NavList:

Frank,

In my last post I raised a possible issue about some results of the USNO web app clone you recently mentioned here.

Further to a private communication from another Navlist Member received meanwhile, I simply wish to be more explicit about this here.

Frank, from your indicated position at 41°18.4' N and 72°04.6' W, I submitted that the UT time at which the Picture was taken is 2021 Oct 19th at 21:57:31 UT.

BTW, how are you rating this result ?

Just 2 minutes later at 21:59:27 and for a slightly different position - same latitude at N41°18.4' and longitude at W072°30.0' - the USNO Web Application clone yields the following results.

Celestial Navigation Data for 2021 Oct 19 at 21:59:27 UT           
                                                                               
                 For Assumed Position:  Latitude    N 41 18.4                 
                                        Longitude   W 72 30.0                 
                                                                               
                        Almanac Data                |    Altitude Corrections  
 Object       GHA        Dec         Hc       Zn    |   Refr   SD    PA    Sum 
              o   '      o   '      o   '       o   |     '     '     '     '  
 SUN        153 39.1   S10 18.4   - 0 15.2   256.4  |  -31.0  16.1   0.1 -14.8 
 MOON       339 48.3   N 4 02.6   + 0 38.9    85.2  |  -24.0  15.0  55.2  46.2 
 VENUS      106 55.5   S25 40.0   +15 49.0   212.0  |   -3.4   0.0   0.2  -3.2 
 JUPITER     33 22.6   S15 08.8   +22 57.4   138.6  |   -2.3   0.0   0.0  -2.3 
 SATURN      48 53.7   S19 19.7   +25 32.1   155.2  |   -2.0   0.0   0.0  -2.0 
 ARIES      358 27.2                                                           

                 Moon phase is waxing,  99% illuminated.    

What is interesting to observe is that in the real world not affected by refraction anomaly for the position and time just here-above - both chosen on purpose actually - and with HOE = 0 ft, both the Sun LL and Moon LL heights are very close from 2.0' above the local horizon.

Peter, Paul and Andrés or other[s] and please be so kind as to confirm this.

Hence for both bodies the refraction correction should be almost identical at -31.0' (NAL value). The Application gives :

Sun : Ref -31.0 ' (I agree) , and:

Moon : Ref -24.0' (I cannot agree). From both the 1982 NAL and the French Ephémérides Nautiques :-24.0' seems to be the inverse refraction correction for an angle very close from 38.9'  (i.e. very close from Hc)

Hence it looks that at some point in the Reverse-Engineering Process likely timplemented in the USNO Web Application clone Refraction and Parallax corrections might not be performed in the right sequencing order.

On all other bodies, such proper sequencing order is less important because the parallax effect is almost negligible. For the Moon, and especially when close to the horizon this is not quite so.

This requires some fine tuning which should be rather easily corrected.

Antoine M. Couëtte

antoine.m.couette[at]club-internet.fr