[LEAPSECS] UT1 offset

[Steve Allen]

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                    <sla at ucolick.org>              WGS-84 (GPS)
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