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Re: Lunars for dummies like me
From: Jan Kalivoda
Date: 2004 Sep 24, 07:42 +0200
From: Jan Kalivoda
Date: 2004 Sep 24, 07:42 +0200
Two small addenda to George's response: The ship clock was regulated according to time sights of Sun and trusted between on sailing ships, at least if there wasn't any chronometer onboard. So the application of the Equation of Time wasn't important at sea without a chronometer, at least before 1833 (or 1832/34?), when GAT was superseded by GMT as the argument of the Nautical Almanac. This was caused by growing numbers of chronometers at sea, of course. Previously even the lunars were given in GAT. Theoretically, even without a previous or subsequent Sun time sight, it was possible to gain the longitude by a twilight lunar distance from a star or a planet without any clock. From the time sight of a star (that one used for the lunar distance or another, nearer to the first vertical, if needed), the local star time can be obtained easily. The GAT (before 1833) or GMT (after) was needed for changing LST to LAT (to be compared for longitude with GAT obtained from the cleared lunar distance before 1833), but it can be assessed very roughly - the error of one hour in it caused an error of 2.5 nm in the longitude obtained on the equator, less elsewhere. (According to the changing difference of LST and LAT/LMT, growing for cca 10 sec during an hour. ) A trifle with lunars. This procedure was taught in manuals. Of course, the reality differed. As Frank Reed wrote here once on the basis of old whalers' logbooks, the great majority of lunars was taken from the Sun and probably for the great majority of the minority of lunars taken from stars, the George's procedure (combining the lunar observation with a previous/subsequent Sun time sight by a clock) was used as more accustomed to. And it was the only option, if Moon's and/or other body's altitudes were not observed, but computed for clearing the lunar observation, as needed above the indistinct night horizon. Jan Kalivoda