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    Re: Jupiter satellites
    From: Alexandre Eremenko
    Date: 2005 Apr 5, 09:34 -0500

    Dear George,
    Thank you for so many interesting details.
    I afraid that my project with Jupiter satellites
    will fail because my binocular is not powerful enough
    for timing with any reasonable precision.
    The sky, as seen from my balcony, is dark enough to
    see the satellites, but a major obstacle seems to
    be Jupiter itself: it is too bright and obscures
    the satellite which is closest to it.
    Alex.
    
    On Tue, 5 Apr 2005, George Huxtable wrote:
    
    > Alex asked-
    >
    > >This night I saw Jupiter satellites
    > >from my balcony, all 4 of them.
    > >Is there any web almanac for them, to
    > >practice in longitude from Jupiter satellites?
    > >(I imagine that timing their ocultation will be hard
    > >with my 10x50 binocular, but I would try).
    > >Alex.
    >
    > =============
    >
    > Alex must enjoy nice dark skies from his balcony.
    >
    > When he sees Jupiter satellites in his 10x50 binocular, they are then at
    > full brightness, but to time them as they vanish, at much lower light
    > levels, a larger-aperture instrument would be better. And some sort of
    > mounting would make observation easier.
    >
    > I don't know about a web version, but a printed almanac of such Jupiter
    > events is available in the "Astronomical Almanac", jointly produced in
    > London and Washington. That was the case a few years ago, anyway: I haven't
    > consulted that almanac in recent years.
    >
    > However, times of these events are printed only to the nearest minute,
    > although Jupiter satellite timings were, and are, useful to a better
    > accuracy than that. In contrast, even Maskelyne's predictied times of such
    > events were given to the nearest second of time, right back to the first
    > Nautical almanac of 1767, though I have doubts whether his calculations
    > were correspondingly precise.
    >
    > When I needed to know about timing of Jupiter satellite events a few years
    > ago, Catherine Hohenkerk of the Nautical Almanac Office kindly pointed me
    > to a source of precise information on phenomena concerning the satellites
    > of Jupiter. This is-
    > "Satellites Galileans de Jupiter", a supplement to the "Connaissance du
    > Temps", published by the Bureau des Longitudes, Observatoire du Paris, each
    > year. It's likely that many actively-observing observatories will get a
    > copy of that publication, and may be willing to share it.
    >
    > A website was given, then (2001), at http://www.bdl.fr  and an
    > anonymous.ftp site at ftp://ftp.bdl.fr, neither of which I have visited.
    >
    > This publication provides precise timings TO THE SECOND of all phenomena
    > involving the four bright satellites of Jupiter, distinguishing carefully
    > between the various stages of immersion into umbra and penumbra, and
    > occultation. It gives precise calculated values for the current year and
    > algorithms for calculating less precise predictions for the succeeding
    > year.
    >
    > The text is given in both French and English.
    >
    > Alex will perhaps discover a difficulty in using this information for
    > precise timing. As I recall, the tables relate to the "geometrical" instant
    > of vanishing or reappearance of the satellite. In practice, the brightness,
    > as the last sliver of the satellite goes into the deepest umbra of the
    > planet, falls from its maximum value over a period of a few minutes, and
    > the final extinction is a gradual process lasting over some tens of
    > seconds. Those tabulated timings relate to an observation by a
    > theoretically perfect telescope that can detect the last few photons. In
    > practice, the satellite will disappear from an observer's view, many
    > seconds earlier, depending on the aperture of his instrument and,
    > crucially, on the darkness of his sky.
    >
    > Jupiter satellite timings were used extensively by land explorers and
    > geographers in the 17th-18th century, and gave a more precise result than a
    > lunar distance, but were only observable from on land, not at sea, because
    > the motion of the vessel made it impossible to get a good enough view of
    > the satellites in a telescope. Much of the world was mapped by this method.
    > Cook used the method occasionally, from on land, in his circumnavigations.
    > And the biggest advantage over lunars? It's simple and straightforward, and
    > involves no reams of calculation.
    >
    > One snag. Over a period of a couple of months each year, Jupiter is too
    > close to the Sun for the satellites to be useable, and for a couple of
    > months around that period, Jupiter events when the sky is sufficiently dark
    > are rare. For the rest of the year, timeable events occur frequently. Those
    > using the two inner satellites are best, because extinctions occur more
    > suddenly in time, and in the early days were better predicted by the
    > observatories.
    >
    > Maskelyne did not have available the precise information used by the BdL
    > for their modern predictions, and presumably his predictions of
    > disappearances were based on what observers could see in the instrument
    > used at Greenwich (whatever that may have been: Maskelyne doesn't say, in
    > the 1767 almanac). Perhaps some empirical relation was used by travelling
    > observers to correct for the difference between the aperture of their own
    > telescopes and that used at the observatory (Paris or Greenwich) but I
    > haven't yet found a reference to that matter, and would be pleased to learn
    > if anyone else has.
    >
    > I would be interested to learn how well Alex gets on, and how closely his
    > timings correspond with the predictions.
    >
    > George.
    >
    >
    >
    > ================================================================
    > contact George Huxtable by email at george---.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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