NavList:

To the attention of Greg Rudzinski and Frank E. Reed
Dear Greg and Frank,

In [NavList 13860] Re: DSLR Venus Lunar From: gregrudzinski---com Date: 13 Sep 2010 17:39, Greg you requested to Frank :

" Frank does your program compensate for the difference of visible Venus and the center of Venus ? I am assuming that it does. "

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Pending Frank's reply on this specific subject, I have dug out in my archives a quite interesting Moon-Venus Lunar. This example is interesting both because it was a daylight Lunar with Venus rather close from the Sun being difficult to visually acquire in the (preset) sextant, and also because the difference between Venus apparent center and apparent center of light (phase effect) exceeded 0.3', not to mention that on this late afternoon (Sun close to the horizon) the Moon declination had a rather high unusual South value (S 28°16'9).

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1 - Here are the data of this Venus Lunar :

Date : 26 Jul 2007,
UT and GPS position at instant of Lunar Distance observation: 19h43m53s3 and N4732.8W00517.6
Height of Eye: 5.20 m (17 ft), Temperature 21°C (69.8°F), Pressure 1018.2 mb (30.07 In Hg)
Observed Sextant distance between Venus center of light and the Moon near limb: 110°54'4 (corrected for Sextant Error)

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2 - With a DeltaT Value of 65.7 seconds of time, and with the following corrections from Center of Planet to Center of illuminated planet (phase effect):

RA correction = -0'300 and Declination correction = +0'155

my computed Lunar Distance error was 0.0' (lucky day!)

NOTE: the "phase effect" correction values given hereabove are computed through assuming that the "planet center of light" is the "mid-point of its illuminated disk as seen from the Earth" . This computation method seems a customary and well accepted one (See Jean Meeus's "ASTRONOMICAL ALGORITMS", Chapter 40 "Illuminated Fraction of the Disk and Magnitude of a Planet")

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3 - In order to understand how Frank's Computer currently deals with this phase effect, I just tossed the numbers given in 1 hereabove into Frank's computer and I got the unexpected following results:

--- Error in Lunar : 0'2 , Approximate Error in Longitude : 0° 05'1 ---

Why is this result an unexpected one for me? Simply because almost 100% of the time - at least for Stars - I compute Lunar results just the same as the ones derived by Frank's Computer. In other words, I almost always get the following comments from Frank's On-Line Computer, at least for Stars :

--- Error in Lunar 0°00'0 ---

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4 - Luckily enough, on the same day and from the same position, Antares - although not visible at all then - was above the horizon. So I just computed a (fictitious) Moon-Antares Lunar distance (i.e. Sextant Distance with no instrumental error). For the exact same conditions as in 1 hereabove, I got a distance of 14°33'7

When processing all these Moon-Antares Lunar numbers through Frank's computer, I am getting the Following result :

--- Error in Lunar : 0'0 , Approximate Error in Longitude : 0° 00'0 ---

As expected, this result is in general agreement with the ones I get by comparison to Frank's results.

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5 - I then decided to investigate this case a bit further through asking myself :

What would have been the Observed Moon-Venus Lunar distance, had it been possible to observe Venus geometrical center instead of its light center?

I can easily compute such case by simply removing the phase effect corrections indicated in 2 hereabove. As a result, I get a "modified" Moon-Venus Lunar Sextant distance equal to 110°54'0

When I reprocess this "modified" distance in Frank's Computer, I get the following result :

--- Error in Lunar : -0'2 , Approximate Error in Longitude : 0° 07'1

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6 - Before going any further, I decided to check that the significant differences between our results can NOT be attributed to significant error in my apparent coordinates computation.

I therefore crosscheck my computed data against INPOP08 (http://www.imcce.fr/fr/ephemerides/formulaire/form_ephepos.php)

For Delta-T=0, at TT=19h43m53s3, I am computing the following values:

Moon RA=17h34m28s90 , Dec=-28°16'51"57 , HP=55'802 , and
Venus RA=10h14m06s34 , Dec=+ 6°54'54"15 , Dist=0.356303 UA

While the INPOP08 results for the same conditions are as follows:

Moon RA=17h34m28s95 , Dec=-28°16'51"23 , HP=55'798 , and
Venus RA=10h14m06s05 , Dec=+ 6°54'55"85 , Dist=0.356325 UA

So I can rule out any error in the coordinates I am using. They all comply to my own 0.1' accuracy criteria.

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7 - AS A RECAP

7.1 - I simply wanted to check how your on-line computer, Frank, deals with the Planets phase effect.

7.2 - In 1, 2 and 3 hereabove I compared computations of a same Moon-Venus Lunar in which the Venus phase effect is quite significant.

7.3 - From earlier posts, since you Frank compute apparent coordinates with an accuracy (much) better than 6", it was necessary for me to rule out any significant error in the accuracy of the Apparent Coordinates I am using. This check has been addressed in 6 here-above and to a lesser extent in 4 here-above where we get exactly the same results for a (fictitious) Moon-Antares Lunar. Check is fully OK.

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8 - CONCLUSION

And finally, by 3 and 5 hereabove I have come to and I am submitting the conclusion that the phase effect you are computing in your on-line computer seems to fall halfway between "no phase effect at all" and "phase effect equal to mid-point of the illuminated planet limb" as earlier referenced.

Last note to Frank (not quite off topic) : thank you for having (lately ?) introduced the ability to use your On-Line Computer with FULL DECIMAL VALUES for time and distances.

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Please, be so kind, Frank, as to comment on the analysis I have made here-above and as to confirm to our community how and to which extent your on-line computer deals with the planet phase effects.

Best Regards to you all

Kermit

Antoine M. "Kermit" Couëtte

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