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    Re: Pre-setting sextant; Wulf's Grid.
    From: Frank Reed CT
    Date: 2004 Oct 20, 01:13 EDT
    Alex E wrote:
    "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?"

    For lunars the historically recommended procedure, and the one that I use today, is nothing more complicated than looking up the distance in the almanac (or equivalent software). A navigator is never completely lost so you know GMT within a few minutes. You open the almanac to the right date, eyeball interpolate for your GMT and then preset your sextant to that angle. Then you look through the sextant and, unless you're dealing with an object well off the ecliptic, you rotate the instrument so that a line through the horns of the Moon is perpendicular to the frame of the instrument. In most cases, the other body will pop right into the field of view, and you'll be within one degree of the correct apparent distance. One could get fancy and make a rough guess for parallax. This would give a more accurate preset distance but it's not usually necessary.

    And:
    "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."

    I have access to a planetarium with a thirty-foot dome to play with and sometimes, just for fun, I read off angles directly from the dome, but yeah, it's a little inconvenient aboard ship. <g>

    Of Wulf's Grid:
    "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."

    This sounds fun. I'm having a little trouble picturing it. Can you post a photo?

    Frank R
    [ ] Mystic, Connecticut
    [X] Chicago, Illinois
       
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