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    Peculiar definition of sun set/rise
    From: Paul Hirose
    Date: 2019 Jun 21, 17:45 -0700

    In a 1917 issue of The Observatory I saw this letter complaining about
    the definition of sunrise and sunset in the tables of an ephemeris. I
    must admit my inability to grasp the "parity of reason" in his argument,
    but it does show that "obvious" notions of rise and set are not
    universal. (The "Companion" mentioned in the letter apparently refers to
    a supplement to the magazine.)
    
    "There can be little doubt as to the usefulness of the tables of rising
    and setting of the Sun and Moon, lately published as a supplement to the
    American Ephemeris... From the figures in these tables it appears that
    sunset is defined as the instant when the upper limb is on the visible
    horizon. This, therefore, implies that sunset does not happen until the
    whole of the disc has disappeared, and it might be expected by parity of
    reason, that sunrise should not happen until the whole of the disc has
    reappeared, but in these tables, on the contrary, the times given are
    again those when the upper limb is on the visible horizon. By defining
    sunset or sunrise as the instant when the centre of the disc is on the
    visible horizon, as is done in your 'Companion,' any inconsistency of
    this kind is avoided."
    
    
    http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?bibcode=1917Obs....40..345H&db_key=AST&page_ind=0&data_type=GIF&type=SCREEN_VIEW&classic=YES
    
    A later letter supports the "upper limb" definition:
    
    
    http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?bibcode=1917Obs....40..410O&db_key=AST&page_ind=0&data_type=GIF&type=SCREEN_VIEW&classic=YES
    

       
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