NavList:

Brad Morris wrote:

"Can someone explain what the purpose(s) are of the ABC Tables? If its any help, A=Cot(HA)Tan(Lat) B=cosec(HA)Tan(dec) and C=Cot(Az)Sec(lat)"

From Notes On The Tables in my 6th Edition of Burton's Nautical Tables:

The ABC tables "provide one of the quickest and most convenient means of finding the azimuth; and also provide, with equal facility, great-circle courses and the 'Longitude Correction'."

These tables were once in common use in British ships and are still contained in Reeds Astro Navigation Tables, published annually in the UK.

"'C' (the combination of the values A and B) is the error of longitude corresponding to an error of one minute of latitude. It must...be multiplied by the total error of latitude to obtain the total error of longitude."

A full description of their use follows in my copy of Burton's. Basically a noon estimated latitude obtained by course and distance run from (usually) an a.m. sun intercept terminal point was compared to the noon latitude by observation. The difference was multiplied by the 'C' correction to obtain the 'correction' to the estimated longitude. This is just the equivalent of a running fix by plotting.

If anyone is interested, I can scan and post the full explanation.

Burton's Tables do everything that Nories do, but in my opinion are superior in terms of layout and clarity.

Regards to all

John

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