate Longitude Error : 1.9' (no sign indicated, should be opposite to the previous one) , and

3 - With no Oblateness and only Flattening : Error in Lunar +0.0' Approximate Longitude Error : 1.4' (no sign indicated, which way did it go ???) , and

4 - With no Oblateness and no Flattening : Error in Lunar +0.1' Approximate Longitude Error : 4.2' (no sign indicated)

DISCUSSION OF THESE RESULTS :

Again, and since I always compute with Oblateness and Flattening, the options of Frank's Computer are very nice to evalute the effects of both Flattening and/or Oblateness.

As we can observe, the Flattening and/or Oblateness effects account for differences in Longitude well above the "One Mile" position difference which you had earlier mentioned.

Everything else being absolutely equal, if we use 2 different methods to clear a Lunar, a "One Mile" position difference derived from this Lunar (at least the Longitude part of it) is the result of about 1/30' (only 2" at the most) difference in the resulting Geocentric Centers Distances obtained once this Lunar has been cleared by these 2 methods.

I would therefore conclude that - since in this "fast computation" you elected not to take in account either Flattening or Oblateness - their combined effects would most probably exceed the results of a sole difference in Refraction models, and they would most probably also be the main reason for this "One Mile" difference observed between our both positions.

Thank you for your Kind Attention and

Best Regards

Antoine M. "Kermit" Couëtte

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