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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.

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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.



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