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    Re: Fix by equal altitude sights around local apparent noon
    From: Geoffrey Kolbe
    Date: 2009 Oct 13, 21:35 +0100

    In [NavList 10109) John Huth wrote
    >For my class, I made a rudimentary quadrant, where the angular scale
    >came from successively bisecting angles to get 64 divisions.   I'm
    >not claiming great accuracy with this, perhaps 30 arc-minutes at
    >best.    I measured the sun's altitude multiple times when it was
    >rising, during meridian passage (for latitude) and then when it was
    >setting.  In terms of getting the LAN, the midpoint of the rising
    >curve and the setting curve turned out to be reasonably accurate,
    >given how rudimentary my quadrant was.   I got to within 20
    >arc-minutes of my longitude (which might not sound like anything
    >great to people accustomed to sextants).
    >Obviously this doesn't work for a moving vessel, but I found it
    >interesting that a crude instrument, coupled with a watch could do a
    >reasonable job in determining longitude.
    >  John Huth
    It may be of interest that the method of equal altitudes is probably
    the most precise method of determining a position using a "portable"
    optical device.
    The device - called a precision astrolabe in Europe and an
    'equiangulator' in the United States - is quite simple. A telescope,
    a beam splitter and a pool of mercury. By noting the elapsed time
    between the moments when the reflected and direct image of a given
    celestial object were seen to coincide in the eyepiece, it was
    possible to determine position to great accuracy
    See, for example, http://www.aggregat456.com/2006/07/geodet.html for
    more information.
    Geoffrey Kolbe
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