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    Re: Alnair identification
    From: Antoine Couëtte
    Date: 2023 Nov 19, 09:50 -0800


    Peter, in my previous post I indicated that :

    (1) - All straight lines on this picture can be considered as Great Circle portions.

    (2) - The very central part of this same picture, i.e. the one surrounding the immediate vicinity of Peacock, is conformal.

    As I use only my 1970 Règle Cras to measure angles from either paper printed copies or directly from laptop screens, I cannot obtain better than approximate results only.

    3 - This time I measured the (Peacock - Z Cap) PA "Position Angle" from my laptop screen and I am getting PA = 64.7°, most probably within +/- 0.5°.

    Pending further electronic and better determination expected to be more reliable, let's us use this provisional value of 64.7° as (Peacock - Z Cap) PA.

    On the (Peacock - Z Cap) pair this time, let us again perform the same computations as the ones just earlier performed onto the Peacock - AlNair pair then.

    Again, all values here are the real world ones as would be seen in a sextant with zero instrumental error. Therefore they are subject to both Dip and Refraction (the latter being a function of both Temperature and QFE), here with the same unchanged values : HoE = 15 ft, QFE = 1013.25 hPa / 29.92 " Hg and T = 25°C

    (3.1) - At UT = 05h50m00.0s

    Z Cap Height/Azimuth=29.76016°/251.63363° and

    previous Peacock Height/Azimuth=17.61023°/214.32697°, yielding Distance = 2164.1', PA=63.33634°  (and HR=1.68994)

    (3.2) - At UT = 05h56m00.0s  ( i.e. 6 minutes later)

    Z Cap Height/Azimuth=28.27358°/251.54493° and

    previous Peacock Height/Azimuth=16.78676°/214.35381°, yielding Distance=2164.2', PA=64.60268° (and HR=1.69018).

    Backwards interpolation on this (Peacock - Z Cap) PA gives us : Picture time = 05h56m27.7s .

    It is again well understood and accepted that so many digits are not meaningful.

    Nonetheless, this archaic (Peacock - Z Cap) PA determination already put us within 30 s of our previous (Peacock - AlNair) PA determination expected to be a more reliable one.

    This current provisional (Peacock - Z Cap) PA should be improved through some electronic reading/measurement.

    Assuming no other error, since the (Peacock -  Cap) distance is about twice the (Peacock - Al Nair) distance, we cannot rule out some more significant picture distortion due e.g. to the lenses. Hence, even with a good and solid electronic reading of this (Peacock - Z Cap) PA, chances are that, per se / in essence, the use of this (Peacock - Z Cap) pair is a little less performing than the (Peacock - Al Nair) pair/


    4 - To recap :

    We now have 2 reliable independent determinations of UT :

    4.1 - The accurately determined Z Cap height at 28°22' yielding UT = 5h56m02s

    4.2 - The accurately read (Peacock - Al Nair) PA yielding UT = 5h56m04s

    4.3 - To which, pending further and better PA determination, we may already add the current rough (Peacock - Z Cap) PA yielding UT = 5h56m28s

    4.4 - To which we may also add the Peacock and Al Nair heights once better known (see § 5 here-under).

    4.5. - To which we may also add the AlNair/Peacock and Z Cap/ Peacock heights ratios once better known (see § 5 here under).

    5 - So, for you Peter, would you be so kind as to indicate the following 3 data electronically read from the picture itself (therefore including the effects of Ref and Dip since my benchmark values do include these as per § 3 here-above):

    5.1 - The Peacock height, and

    5.2 - The Al Nair height, and finally

    5.3 - The Peacock - Z Cap Position Angle.

    Thanks in advance,


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