NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Trombone Kamal Prototype
From: Greg Rudzinski
Date: 2009 Jun 5, 16:11 -0700
From: Greg Rudzinski
Date: 2009 Jun 5, 16:11 -0700
Frank, The Polynesians did have a version of the kamal which was a straw with a loop on top. A set of straws was made by the navigator while he was on a given island for a set of star transits. When a return trip to a given island was done then the designated straw and star combination was used to arrive at the correct latitude then go east or west from there. Greg On Jun 5, 3:28�pm,wrote: > Greg, George H. wrote: > > "Greg hasn't (yet) told us at what angle his plastic plate should be held. It > doesn't matter (much) for his present small angles, but it will become more > important as angles increase. Should it be held vertical, at right angles to > the observer's sight-line to the horizon?" > > This is a good point. I have assumed that you're holding the plate perpendicular to the tape measure. Is there some way to guarantee that? In a traditional cross-staff, the design ensured that the cross-piece was very close to perpendicular to the staff. > > And he wrote: > > "So, the dimension that needs to be measured, if the tan formula is going to work, is between the cross-piece (or plastic plate) and the centre of the eyeball. Not a practical proposition, obviously, but what is the best that can be done? There's a notch in the edge of the skull-bone, just close to the side of the eye, which isn't far from a plane drawn through the centre of the eyeball. Just the spot where the user of a cross-staff would press the end of the staff. There was much argument, then, about whether different individuals should shave off a suitable amount from the staff, appropriate to their own eye-socket dimensions." > > This is what I was getting at about not knowing the exact length of the baseline. But rather than try to figure it out theoretically, we today, with our experience with sextants, can treat this as a calibration question comparable to checking index error, or maybe arc error, in a sextant. You measure a series of known angles, possibly with the plate clamped onto a fixed mounting, and then find whatever additional length added to the observed baseline yields minimal error in the observations. You can then hold the end of the tape (or the end of the staff in a cross-staff) wherever you find comfortable, as long as it's repeatable, and then add in that "index correction" distance to every observed baseline. > > And George wrote: > > "in this aspect, the cross-staff wins, hands-down, on convenience, being direct-reading. You read the angle straight off, marked on one of a number of scales, each dependent on the chice made from a range of transoms of varying lengths." > > Once it's calibrated, the angle conversions could be printed up in a small table, one for each of the three horizontal lines. The slide rule would save you an interpolation step if the printed table isn't fine enough. > > Crossing the threads and tangling them up a little, why didn't those Polynesian navigators a thousand years ago ever invent the cross-staff? Is it possible that the "dead reckoning" cues in the tropical Pacific, things like bio-luminescence, are so much better there than in the Atlantic that Polynesian navigators simply never had any practical reason to go beyond the tools of dead reckoning (and possibly basic zenith stars)? > > -FER --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---