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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: measuring sextant instrument error
From: George Huxtable
Date: 2000 Sep 17, 5:31 AM
From: George Huxtable
Date: 2000 Sep 17, 5:31 AM
Bill Murdoch wrote, about his plan for calibrating a homemade octant- >I think I can do it in my garage like this: (1) Fix a block of wood on the >floor with a 1/4" hole drilled in the wood. (2) Strike an arc just over 100 >degrees in length with a radius of 6,875.5 mm centered on the hole. (3) >Stand a steel tape measure on edge along the arc so that each 1 minute of arc >is 2 mm on the tape. Replace the pivot bolt in the octant with a longer bolt >and place the octant over the block of wood so that the bolt slips into the >hole. That places the index mirror over the center of the circle. (4) Read >the steel tape scale in the horizon glass for reference. That reading >becomes the horizon. (5) With the index arm set to 0 degrees read the steel >tape scale at the spot that is superimposed over the horizon reading. That >begins the calibration. (6) Move the index arm and note the scale reading >viewed in the index mirror that is now superimposed over the horizon reading. > If the scale can be read to a half millimeter, the sextant error can be >measured to 0.25 minutes of arc. That is four times better than my usual >shooting accuracy and I think good enough for checking a homemade instrument. > >Think that would that work ? > >Bill Murdoch ================================== I think Bill is making heavy weather of all this. A manufacturer (or the Navy) would need such a setup where many instruments could be calibrated at any time under controlled conditions. Unless Bill is setting up to manufacture his octants in quantity, he needs no such thing. He simply wishes to make a one-off calibration. Nature has provided a well-illuminated object at infinity, known as The Sun, and a well-defined plane near the horizontal, known as The Horizon. Furthermore, Nature has kindly arranged that over the course of a morning, the angle between this Sun and this Horizon sweeps over a wide range of values from zero to a maximum at the meridian. Astronomers have gone to some trouble to predict the precise motions of this body, and almanacs are readily available with the result of their predictions. Small corrections need to be made to the predictions, and these are well-known and readily applied. Bill has written a calculator program which computes all this stuff. The only remaining uncertainty relates to abnormal refraction in the atmosphere possibly affecting the altitude of the Sun and (particularly) the Horizon. A company named Freiberger has taken pains to manufacture an accurate sextant, which has already been accurately calibrated, and this is an instrument that Bill Murdoch happens to possess. What's more, the US military have provided a system, free to any user, which allows him to detrmine his precise position on the surface of the Earth. No doubt Bill possesses a simple Quartz watch which he can set to correspond with Greenwich time signals, which are broadcast by radio. All these resources are available to Bill. All he needs to do is to take a trip to the seaside (or a large lake) on a sunny day, and spend a morning by the shore, or on a jetty, with his octant and his sextant. And a sunshade. And some way to check his height above water level. He needs to find a spot which has a clear view of the horizon, from somewhere near East through to South, and to be there by Sunrise. There he can spend his whole morning recording times and octant altitudes of the Sun, as close together in time as he wishes. He should frequently record (but not adjust) any changes in the zero-error, by observing the apparent altitude of the horizon. He can interpose (perhaps even alternate) Sun altitudes from his precise sextant, to check whether refraction conditions in the atmosphere are changing noticeably. He should bear in mind that at low altitudes the predictions become unreliable due to to the variable effects of refraction. When Bill gets home again, he can plot out any deviations between the (corrected) altitude predictions from the almanac, and the observations from his sextant and octant. This would result in a table of deviations, similar to what one uses with a magnetic compass, but hopefully much smaller in amount. If a convenient viewpoint sees the sea to the West and South, instead of East and South, then the job would be done in the afternoon rather than the morning. Of course this procedure will calibrate the octant only up to the meridian altitude that the Sun reaches at that spot and date. I don't know where Bill Murdoch lives, but if it's at a high latitude, he could take his holidays at an appropriate spot and date within the Tropics and calibrate his sextant there. More easily, he can, at home, take angles between the Sun and its reflection in a tin tray containing some treacly liquid, such as old black engine-oil. The reflection doubles the angular divergence, so for a Sun altitude of 45 degrees, the apparent "altitude" between these directions is then 90 degrees. This method could be used for the whole operation, intead of going to the seaside, and has some advantages, in that the horizon (with its problems of uncertainty of dip) doesn't come into the question. However, the method presents some difficulies at low altitudes. Otherwise, he could arrange to be back at the seaside at dawn or dusk, and arrange beforehand to identify a few stars which will then be visible over his sea-horizon. They should then have altitudes that allow the as-yet uncalibrated gap in the octant's scale (between the Sun meridian altitude and the vertical) to be filled in at a number of spot values. The measurements could be continued over as long a time as the visibilities of the star and the horizon allow. I think that Bill Murdoch would be able to achieve an accurate calibration of his octant by these methods in less time than it would take him to clear his garage enough to provide space for the method he proposes. George Huxtable. ------------------------------ george@huxtable.u-net.com George Huxtable, 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. Tel. 01865 820222 or (int.) +44 1865 820222. ------------------------------