On Thu 2023-12-28T09:23:30+0000 Poul-Henning Kamp hath writ: > Also: When&where celestial navigation is possible, most vessels > travel a lot further than 50 meters during the time it takes to make > a measurement of the necessary precision.
As noted by ION https://www.ion.org/Museum/item_view.cfm?cid=6&scid=5&iid=29 "Modern sextants can read the angle to a 0.1 minute level of accuracy" and older sextants to more like 0.5 arcminute. The best navigator under the best circumstances might get to within 0.1 nautical mile. More typical accuracy in actual use was several arcminutes or several nautical miles. When a sextant was used to shoot the moon to determine time half an arcminute accuracy means one time minute accuracy which is 15 arcminutes of longitude, so longitudes in mid-ocean could be off by many nautical miles. Frank Reed teaches the old techniques and if the opportunity presents itself I highly recommend taking his classes. https://reednavigation.com Few living astronomers have held a sextant in hand to shoot stars and the moon. I have, but during my lifetime has grown as disconnect between theory and practice. Some who are responsible for the tabulations in the almanacs are not facile with those techniques. Frank Reed taught me that the reason navigators did not like changes to the almanacs was because the reduction of a lunar was basically a ritual. Navigators did not know the nitty gritty of the celestial mechanics, but they did know a sequence of operations with the almancs which would produce a valid position. The instructional manuals for navigators still teach the equator and equinox and Greenwich meridian, but the equinox was abolished entirely by the IAU in 2003. The values tabulated in the almanacs had ceased to represent those original concepts a century before that, but a navigator who used the tabulations in the almanacs in the classical fashion would still compute a correct result. Gary Miller recently noted that navigators do not know about the existence of DUT1. I suspect that may be because the calculations are no longer being performed according to the ritual found in the old log books of ships. If the calculations are now performed by machines that are connected to telecom systems then all of the nitty gritty details like DUT1 are probably in the computer. -- Steve Allen <s...@ucolick.org> WGS-84 (GPS) UCO/Lick Observatory--ISB 260 Natural Sciences II, Room 165 Lat +36.99855 1156 High Street Voice: +1 831 459 3046 Lng -122.06015 Santa Cruz, CA 95064 https://www.ucolick.org/~sla/ Hgt +250 m _______________________________________________ LEAPSECS mailing list LEAPSECS@leapsecond.com https://pairlist6.pair.net/mailman/listinfo/leapsecs
