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    Re: Unexpected USNO height correction precepts
    From: Antoine Couëtte
    Date: 2018 Sep 24, 07:35 -0700

    Dear Andrés,

    Last - but not least - this is my reply to your post " Unexpected-USNO-height-correction-precepts-AndrésRuiz-sep-2018-g42747 " .

    First of all, I wish to emphasize the practical importance of the [Rude] Starfinder  displayed in your post referenced here-above.

    This was an essential tool for us to best preset our sextants since we were observing Venus in full daylight and quite close from the Sun ( only 30° away from the Sun ).

    worshipped this Starfinder, and I have to confess that - so many moons (almost 500 !) after these sins - and when in a hurry I did not always take time then to compute the CelNav bodies Azimuths, as I would lazily make use of the Azimuths displayed on a carefully and properly set Starfinder.

    Anybody has ever done that ??? 

    Greg, you should try ... I have to say that it works extremely well once the Starfinder is carefully preset and kept current through advancing it 1° every four minutes of elapsed time. The Rude Starfinder is the BEST Azimuth non-electronic computing option and candidate for your EMERGENCY TOOL KIT, would'nt you agree ?

    Also, Andrés, it is interesting and nice to observe that - in the case with the phase correction and SD set to 0.0' which you treat in your numerical example here-under - our intercepts differ by only 0.01 NM.

    *******

    To conclude again, and as a lesson learnt:

    It is very advisable that the USNO Celestial Data Site " http://aa.usno.navy.mil/data/docs/celnavtable.php " should compute the phase effect for the Planets and should also take SD = 0.0' in their published "Sums" because these assumptions best - if not "exactly" - fit the real world sextants usage by Seamen.

    There seems to be very little logic, consistency and good sense for publishing coordinates accurate to +/- 0.1' for all celestial bodies - which otherwise makes a lot of sense because 0.1' represents the extreme very best sextant observations meaningful accuracy - if on the other hand the published height corrections "Sums" for Venus can be off by up to 0.7' in some extreme and rare cases which otherwise could be very accurately solved through stricter - nonetheless quite straightforward and [hopefully] affordable - computations.

    Best Friendly Regards, and thanks to you again Andrés,

    Antoine


     Comparison between Andrés' and Antoine's computations

    18/03/1977
    00:02:17 UT1
    Bap = -27.583333 º = -27º 35.0'
    Lap = 144.895000 º = 144º 53.7'
    NAUTICAL ALMANAC                 Antoine's results
    Venus (Andrés' results)          Venus   Phase Correction δ GHA = + 0.262691'  ,  δ Dec = -0.184298'
    GHA = 156.142468 º = 156º  8.5'  With phase corr. GHA = 156°08.83', without phase corr. GHA = 156.142784° = 156°08.57' (Andrés-Antoine = -1.14")
    Dec = 16.316548 º =  16º 19.0'   With phase corr. Dec = N16°18.82', without phase corr. Dec = N16.316814° = N16°19.01' (Andrés-Antoine = -0.96")
    SD = 0.4181 '                    SD = 0.4181'  NOTE : Since I am using truncated series, I am nonetheless getting apparent positions accurate to better                                                        than 0.07' for Venus under all cases including very short distances from Earth, which is again                                                          fully verified here with Earth-Venus Distance = 0.33262 AU
    HP = 0.4406 '                    HP = 0.4406'

    ALTITUDE CORRECTION
    Limb: center                     Limb: Center of Light including phase effect
    Ref: SeaHorizon                  Ref: SeaHorizon
    Hs = 18.0617 º =  18º  3.7'      Hs = 18.061667 º =  18º 03.7'
    ic = 0.0000 '                    ic = 0.0000 '
    heas = 4.5720 m                  heas = 4.5720 m
    T = 10.0 ºC                      T = 10.0 ºC
    P = 1010.0 hPa                   P = 1010.0 hPa
    Hoi = 18.0617 º                  Hoi = 18.061667 º
    dip = 0.0627 º                   dip Correction = -0.063078 º
    Ha = 17.9989 º =  17º 59.9'      Ha = 17.998589 ° = 17°59.91'
    R = 0.0499 º                     Refraction Correction = -0.049179 ° including Parallax Induced Differential Refraction (PIDR)
    OB = -0.00000816 º               PIDR = 0.000000° (PIDR significant for very low altitude Moon for Celnav, and also for artificial satellites)
    HP = 0.0073 º                    HP = 0.007344°
    PA = 0.00697593 º                PA = 0.006987°
    Without SD correction:
    Ho = 17.9561 º =  17º 57.4'      Ho = Ha + Ref corr. + PA = 17.956397° = 17°57.38' ( Andrés - Antoine = -0.0003° = 0.017' )
    Marcq St. Hilaire LoP            Marcq St. Hilaire LoP
    LHA = 301.037468 º = 301º  2.2'  With phase corr. LHA = 301.042166° = 301°02.5300' , without phase corr. LHA = 301.037784° (Andrés-Antoine = -1.14"), 
    Hc = 17.9689 º =  17º 58.1'      With phase corr. Hc = 17.974212° = 17°58.4527' , without phase corr. Hc = 17.969005° (difference : 0.3124')
    Zn = 59.8 º                      Zn = 059.82°
    p = Ho-Hc = -0.012863 º = -0.771794 ' With phase corr. p = Ho-Hc = 17.956397° - 17.974212° = -0.017815° = -1.068900' = -1.07 NM
                                          With phase corr. : (Andrés - Antoine) intercepts = -0.012863° - (-0.017815°) =  0.004952° = 0.30' = 0.3 NM        
                                          Without phase corr. p = Ho-Hc = 17.956397° - 17.969005° = -0.0126608° = 0.756480' = -0.76 NM 
                                          Without phase corr. : (Andrés - Antoine) intercepts = -0.000202° = -0.012132' = -0.01 NM
    With SD correction:
    Ho = 17.9561 º =  17º 57.8'
       
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