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    Pre-setting sextant; Wulf's Grid.
    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.
    
    
    

       
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