NavList:

Hi Greg,

The method of calculation used by Weems, Comrie, Dreisonstok, for Hc, was found by F. Souillagouët, a french lecturer in navigation, in 1891. In France, the method was quickly forgotten because the use of an assumed position was, at the end of the 19th century, considered as something wrong. It was refound by a Japanese engineer, S. Ogura in about 1920. This method was then imported in England by H. B. Goodwin (a naval instructor in the Royal Navy) and was tabulated in Norie's Nautical Tables (it was named A and K tables) and Inman's Nautical Tables (1922, following calculations by Smart and Shearme). Simultaneously, the method was known in USA and tabulated by Weems in his Line of Position Book (1927), Dreisonstok in HO 208 (1928), Gingrich in Aerial and Marine Navigation Tables (1931), Ageton in his Manual of Celestial Navigation (1942) etc. We find also this method in Germany (F-Tafel, 1937), England (Comrie in 1938, Myerscough and Hamilton in 1939), Holland (Lieuwen, 1953), Italy (Tavole-H), Portugal (Pinto) ... but not in France ! In all those tables, the method of calculation is the same for Hc, but each table have a special process for the azimuth calculation (Rust diagram, ABC tables or specific formulation, more or less complicated).

I agree with you Greg, starting from LHA, Lat and Dec, the calculation of Hc with those tables takes about 3 minutes; it's necessary to add the time required for azimuth calculation.

Regards.

Paul.