A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Henry Halboth
Date: 2011 Feb 13, 20:56 -0800
Thank you for simply stating what has apparently escaped so many posters on this List. The fact that GHA, as tabulated in arc units against GMT, is actually GAT +/- 12 hrs or 180d, as the case may be, for the given GMT, allows easy conversion between the two times. Actually, the GHA at 0000 or 1200 hrs, converted to time, is the Eqt as elsewhere tabulated - it is the meridian angle (t) of the true sun at noon or midnight GMT. The confusion, if any, arises in the fact that Hour Angle is measured from the upper branch of the meridian, while Time is measured from the lower branch.
Anyone professing to be a competent celestial navigator must be fully conversant with the various manipulations involved in converting between the various times employed, i.e., mean time, apparent time, sidereal time, etc., both local and Greenwich. This is especially true in the study of historical methods where apparent time, sidereal time, and right ascension played a greater role. The study of celestial navigation begins with the study of time and the use of the time diagram, not with the purchase of a fancy sextant.
While on the subject, and responsive gl's comments on time of LAN prior to the use of chronometers, it might be well to note that one of the by-products of the Time Sight, as originally employed, was the meridian angle (t) of the body observed, or, in the case of the Sun, the time to LAN, corrected as necessary for run E or W - whether the time was kept by hour glass, pocket watch, or otherwise. Generally speaking the Time Sight has received "short shrift" from members of this List - actually it is a perfectly legitimate solution of the astronomical triangle, can be and was used in LOP methodology, and given the various short tabular methods developed for its solution was every bit as convenient as HO 211, HO 208, etc.; actually a short method was proposed about 1848 which may have been shorter that HO 249 ??? , although it never really caught on.
--- On Sun, 2/13/11, Paul Werner <firstname.lastname@example.org> wrote:
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