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Re: How Many Chronometers?
From: George Huxtable
Date: 2009 May 8, 14:40 +0100
From: George Huxtable
Date: 2009 May 8, 14:40 +0100
Paul Hirose pointed us to an interesting paper in the monthly notices of the royal astronomical society, 1853, at http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?journal=MNRAS&year=%3f%3f%3f%3f&volume=..14&letter=.&db_key=PRE&page_ind=19&plate_select=NO&data_type=GIF&type=SCREEN_GIF&classic=YES&high=4899d32e1929742 This was "On rating chronometers by lunars", by H Toynbee, commander of the Gloriana, East Indiaman.. It had a few surprises, for me. It argued (contrary to an opinion I had recentltly expressed in a posting, that lunars were insufficiently accurate to use for rating chronometers) that lunars, taken at regular fortnightly intervals, provided useful chronometer information. Anyone interested in this matter will find his figures worth examining. The paper referred only to a single chronometer on board, made by Dent. There seem to have been others, though, because he writes, at the end of the voyage, "the chronometers were taken by the maker", but for some reason other chronometers were not discussed. East India Company vessels were usually regarded as being lavishly fitted-out, and as late as 1853, I would expect a set-of-three, as we've discussed before. She was making a return passage from Melbourne, via Madras and St Helena. She had taken all of two months from Madras to St Helena, which I would have thought to be slow, even by East India Company standards. Their ships had a reputation of being safe and reliable, but ponderous and slow. The approach to St Helena is of interest. "We hove-to at 4am, July 12th, and at daybreak found the island bearing north-west 10 miles, as expected.". Shows some confidence in their astro navigation, doesn't it? That they press on in the dark towards a rocky steep-to island until they reckon to be within 10 miles, and only then do they heave-to. Would we be as bold, I wonder? It's interesting that St Helena had its own time-ball, in 1853, which ships would use to check their times. That would depend on knowing a precise longitude for the island. I wonder how that had been ascertained? Jupiter satellites, perhaps, or Moon occultations? Might it have come from Halley's observations there, 170 years before? But I'm puzzled by their tables of numbers, and I wonder if Paul Hirose, or anyone else, can throw light on it. The observations appear to have been taken at dates in 1853 of May 14, May 30, June 12, and so on, at fortnightly intervals when the Sun was in quadrature with the Moon; the best times for Sun Lunars, at first or last quarter. And yet, for each of these dates, two values of deduced clock error are shown, each against one or other of the following labels- either 1. (Sun symbol) E. (Moon symbol) or else 2. (Sun symbol) W. (Moon symbol) where what I take to be the Sun symbol is a circle with a dot in the centre, and the Moon symbol is a crescent. In alternate fortnights, as at May 14th, line 1 is above line 2, and vice versa on May 30th etc. However, over a particular quadrature, the Sun can be only East or West of the Moon, not both. On May 14th, it was East. On the 30th, West. So what are those two lines of data? What am I misunderstanding? Of course, if star lunars were being observed, they could be either East or West of the Moon, but that's not what those symbols indicate, as I see it. And Toynbee's words "In several instances lunars were obtained on only one day of the sun and moon's continuing in distance" imply to me that he was taking Sun, not star, lunars: otherwise, those words "in distance" wouldn't apply. I'm also puzzled about his numbers for "maker's error" near the foot of his table, and somewhat suspicious about his comment that "the chronometers were taken by the maker on the 14th, and the daily rate gradually decreased to 6.5s during the first five days." If anyone else takes a look at this interesting paper, I would be pleased to see any comments. George. contact George Huxtable, at george@hux.me.uk or at +44 1865 820222 (from UK, 01865 820222) or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---