NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: sextants on aeroplanes
From: George Huxtable
Date: 2008 Dec 13, 16:27 -0000
From: George Huxtable
Date: 2008 Dec 13, 16:27 -0000
Gary LaPook wrote in [6658]- "There is a distinction between sextants, octants and quadrants. Because of the double reflection operating principle of the marine instruments it is possible to measure an angle of 90�, one quarter of a circle, with an instrument having a calibrated arc of only one-half that, 45�, one-eight of a circle, an instrument called an octant. If the arc is one-sixth of a circle, a sextant, you can measure angles up to 120� and if the arc is one-quarter of a circle then you can measure an angle of half of a circle, 180�, and you then have an instrument called a quadrant." ======================== In trying to resolve these matters, I fear that Gary has added to the confusion. Historically, instruments which could measure up to 90�, were known as "quadrants". That applied to instruments without a mirror, which then required an arc of 90�, and, also later, to Hadley's 2-mirror invention, which did the same job with an arc of only 45�. It was called a reflecting quadrant, a Hadley quadrant, or sometimes just a "Hadley". A bit later, the working range was extended to 120�, which required an arc of 60�, and that being a sixth of a circle, it was christened a "sextant". (Later still, a few instruments were made with a range extended to 144�, and an arc of 72�, known as "quintants") But all this was logically inconsistent with the use of the word "quadrant", for an intrument measuring to 90�, so those instead became known as "octants", because their arc was an eightth of a circle. There is no difference at all between a reflecting quadrant and an octant; they are two names for the same thing. So Gary is wrong when he writes- "Quadrants must be made larger than sextants which are larger than octants. An octant is sufficient to measure any angle from horizontal to straight up so can be use for normal celestial navigation and this is how they are used in aircraft. Sextants and quadrants allow a greater range of measurement which might be useful for lunar distance measurements, horizontal sextant angles for coast wise navigation and in the rare case of a body within 30� of the zenith with an obstructed horizon below it but a clear horizon in the opposite direction in which case the navigator could turn his back to the star and use the opposing horizon for the sight. (I don't know if this was ever actually done in real life.) Since these types of sights are never taken from an aircraft it is never necessary to have more than an octant on an aircraft. My tamaya sextant actually measures up to 125� and my SNO-T goes all the way to 140� but these are both sextants." The first account I have read of that trick of measuring up from the opposite horizon was on the Malaspina exploration, near Callao, in 1790, when the morning Sun was visible to the West, but the horizon beneath it couldn't be seen because of the nearby coast (a dip-short situation, really). Usually, if you're close to visible land, celestial navigation isn't called for: except when, as here, surveying an unknown coast. Malaspina was pleased to have, in his kit, an early quintant, perfect for such a task. Gary's SNO-T sextant, measuring up to 140�, is only 4� short of being a true quintant. Quintants were mostly used by hydrographic surveyors, to measure wide horizontal angles. Lecky, in his "Wrinkles", was keen to push quintants for navigators, but I've never really understood why. I wonder if, when Gary wrote "Quadrants must be made larger than sextants which are larger than octants.", he was thinking of quintants rather than quadrants. However, in writing "if the arc is one-quarter of a circle then you can measure an angle of half of a circle, 180�, and you then have an instrument called a quadrant", he was describing a device which, as far as I know, has never existed. Not that it's impossible, necessarily. I've been pondering a scheme which might allow a two-mirror reflecting instrument to measure angles up to 180�, which would allow wall-to-wall observations from one horizon to the opposite one, and so allow dip to be measured. More about that later, unless it turns out to be nonsense. 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 -~----------~----~----~----~------~----~------~--~---