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    Re: Jupiter satellites
    From: George Huxtable
    Date: 2005 Apr 5, 10:45 +0100

    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 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
    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
    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.
    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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