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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Time-sight time conventions
From: Paul Hirose
Date: 2018 May 20, 00:05 -0700
From: Paul Hirose
Date: 2018 May 20, 00:05 -0700
On 2018-05-17 11:15, Ed Popko wrote: > Look towards the bottom in the final steps. The example shows Apparent Time at the Ship to be 3:56:42 and Greenwich time 7:45:27. Given the date above, 1880, we know we are dealing with GMT Civil Time or 12:00:00 at noon. I don't think so. Didn't the Almanac in that era follow the astronomical time convention? I.e., GMT was 12 hours behind Greenwich Civil time. In a controversial step adjustment, GMT changed to the civil time convention at the beginning of 1925. In the example from Lecky, ship time is 1880 June 25, "about 4 p.m." At the observation, the chronometer reads 7:47:57 after correction for rate. Longitude by DR is 57 12 W, so GMT (modern style) is about 4 hours ahead of local time. Thus chronometer time corrected for rate is 19:47:57 GMT modern style, or 07:47:57 GMT old style, June 25 in both cases. At that time I get these values from the USNO MICA software: 14 05 14.2 Greenwich apparent sidereal time 6 19 47.0 Sun apparent right ascension ---------- 7 45 27.2 Greenwich apparent solar time (old style) That matches the "Apparent Time at Greenwich" near the bottom of the page in Lecky. Since I calculated solar time in the old style, the 12-hour adjustment required by the modern style isn't necessary. In other words, old style apparent solar time is simply the GHA of the Sun.
