NavList:

   

Reply

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@huxtable.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.
> ================================================================
>
>

   

Reply