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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Frank Reed
Date: 2023 Dec 19, 06:19 -0800
What portions of a typical almanac page for Moon-culminating stars would you need to reproduce today or might wish to reproduce for a sort of historical re-enactment? Below I'm attaching a page from 1864 from the Nautical Almanac and Astronomical Ephemeris (that's the British astronomers' almanac, which at that time was mostly comparable to the modern "Astronomical Almanac"). How were the stars selected? Should a modern observer (for the sake of historical re-enactment) use stars from the lists that they used in the era, or would it be acceptable to work with any reasonable stars? And what qualifies as a reasonable star for this process?
Would anyone care to walk through an explanation of this almanac data page? I'll throw in one detail. There's a column that shows the magnitudes of the stars. There was no photometry to speak of at this time so magnitudes were listed to the nearest integer usually and sometimes the nearest half magnitude. If you're accustomed to a star being listed with a magnitude of, let's say, 5.21 in a modern table, here it would be just plain "5". But in that same column for magnitudes, there is an entry for the Moon whenever it's listed. This is not a magnitude. Instead it's the age of the Moon in days, tucked in there to avoid adding another column to the table.
On a general historical note, timings of observations of the Moon and nearby stars on the meridian (these so-called Moon-culminating stars) had a rather short window of practical value to astronomers and astronomical surveyors. When were they used? Who used them? What replaced them? And does their presence in an astronomical almanac imply that they were in wide use? And finally, why not just shoot lunars (logitude by lunar distances)? What are the advantages?
Frank Reed
