A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Newton and Halley
From: George Huxtable
Date: 2007 Nov 20, 22:48 -0000
From: George Huxtable
Date: 2007 Nov 20, 22:48 -0000
----- Original Message ----- From: "Michael Daly"
To: Sent: Monday, November 19, 2007 11:42 PM Subject: [NavList 3927] Re: Newton and Halley | | George Huxtable wrote: | | > And again, I'm told that Halley wrote, in Phil. Trans. 37, (1731), 185-95, | > that he obtained his longitudes at sea by observations of "a near Transite | > of the Moon by a known fix'd star", using "a five or six foot telescope, | > capable of showing the appulses or occultations of the Fix'd stars by the | > Moon, on shipboard, in moderate weather." How he managed such a telescope, I | > have no idea, but at a guess it must have been suspended from the rigging at | > the far end. Mike replied- | Anyone with experience at sea knows that you can't steady an instrument | by coupling it to the vessel - you have to isolate it. That's of some interest. Not that a disagree with that assessment. It is indeed better if you can isolate it. But if Mike is claiming special authority as a result of his experience at sea, it would by nice to learn how much sea experience, as a celestial observer, he actually has. Within our existing community we have got to know, over the years, each other's personal strengths and weaknesses in that regard, but Mike is rather an unknown quantity at present. Tell us more | That's why | Galileo suggested nested hemispheres as an observing platform for his | celatone and Maskelyne tested a gimballed chair. Yes, that's why they tried them, and they were both complete failures, quickly abandoned | Suspending a telescope | from a yardarm would be futile. It would indeed. Nobody has suggested that, however. It would be quite absurd, as anyone with any knowledge of square rig would realise, to suspend from a yardarm. What I wrote was- "How he managed such a telescope, I have no idea, but at a guess it must have been suspended from the rigging at the far end." On second thoughts, I would alter that a bit, to suspend it by a sling from near its mid point, or its centre of gravity, rather than its far end. But the obvious part of the rigging to sling it from would be between two shrouds (which are set up bar-tight), through a gap between the ratlines; never from a yardarm. The earliest brass sextants, 18 or 20 inches radius, from Bird or Ramsden, were too heavy for a man to operate properly, as was Mayer's circle. A staff was provided to take the weight, with a ball-joint at the top, which was supported either from the deck (like a 'cello) or from a belt round the observer's middle (like a modern banner). Constrained in that way, it must have made operating the instrument particularly difficult, but not impossible. It must have been easier for navigators when the size was reduced to 12 to 15 inches, as brass sextants became during Cook's time, and so could be used without a staff, though a navigator still needed a powerful right arm. It's amazing what people can learn a way to do if there's no alternative. Similarly with Halley's telescope. | | At the end of the 17th century, telescopes were typically made with | pasteboard covered with shagreen, vellum or paper. Lens cells and | fittings were brass but the overall weight of such a telescope was low | compared to a brass tube. In small apertures, such a scope would be | very awkward due to its size, but not due to its weight. Halley could | have managed one for lunar occultation or appulse observations. | On the other hand, Newton's instrument called for brass and a stiff tube | would be required to support the brass plate. That sentence appears to have been carefully constructed to mislead the reader into thinking that Newton had specified a brass telescope, but not so. These were Newton's words- " In the attached scheme, PQRS denotes a plate of brass, accurately divided ..." (Size not mentioned) AB, is a telescope, three or four feet long, fixt on the edge of that brass plate" (material not mentioned) And (especially if we were to take the Royal Society's engraving at face value, as Mike seems to wish us to do), there would be no call for a stiff tube to support the brass plate. That would be the tail wagging the dog. It's the brass plate, of whatever size, supporting the telescope, whatever it's made of. | Using Newton's instrument, as described in his notes, would be basically | impossible. It's just too big and heavy. Unless some kind of smaller | instrument that yields a suitable accuracy can be identified, it remains | that the instrument that Halley used was unknown. That applies only to Mike's special interpretation of the instrument, and to the Royal Society engraving. To an open mind things are different, with our new understanding of how Newton's note can be read.. George. contact George Huxtable at george---.u-net.com or at +44 1865 820222 (from UK, 01865 820222) or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. --~--~---------~--~----~------------~-------~--~----~ To post to this group, send email to NavList@fer3.com To unsubscribe, send email to NavListemail@example.com -~----------~----~----~----~------~----~------~--~---