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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Suitable Sextants - Mirrors
From: Herbert Prinz
Date: 2005 Oct 14, 15:01 -0400
From: Herbert Prinz
Date: 2005 Oct 14, 15:01 -0400
Yourname Here wrote: > here is a link to photographs of page 108, Figure 6-4 of Bauer's " The > Sextant Handbook". [...] For those interested, Bauer goes into great > detail in the methodology of swining an arc on pages 105-108 which > are the same as my own. > > http://www.villagephotos.com/pubbrowse.asp?folder_id=1485991 > Joel, I hate to burst your bubble, but Bruce Bauer has it all wrong, and so do you. His figure 6-4 does not show clearly how he is swinging. But he tells us on p.106, which you chose not to reproduce in your post: "Swinging the arc is also called rocking the sextant and simply means rotating the instrument from side to side around the line of sight to the horizon." I think that there has long been a consensus on this list (at least since Frank Reed brought the attention to the subject a while ago) that this is the wrong way of doing it. Actually, it's an impossible way of doing it (see below). Instead of trusting a particular book, let's use common sense. The goal of swinging the arc is to find the exact point where the vertical line through the star intersects the horizon. If you pick a point on the horizon arbitrarily and then start swinging the sextant around the line of sight to this point, how do you ever find the correct point?? That this wrong way does not work is further evidenced by Bauer's own claim that he cannot make it work for any other than medium altitudes. See p. 108, "One more wrinkle on swinging arcs [...]", full quote on the page you posted. Bauer says "At high altitudes the arc becomes hard to manage because of its sharp curve." Not at all. The very problem with a star at high altitude is that it is difficult to guess the azimuth because the curve is so _flat_! Of course, if you decide on the wrong azimuth prematurely, then face that direction and start swinging the arc on a horizontal axis, the star shoots out quickly on eiter side of the mirror. Neither will its arc ever touch the horizon - it may intersect. Even a Venetian wall mirror on your sextant will not help you solve this problem. Why Bauer says that swinging does not work for low altitudes, because of the arc flattening out, is beyond me. It makes me wonder whether he ever swung a sextant. For sufficiently low altitudes one can actually swing the object full circle (!) without loosing its view. Herbert Prinz