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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Pre-setting sextant; Wulf's Grid.
From: Alexandre Eremenko
Date: 2004 Oct 16, 23:44 -0500
From: Alexandre Eremenko
Date: 2004 Oct 16, 23:44 -0500
In certain observations, like lunar distances, or altitudes with art horizon, it is not easy to catch both objects in the field of view (and easy to catch some wrong star instead of the right one!) Pre-setting the sextant on the approximate angle helps. I know the following methods of quickly calculating this approximate angle. Lunar distances experts: how do you do it? 1. By using a navigational calculator (but this is electronic equipment, and I do not discuss it:-) Though my calculators are all solar powered and use no bateries:-) 2. By measuring the needed angle directly on a star globe. This seems to be the fastest and most convenient method. But a star globe is bulky and expensive. 3. For altitudes, the Rude star finder will do the job, but NOT for distances! 4. The Stereographic Grid. I don't know whether this tool (or something similar) is known in the West, so let me describe it. It is almost as quick as the Star Globe, has approx 1 degree precision, costs nothing, and can be stored between the pages of a book. It solves all problems of spherical geometry in no time, with 1 degree precision. In Russian literature it is credited to Professor G. Wulf (or maybe Woolf, Wolf, Wolff, transliteration from Russian is not unique) who published a booklet about it in 1909 in Russia. (I have never seen the original book, but many Russian manuals mention it as a replacement of the Star Globe. The device is easy to make yourself. The device consists of two sheets of paper, one permanent and one replacable. On the permanent sheet a spherical coordinate system is drawn in Stereographic Projection, with circles for each even number of degrees. The diameter of the picture is 20 cm. The replacable sheet is made of transparent paper (you can draw on it), a picture of the circle of the same diameter is made on it and the center of the circle is marked. The sheets are connected by a pin passing through the centers of the circles so that one sheet can be rotated with respect to another, so the thing looks very much like the Rude starfinder. The operation is very simple and evident. To measure the distance between two celestial bodies, you mark them with a pencil on the transparent sheet, according to their co-ordinates (SHA and Dec, or GHA and Dec). Then you rotate the transparent part to bring the two marks on the same meridian. And read the distance along this meridian. Of course one can make this device of plastic too, then it will be water resistant. A simpler and cheaper option is to make many photocopies of the grid, so that both sheets will bve disposable. One can use it to solve other problems as well, where the high precision is not required. Like finding azymuths etc. A challenge for math lovers: why it works? :-) Alex.