NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Suitable Sextants - Mirrors
From: Frank Reed CT
Date: 2005 Oct 12, 20:29 EDT
From: Frank Reed CT
Date: 2005 Oct 12, 20:29 EDT
Joel Jacobs wrote: "Your analysis though interesting, fails to take into account that a sextant's mirrors are not used in a static state, and hence size does make a difference. Consider that the platform is moving directionally, and rolling and pitching all at the same time." But since the mirrors, telescope, and other components of the sextant are all experiencing the same motion, this really isn't relevant to mirror size. They're either big enough to fill the field of view, or they're not --no matter how much pitching and rolling there is. And: " There also is motion introduced by the user when he rocks his arm to align for perpendicularity." Why would that depend on mirror size? Many people mistakenly rock the sextant about the axis to the horizon in which case the Sun or star's image sweeps across the field of view from one side of the horizon glass/mirror to the other. When "rocking" for perpendicularity, the sextant is supposed to be rotated about an axis that points to the Sun or star. And when this is done correctly, the Sun or star remains centered in the field of view. For objects that are low in the sky this is nearly the same motion as the "incorrect" procedure. But for objects at high altitudes, it's very different. If the Sun or star is in the zenith, "rocking" the sextant amounts to spinning about vertically. And in every case, the Sun or star stays centered in the field of view so mirror size does not appear to be relevant. Just so there's no misunderstanding, I agree with you that a sextant should have (or at least potentially accept) a telescope with a relatively large aperture and the mirrors should be compatible in size with that --large enough to fill the field of view. I don't agree that it is because the mirrors "collect more light" but this theoretical issue is really a minor concern compared to your experience in actual use. -FER 42.0N 87.7W, or 41.4N 72.1W. www.HistoricalAtlas.com/lunars