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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: GMT from Lunar Photo
From: Antoine Couëtte
Date: 2022 Jan 7, 00:04 -0800

RE (1) - http://fer3.com/arc/m2.aspx/GMT-from-Lunar-Photo-FrankReed-jan-2022-g51845

(2) - http://fer3.com/arc/m2.aspx/GMT-from-Lunar-Photo-Couëtte-jan-2022-g51858

(3) - fer3.com/arc/m2.aspx/GMT-from-Lunar-Photo-Miseta-jan-2022-g51859

(4) - http://fer3.com/arc/m2.aspx/GMT-from-Lunar-Photo-MurrayPeake-jan-2022-g51860

Hello to all,

(A) - In (4) Murray Peake gives an interesting information. What is then identity of this star being occulted ? (2000.0 Catalog data would be appreciated).

(B) - As per (2), still unable to correctly identify star at "4 o'clock" on picture from Ref (1).

If somebody can help, that would be great (2000.0 Equatorial Catalog coodinates and proper motions) ! Thanks in advance.

I keep thinking that performing a simple Far Limb Lunar with this star should probably yield a quite decent UT determination from the picture.

(C) - Then remains the very nice idea by Tibor Miseta (Well done Tibor !) : trying to best estimate the Moon Phase angle from the picture.

Tibor measures Diameter 732 pixels, Dark side 672 Pixels, hence from Meeus k = (732 - 672) / 732 = 0.0820 .

With i = Phase angle, Meeus gives : cos i  = ACOS (2k - 1)  , which here yields i = 146.727

I am not sure whether this Meeus formula is fully applicable here because it seems to be a geocentric formula, while it should certainly need refinements to actual topocentric value.

The idea to solve it would be to compute exact topocentric "k" values at regular time intervals and then backwards interpolate to the requested UT. Would it be as accurate as a star occultation or a star Lunar ???

(D) - Another expeditious way is to compute geocentric Phase angles values at regular intervals from the IMCCE Server .

Example of results :

Moon, 2022-01-05T01:13:00.00, 21 21 12.78807, -20 42 56.2526, 0.002451293, -6.56, 147.98

- Moon, 2022-01-05T02:13:00.00, 21 23 40.76108, -20 32 32.9188, 0.002452739, -6.62, 147.42

- Moon, 2022-01-05T03:13:00.00, 21 26 8.21228, -20 22 2.5327, 0.002454197, -6.68, 146.86

- Moon, 2022-01-05T04:13:00.00, 21 28 35.14146, -20 11 25.2191, 0.002455669, -6.74, 146.31

Inverse interpolation yields a UT time somewhere between 03:13 and 04:13 . it cannot be so since by this time Moon is no longer visible ... So we certainly need to compute accurate topocentric data.

(E) - As a provisional result :

- Getting to know which exact star shows up at 4 o'clock seems the most accurate way to solve for the requested UT.

- By default, we should be able to solve through the disk illuminated fraction, through comparing it with refined topocentric values. Likely to be less accurate though because of the inherent inaccuracy on the starting data (measurement of the K value)

- An alternate way would be to carefully check for the Moon libration as seen from the picture. Again I am not sure that it would yield super accurate results.

- Any other method ?

(F) - Conclusion

Let's go from a real world Lunar ! Somebody gives me the 2000.0 equatorial coordinates and proper motion of the mysterious 4 o'clock star and - after the upcoming week-end - I should be able to compute UT in just a couple of minutes of time afterwards..

Antoine M. Kermit Couëtte

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