A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Eclipses of Jupiter's moons: Did ships tend to carry the requisite equipment?
From: Patrick Stanistreet
Date: 2004 Feb 17, 05:41 -0800
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 firstname.lastname@example.org, by phone at > 01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy > Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. > ================================================================ > >