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    Re: Eclipses of Jupiter's moons: Did ships tend to carry the requisite equipment?
    From: George Huxtable
    Date: 2004 Feb 14, 00:28 +0000

    Carl Zog asked about detemining time, and longitude, from observation of
    Jupiter's moons-
    >Apparently, in one of his books, O'Brian's main character is carrying a
    >fairly substantial telescope on board with which to determine longitude by
    >the eclipses of Jupiter's moons.
    >Presumably, he would be conducting these measurements on some remote
    >coastline and not on board. But even assuming that much, the question was,
    >how likely was a British naval ship or its captain to have carried or used
    >such equipment circa 1800?
    I think it's plausible, if he was on some exploratory geographical voyage
    intended to ascertain longitudes of various islands, capes, and headlands,
    for marking on the chart, but certainly not for a run-of-the-mill posting.
    Using Jupiter satellites was probably the most accurate way to do that job
    on land, until the days when chronometers were precise and cheap enough to
    allow dozens of them to be carted to and fro. But there were periods of a
    couple of months in each year when Jupiter couldn't be seen at all.
    Each observation would involve going ashore with the telescope. A 2-foot
    long reflector would do the job adequately. Predictions of Jupiter
    satellites remained in the almanac until at least my copy of 1864.
    Cook occasionally observed immersions and emersions of Jupiter satellites.
    Malaspina, in a Spanish Navy scientific expedition into the Pacific, was
    getting very accurate longitudes of Pacific islands using Jupiter
    satellites, in 1792. See vol 2 of the Hakluyt Society's Malaspina Journals,
    ed. Andrew David, page 276 ff.
    Around that date this method was, in my opinion, the most precise way of
    obtaining longitudes, for expeditions that would be away from Europe for
    long periods, because over that time, chronometers were likely to drift
    off. And the great advantage, compared with lunar distances, was that it
    involved very little computation to get to the answer.
    There's an article in "The Quest for Longitude" (ed. Andrewes, Harvard,
    1996) by Van Helden, "Longitude and the Satellites of Jupiter", which ends
    "but if this method was not practical at sea, we must not forget that in
    was at the center of the revolution in mapping of the seventeenth,
    eighteenth, and nineteenth centuries." Note the "nineteenth".
    But how seriously should you take O'Brian's revelations? I admit that he
    isn't my favourite author of such Napoleonic Wars tales; Forester is. I get
    the feeling that O'Brian tended to browse a library for some obscure fact,
    which he has then determined to work into a story to make it sound
    convincing, and to make himself appear more learned.
    In a later posting, Carl Herzog made some statements that puzzle me-
    He said-
    "It was first proposed by John Flamsteed in the late 1600s and the first
    calculated distances appeared in the British Nautical Almanac of 1765."
    The first Nautical Almanac was for the year 1767, though it was printed a
    couple of years before. But long before then, Jupiter satellite timings
    (not distances) had appeared in the French "Connoissance des Temps"from
    1690 after calculations by Cassini. The method itself was first proposed by
    Galileo in 1616, within a few years of his discovery of those moons.
    He added-
    "As John Forrester indicates, there were numerous impracticalities associated
    with this technique. Compounding those he mentioned are the need to discern
    the semi-diameter of the lunar body in order to accurately gauge the
    beginning of the eclipse and the need to determine the errors caused by
    atmospheric refraction."
    I haven't seen anything from John Forrester. But there was no chance to
    "discern the semi-diameter of the lunar body". In those telescopes the
    satellite would appear as no more than a point of light, never a disc. The
    best you could do was to determine the moment when its light finally
    snuffed out, which depended, to some extent on the light-gathering power of
    your telescope, compared with that at Greenwich Observatory. Atmospheric
    refraction didn't come in to the observation at all.
    Carl said-
    "Despite these shortcomings, astronomers viewed this as a better method for
    determining longitude than the use of our own lunar eclipses -- based on
    difficulties prediction our moon's orbit".
    Well, by 1800, the lunar predictions had got pretty good, and the real
    difficulty was observing the lunar distance itself to sufficient accuracy.
    "As in all astronomical methods, you compare the local time of your sight
    with the time in the almanac. Local time was calculated by observation of
    the sun and a little trig -- determing the angle between the sun's azimuth
    and your meridian provides the time before or after local apparent noon."
    No, there's no way to obtain those time-differences with sufficient
    accuracy by measuring azimuths; the azimuth doesn't enter into it. Some
    sort of reasonable timepiece is required to interpolate times, though it
    doesn't need to be a full-blown chronometer to do that job.
    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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