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Subject: Re: occultations of jupiter's satellites
From: George Huxtable (george@XXX.XXX)
Date: Sun Apr 06 2003 - 20:06:41 EDT
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
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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
sea.
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
timing.
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
flexible.
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
him.
George.
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contact George Huxtable by email at george@XXX.XXX 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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