A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Frank Reed
Date: 2018 Jul 17, 11:29 -0700
Tony, you wrote:
"it requires to calculate the DD from a calendar date"
That may be less trouble in a "real practical" case than it seems (however we may interpret "real" and "practical" in this context). Just count the days...
If you prefer, you could also create something much like that old rule involving the int() function that you found in Henning Umland's code. That trick goes waaay back. That was the basis of one my very first serious computer programs, by the way. :) In 1978, when I was 15, I learned to code in Fortran on the mainframe computer in an evening class at the local college. The textbook was "Fortran for Humans". It's around here somewhere... Naturally, the program was on punched cards. I think I found that algorithm for calendar calculations in the back of the Explanatory Supplement to the American Ephemeris and Nautical Almanac. It seemed magical back then. Makes sense now --though still pretty clever, and you can develop similar tricks for other calendar problems.
For some problems, you can just read LHA Aries from the sky itself. You can look at the position of the Big Dipper and other circumpolar star patterns relative to the North Star. That's plenty good enough for a Polaris altitude correction to get latitude.