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    Re: Eclipses of Jupiter's moons: Did ships tend to carry the requisite equipment?
    From: Patrick Stanistreet
    Date: 2004 Feb 17, 05:41 -0800

    I plan on testing the method of Jupiters moons, at 4 am
    this morning about 60 miles north of Los Angeles, CA
    I could see three moons for about 30 minutes then clouds
    rolled in ending the observations. My telescope is
    rather small so I wont be going for much accuracy.
    Since I dont have any tables I tried a search on
    google using jupiter moons ephemeris
    
    First link was helpful
    
        http://ephemeris.sjaa.net/0206/d.html
    
    He had a link to some tables and source code which I have
    not tested at this time but will be using the tables to
    time events.
    
        http://www.projectpluto.com/jevent.htm
    
    I am curious what type of  data appears in George Huxtable's
    almanac of 1864 as compared to the tables at projectpluto.
    
    
    George Huxtable wrote:
    > Carl Zog asked about detemining time, and longitude, from observation of
    > Jupiter's moons-
    >
    >
    >>Apparently, in one of his books, O'Brian's main character is carrying a
    >>fairly substantial telescope on board with which to determine longitude by
    >>the eclipses of Jupiter's moons.
    >>
    >>Presumably, he would be conducting these measurements on some remote
    >>coastline and not on board. But even assuming that much, the question was,
    >>how likely was a British naval ship or its captain to have carried or used
    >>such equipment circa 1800?
    >
    >
    > ==================
    >
    > I think it's plausible, if he was on some exploratory geographical voyage
    > intended to ascertain longitudes of various islands, capes, and headlands,
    > for marking on the chart, but certainly not for a run-of-the-mill posting.
    > Using Jupiter satellites was probably the most accurate way to do that job
    > on land, until the days when chronometers were precise and cheap enough to
    > allow dozens of them to be carted to and fro. But there were periods of a
    > couple of months in each year when Jupiter couldn't be seen at all.
    >
    > Each observation would involve going ashore with the telescope. A 2-foot
    > long reflector would do the job adequately. Predictions of Jupiter
    > satellites remained in the almanac until at least my copy of 1864.
    >
    > Cook occasionally observed immersions and emersions of Jupiter satellites.
    > Malaspina, in a Spanish Navy scientific expedition into the Pacific, was
    > getting very accurate longitudes of Pacific islands using Jupiter
    > satellites, in 1792. See vol 2 of the Hakluyt Society's Malaspina Journals,
    > ed. Andrew David, page 276 ff.
    >
    > Around that date this method was, in my opinion, the most precise way of
    > obtaining longitudes, for expeditions that would be away from Europe for
    > long periods, because over that time, chronometers were likely to drift
    > off. And the great advantage, compared with lunar distances, was that it
    > involved very little computation to get to the answer.
    >
    > There's an article in "The Quest for Longitude" (ed. Andrewes, Harvard,
    > 1996) by Van Helden, "Longitude and the Satellites of Jupiter", which ends
    > "but if this method was not practical at sea, we must not forget that in
    > was at the center of the revolution in mapping of the seventeenth,
    > eighteenth, and nineteenth centuries." Note the "nineteenth".
    >
    > But how seriously should you take O'Brian's revelations? I admit that he
    > isn't my favourite author of such Napoleonic Wars tales; Forester is. I get
    > the feeling that O'Brian tended to browse a library for some obscure fact,
    > which he has then determined to work into a story to make it sound
    > convincing, and to make himself appear more learned.
    >
    > ==============
    >
    > In a later posting, Carl Herzog made some statements that puzzle me-
    >
    > He said-
    >
    > "It was first proposed by John Flamsteed in the late 1600s and the first
    > calculated distances appeared in the British Nautical Almanac of 1765."
    >
    > The first Nautical Almanac was for the year 1767, though it was printed a
    > couple of years before. But long before then, Jupiter satellite timings
    > (not distances) had appeared in the French "Connoissance des Temps"from
    > 1690 after calculations by Cassini. The method itself was first proposed by
    > Galileo in 1616, within a few years of his discovery of those moons.
    >
    > He added-
    >
    > "As John Forrester indicates, there were numerous impracticalities associated
    > with this technique. Compounding those he mentioned are the need to discern
    > the semi-diameter of the lunar body in order to accurately gauge the
    > beginning of the eclipse and the need to determine the errors caused by
    > atmospheric refraction."
    >
    > I haven't seen anything from John Forrester. But there was no chance to
    > "discern the semi-diameter of the lunar body". In those telescopes the
    > satellite would appear as no more than a point of light, never a disc. The
    > best you could do was to determine the moment when its light finally
    > snuffed out, which depended, to some extent on the light-gathering power of
    > your telescope, compared with that at Greenwich Observatory. Atmospheric
    > refraction didn't come in to the observation at all.
    >
    > Carl said-
    >
    > "Despite these shortcomings, astronomers viewed this as a better method for
    > determining longitude than the use of our own lunar eclipses -- based on
    > difficulties prediction our moon's orbit".
    >
    > Well, by 1800, the lunar predictions had got pretty good, and the real
    > difficulty was observing the lunar distance itself to sufficient accuracy.
    >
    > "As in all astronomical methods, you compare the local time of your sight
    > with the time in the almanac. Local time was calculated by observation of
    > the sun and a little trig -- determing the angle between the sun's azimuth
    > and your meridian provides the time before or after local apparent noon."
    >
    > No, there's no way to obtain those time-differences with sufficient
    > accuracy by measuring azimuths; the azimuth doesn't enter into it. Some
    > sort of reasonable timepiece is required to interpolate times, though it
    > doesn't need to be a full-blown chronometer to do that job.
    >
    > 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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