NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
An "alternative" sextant?
From: George Huxtable
Date: 2008 Dec 15, 00:15 -0000
From: George Huxtable
Date: 2008 Dec 15, 00:15 -0000
The following idea may be a bit daft, and if it is I hope Navlist members will identify the snags and point them out. What follows is an alternative way to reassemble the basic components of a sextant in a new geometry, which will give it different characteristics from the standard instrument. It's difficult to illustrate the notion without making a perspective drawing (at which I am hopelessly bad), so instead I'll try to paint a word-picture. Start by taking a normal sextant and rebuild it. I'm not asking you to demolish your favourite instrument; just imagine it in your head, which is all I've done, so far. The angling of the index mirror has to change. Instead of its plane being perpendicular to the frame, it's now placed at exactly 45� to the frame, in such a way that incoming light from the observed body is now reflected to travel along the pivot line of the arm. The horizon mirror is shifted and fixed to a bracket that places it so that the pivot line passes through its centre, with a few centimetres spacing between the two mirrors. The horizon mirror, too, is angled by exactly 45� to the plane of the frame (about a vertical axis). In that way light from the observed body, travelling along the pivot line, is reflected into the telescope, which is aligned to point to the horizon as normal, and relocated to centralise the horizon mirror in its view. The horizon mirror is half silvered, as normal, so the observer sees, in the telescope, a direct view of the horizon and superimposed, a view of the celestial object, reflected via both mirrors, just as with a sextant. The essential feature of the Hadley 2-mirror invention is then retained; that after two reflections the image of the observed body will shift about in the view exactly the same as the direct view does, so that motion of the instrument in a seaway will not cause any relative motion between the two views. At the index-arm position when the two mirrors become exactly parallel, it should be at its zero mark, and the two images of any distant object should coincide, allowing checking of index zero-error as with the usual sextant. However, there's another "essential" feature of the normal sextant that will no longer apply. Normally, as the angle of the index mirror changes, the deflection of light changes by twice that angle. But that law is only true when all light paths are kept in the same plane. With this alternative instrument, the light paths are in very different planes (except at the zero-check position), such that the deflection of light becomes equal to, not double, the angling of the mirrors. (If anyone can offer disproof, please do so). Horizon shades will be placed in the direct view line through the horizon glass, as usual. Index shades will go into the space between the two mirrors. What would be the consequences of the new geometry? Most obvious is the doubling of the angular motion of the arm, that's called for to achieve the same angular range. A standard sextant frame, only 60 � wide, would only be able to measure up to 60� of arc, not 120� as with the standard sextant, so this would clearly be insufficient. The arc length would need to be doubled to subtend 120�, or even (for reasons we will see) 180�. That would lead to a large, clumsy instrument, except for the following... Because each measured degree now corresponds to a whole degree subtended on the engraved arc, and not half a degree as with the normal sextant, the degree markings, and the teeth of the rack, now become twice as far apart as they were. Or, to put it another way, the same precision as before can now be achieved by halving the radius of the arc. So a standard sextant with a 60� arc and 18cm. radius could be replaced by one of 180� arc and 9 cm radius, which would be no less compact. There would be a few advantages, if only minor ones. Because each mirror always reflects through 90�, there's no shrinkage in the view to a letter-box shape, as you get with a sextant at large angles. That implies you could use the instrument right up to 180�, wall-to-wall between horizons. to measure dip, as long as one proviso has been met. That is, that the sideways displacement between the two sight-lines is enough to allow incoming light from astern to miss the ear of even the most jug-eared navigator. That's why there has to be a certain minimum spacing between the two mirrors. Another minor advantage would be that there's no vertical offset between the two view-lines, only a horizontal one instead. So instead of calling for a distant horizon to zero-check on, to avoid parallax errors, that job could be done with something quite close-up. Also, it means the same instrument could be used for on-land close-up angular measurements for surveying purposes, which presently call for a theodolite because of the parallax error that offset causes.. I haven't yet discovered any major drawbacks to such a redesign, and hope that Navlist members will point out any there may be. For example, is its calibration unduly sensitive to the exactness of those 45� twists? Drawbacks there must be, I presume, if in all the years of sextant development, nobody has tried, or even proposed, such an alternative construction. Perhaps someone has, that I'm unaware of; dicovered the snags, and dropped the notion. The era of the sextant has been and gone; now is not the moment to be proposing a redesign, and that's not what I am doing. I am just asking the question; could sextants have been made in quite a different way, and if not, why not? If no serious objections emerge to the principles of what's been suggested, I can proceed to the practical details of how such a very-different instrument might have been constructed, which I've been pondering on, a bit. George. contact George Huxtable, now at george@hux.me.uk or at +44 1865 820222 (from UK, 01865 820222) or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---