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    Re: How Many Chronometers?
    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.
    
    
    
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