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    Re: Sign of d in HO249
    From: David Pike
    Date: 2020 Apr 22, 14:45 -0700

    Philippe Posth You wrote: In the HO249 tables, is d always positive in the SAME pages, and always negative in the CONTRARY pages ?

    It depends upon the latitude of the observer.

    For Volume 3 i.e. 40-89N, your latitude is always greater than the maximum declination the tables cater for i.e. declinations 0-29 degrees, so d is always plus for same and minus for contrary.  

    In volume 2 it’s more fun.  If lat>>dec, d is plus for same and minus for contrary.  If lat<<dec, it’s minus for same and minus for contrary.  But if lat is within a couple of degrees of dec, take care; d is minus for contrary, but for same its sign depends upon LHA.

    The mathematicians will explain why, but I would guess it’s all to do with the fact that the tables solve the PZX triangle the sides of which are great circles,  but entry to the tables involves parallels of latitude and, I suppose, declination, which are small circles. DaveP

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