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    Re: 7 ways to determine longitude
    From: George Huxtable
    Date: 2003 Dec 24, 17:31 +0000

    Dan Allen said-
    >I have just started to read an old book called "Spherical and Nautical
    >Astronomy", a two volume set by William Chauvenet who was a professor
    >at Washington University in St. Louis.  My copy was printed in 1903 but
    >it appears that it is just a reprint of an 1863 edition.
    >Chapter 7 is "Finding Longitude by Astronomical Observation" where it
    >lists seven different methods, which are:
    >"1st method - by portable chronometers
    >  2nd method - by signals
    >  3rd method - by the electric telegraph
    >  4th method - by moon culminations
    >  5th method - by azimuths of the moon, or transits of the moon
    >               and a star over the same vertical circle
    >  6th method - by altitudes of the moon
    >  7th method - by lunar distances"
    >By chronometers it means chronometric expeditions: taking a chronometer
    >from Greenwich to Boston and comparing mean noon times.  By signals it
    >includes eclipses, occultations of Jupiter's moons, and terrestrial
    Chauvenet (presumably, from his name, of French extraction) was one of the
    important names in early American science. (In astronomy, Simon Newcomb was
    the next, to establish the American lead over the rest of the world in
    astronomical matters). Chauvenet's great two-volume work was intended for
    astronomers and surveyors, rather than for the more simple-minded
    navigator, I suspect. It is remarkably thorough and comprehensive. Lots of
    detailed drawings of survey instruments and telescope mounts.
    It's the only published work that I have seen, which assesses the (small)
    effect of the elliptical shape of the Earth on the AZIMUTH of the Moon.
    That doesn't affect any observations involving just the Moon's altitude,
    but it does, of course, affect the oblique lunar distance to another body;
    an effect that's usually neglected.
    I have never seen any note, in a subsequent work, drawing attention to an
    error in Chauvenet, so I conclude he got it all right.
    The chronometer expeditions that Dan mentions were big business before the
    telegraph came in. Literally dozens of chronometers were shipped bachwards
    and forwards between Greenwich and Dublin, time and again, to establish
    their longitude difference. Also between German observatories and Pulkovo
    (St Petersburg). Does anyone know whether chronometers were ever shipped up
    the Mississippi for that purpose, before the days of the telegraph? Or sent
    by pony express, even, if they could withstand the rigours of that journey?
    My own copy of Chauvenet has a text-date on the preface of 1863, like
    Dan's, but otherwise says nothing about its date other than "5th edition".
    Does Dan have an edition-number with his 1903 printing, that might help to
    fix the date of my own copy better?
    contact George Huxtable by email at george@huxtable.u-net.com, by phone at
    01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
    Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.

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