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## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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Re: Longterm Sun's GHA and Dec: what's under the hood?
From: Geoffrey Kolbe
Date: 2018 Dec 6, 23:16 -0800

Robert VanderPol II wrote:

"The manner with which every long term almanac is arranged depends on the arranger and only a few long term almanacs have been created so I don't thnk there is going to be much info on how to create them."

As one of those rare beasts who have actually arranged and published a Long Term Almanac I suppose I should respond. It is not that difficult really. Quoting from the introduction to my own LTA, "These (ephemerides) are based on the fact that approximately correct values for the position of the sun.... may be obtained from an almanac that is exactly four years out of date. By applying quadrennial correction factors (to the GHA and Dec) an ephemeris for any given year may be used to determine the position of the sun.... to good accuracy exactly four years hence and multiples thereof." "The period of validity for these tables has been set at fifty years, that being the period for which simple, linear, quadrennial correction factors will still give good accuracy for corrected table values. In fact, good accuracy should be maintained (for sixty years) but accuracy will deteriorate rapidly thereafter."

As to accuracy, a random sample statistical analysis of the errors in the sun's position over the fifty years for which the tables are valid showed that on average the error would be about 0.25'. In one percent of cases the error would be about 0.5', but the error should never exceed 0.7'

There was a Long Term Almanac published in the 1981 edition of Bowditch (volume 2). The epoch date was 1972, but no period of validity was stated and the quadrennial corrections seemed to be wrong for any period after 1972. The tables were effectively useless. However, I liked the general layout of those tables, so my tables are based on those in the 1981 Bowditch.

Having created a Long Term Almanac for the sun and selected stars, it is tempting to throw in sight reduction tables (which I did), a map of the world, tide charts, long term trends of the stock markets, and anything else that might conceivably be useful. But sanity prevailed and I just stuck with the LTA and sight reduction tables.

Geoffrey Kolbe

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