NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Request for help re sunset predictions.
From: George Huxtable
Date: 2005 Apr 6, 20:05 +0100
From: George Huxtable
Date: 2005 Apr 6, 20:05 +0100
I'm following some whaling journals of William Scoresby the younger, who visited the Greenland Sea (West of Spitzbergen) each year from 1811. Some of his time-sights, to determine LAT, were taken by observing the moment of sunset. I take that to be defined by the last glimpse of the Sun's upper limb above the horizon. Does anyone think differently? To my mind, it's a poor choice of moment to determine time, when the Sun's centre appears to be actually below the horizon, and refraction corrections are large, and rather variable. However, that was what he did, on occasion. It saved the trouble of getting his sextant out, no doubt. He appears to have obtained his local time, at the moment of sunset, from tables into which he entered lat and dec, quoting a resulting time of sunset to the second, e.g. "6h 13m 28s pm". Does anyone know where such tables were to be found, by a navigator in 1811? My earliest such compendium is Raper's "Practice of Navigation", 1864, in which table 26 is "apparent time of the Sun's rising and setting", tabulating lat at intervals of 1deg, but dec at intervals of 2deg, and giving a time to the nearest minute. Not nearly good enough for interpolating a result to the nearest second. Not only that, the time of sunset, for all lats, when the dec is exactly zero, is given as exactly 6pm. That would only be true for a star (with no semidiameter) and if the refraction and dip were exactly zero: or if all three quantities cancelled out to zero. It seems that Raper's table 26 is intended to give no more than a rough notion of time of sunset, good enough for many purposes, but not for a time-sight. I also have an edition of Norie's, tables dating from 1914, which gives table XLIII (43), "semidiurnal and seminocturnal arcs" , giving times from noon to sunset to the nearest minute, and in this case the decs are tabulated in intervals of 1 degree. But this is claimed to handle "any celestial object", and there's no provision to insert a Sun semidiameter, so presumably this table also isn't intended to give any precise timing for the moment of sunset. So I ask any Nav-L members, who own or have access to navigation tables for the early 19th century, whether they can identify any table, anywhere, of sunrises/sunsets, that Scoresby might have used to get his LHA, in 1811 and following years. George. ================================================================ contact George Huxtable by email at george@huxtable.u-net.com, by phone at 01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. ================================================================