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    Re: Latitude by Spica
    From: Kieran Kelly
    Date: 2004 Jan 12, 15:14 +1100

    Frank reed wrote:
    
    >Celestial Navigation almost never involved stars in the 19th century
    (except for lunars). I
    >found ONE example of a star sight. It's from the whaleship Stonington in
    the spring of 1847
    >out in the middle of the Pacific. It's such a rare thing that I thought it
    would be worth
    >separate mention here.
    
    He then mentions the Stonington officers performing a lat observation of the
    star Spica saying
    
    "it appears the officers were looking for some entertainment."
    
    Hate to disagree but the use of Meridian Passage of stars to determine
    latitude was very common in land based exploration.
    
    The Australian explorer Augustus Gregory used meridian passage of stars
    virtually every night during the two years of the North Australian
    Expedition 1855-56. He is on record as saying that he preferred a star's
    meridian passage over the sun as;
    1) It was more accurate
    2) It removed the need for a lengthy noon time halt
    3) It reduced  the need to unpack equipment during the day then re-pack it.
    4) It reduced the possibility of native attack during the noon halt.
    
    His technique was to shoot the meridian passage of at least two stars
    culminating in close succession, work out the latitudes and divide by two to
    get an average latitude. Sometimes he used three stars. I have found his
    latitude work extraordinarily exact - usually under 400 metres.
    
    I would estimate that in the course of his four major exploring expeditions
    (as compared to his everyday surveying work) he calculated latitude by
    culmination of stars on about 800 separate occasions.
    
    I think the technique was also more commonly  accepted in those days than
    you think. My copies of Raper (1849) and Norie (1852) both contain clear
    exposition of "Lat by Mer Passage of a star" and appear to give it equal
    weight to latitude by meridian passage of the sun.
    
    I don't think we should fall into the trap of believing that sailors were
    the only ones to hoist a sextant in the Nineteenth century. I am sure there
    were just as may surveyors and explorers peering into their mercury horizons
    in the middle of the night and struggling to read the vernier.
    
    Kieran Kelly
    Sydney
    Australia
    
    
    

       
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