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    Re: occultations of jupiter's satellites
    From: George Huxtable
    Date: 2003 Apr 7, 01:06 +0100

    Federico Rossi wrote-
    >I've read Arthur Pearson's contribution about Lunar distances and found it
    >very interesting.
    >As far as getting accurate time at sea is concerned, I think another
    >method is worth noting.
    >My father was an Italian merchant marine officer during the '60: an older
    >officer once told him he had still used a method based on timing the
    >occultations of Jupiter's Galilean satellites to get the K of
    >chronometers, probably before WWII when accurate radio timing signals were
    >not always available. A long focus telescope was installed on the bridge
    >and, comparing the timing of the occultation given by the nautical almanac
    >and the time given by the chronometer, one could obtain the correction
    >later used to reduce star sights. This method doesn't even need to take
    >sights with sextant and, in periods of good Jupiter visibility, several
    >occultations occurs every day.
    >Have you ever heard about this technique?
    >Federico Rossi
    George Huxtable responds.
    Yes, it was a good method to check chronometers by, but only from on-land,
    and only accurate to a minute or so of time. I do not think such timings
    would have been of much  value in rating a 20th-century chronometer.
    The method might conceivably have worked from the deck of a large vessel in
    harbour, but I have seen no account of it being used successfully at sea. I
    would be very interested to hear further details of any such reports.
    The difficulty is this: to see the extinction of the light from the inner
    satellites of Jupiter requires a telescope with a magnification of around
    50 or more. With such a telescope, it was impossible to keep the image of
    Jupiter and her satellites in view from the unsteady platform of a ship at
    In the 1760s, Maskelyne tried to test at sea a "mariner's chair", invented
    (not by him) to stabilise an observer by swinging his chair, with
    telescope, in gimbals, above deck, for just this purpose. It was a
    disaster, exaggerating the motion rather than reducing it!
    Used on land, however, the method was more accurate than lunars in
    determining time and thereby longitude. It was used by many surveyors and
    explorers, including Cook, who would use it when he could, after a
    landfall, to check his chronometers.
    The difficulty of accurate timing is this. The observer has to try to
    record the moment when the last glimmer of light disappears from the
    satellite as it goes into shadow. But this fading is a gradual process, and
    an astronomer working from an observatory with a large-aperture telescope,
    in good seeing conditions, will still see a twinkle of light for many
    seconds after the satellite has become invisible to a field-observer in
    poor viewing. So there is always much judgment and subjectivity in the
    Federico added-
    >in periods of good Jupiter visibility, several occultations occurs every day.
    This is overstating it somewhat.
    Of all the predicted satellite events, it's only the immersions that are
    useful for the purpose, and from a particular location only some of those
    will be visible when the Sun is down. In Cook's era, only Io and Europa
    were accurately predicted. For a couple of months each year, Jupiter isn't
    visible at all. For a couple of months either side of that, Jupiter is near
    to the Sun so that only a few satellite events occur in the narrow
    time-window before dawn or after dusk. In that case, suitable immersions
    can be weeks apart. Surveyors could time their period of work to fit in
    with periods of good availability of Jupiter, but voyagers were rather less
    And I have omitted to mention periods of overcast, of course.
    I was involved in the planning of observations, in a recent re-enactment by
    the BBC of Cook's Westerly passage from the Queensland coast to Indonesia.
    Cook had checked his chronometer against a Jupiter satellite event before
    departing from Endeavour River, and the navigation team hoped to do the
    same from their departure point. Unfortunately, Jupiter had only just
    recently emerged as a morning star, and useful immersions were rare. The
    ship planned several landfalls, but none would coincide with a suitable
    satellite event.
    However, an immersion would occur at that location several days before
    departure, so the navigation team arranged to arrive in Queensland early,
    and set up on land to observe Jupiter before dawn next morning. It was a
    beautiful morning, but a cloud intervened at the crucial moment: just as
    you would expect.
    All was not lost, however. Before departure, one of the team (John Jeffrey)
    had tested the principle in a just-before-dawn on-land observation, under
    very difficult viewing conditions, from the UK. He obtained a time for
    disappearance of Io which was within a minute or so of the almanac
    prediction. We thought that was pretty good. Cook would have been proud of
    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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