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    Re: LAN at higher lattitudes
    From: George Huxtable
    Date: 2004 Jan 8, 22:24 +0000

    Murray Campbell asked-
    
    > I though i would begin by learning the noon shot, but
    >i am at 52 north and the sun is so low and flat right
    >now and i'm just learning to use the sextant....i will
    >try to take the sites way before and after noon and
    >average them as George mentioned in the recent LAN
    >discussions.
    >
    > My beginner question is what the practical limits are
    >for measuring altitudes accurately at high lattitudes?
    >If the sun is above the horizon is it always useable
    >or is it best (and more accurate) to switch to a body
    >higher in the northern sky?
    
    If you are at the very early stages of learning to use the sextant, I would
    start off by measuring only the latitude (by the Sun's maximum altitude)
    and put off measurements for longitude until later.
    
    But now for your questions-
    
    The usual recommendation (with which I would agree) is to keep the altitude
    of a body above 10 deg if you can, and certainly above 5 deg.
    
    The problem is that the refraction correction grows at lower altitudes.
    When you get down to 10 deg, the refraction is already more than 5'. That
    would be acceptable if the refraction were completely predictable, but it
    isn't. You get bands in the atmosphere at low angles which sometimes show
    up bt causing the disc of a low Sun (for example) to become rather squashed
    and distorted. So the refraction correction can become inexact. However, at
    10 deg alt, any such error is unlikely to exceed 1 minute.
    
    Even at midwinter from your latitude of 52deg, you could take
    equal-altitude sights about 2 hours before and 2 hours after noon, at which
    the Sun altitude would be about 10 deg, so you shouldn't find your latitude
    to be a serious limitation.
    
    You could make deliberate equal-altitude observations as I suggested, or
    simply measure Sun altitudes morning and afternoon at somewhere around
    10:00 and 14:00 GMT without any need for the altitudes to be exactly equal,
    and simply work out intercepts and cross two position lines (perhaps with a
    third position line at noon). It comes to the same thing, more or less.
    
    =============
    
    Correcting for the time difference between max altitude and local apparent noon.
    
    If you are going to use the equal-altitude method discussed in that
    previous mailing, remember that it gives you the moment of maximum
    altitude, and not moment of local apparent noon. So you have to make a
    correction for the time difference between them. In the previous example we
    considered a ship doing 20 knots toward the Sun, at winter solstice. Winter
    solstice was chosen because then the Sun itself wasn't moving in
    declination.
    
    But even for an observer stationary on the ground, as I presume you will
    be, at times other than the solstices the Sun's declination will be
    changing. At the spring equinox it will be moving toward you at its
    fastest, which is about 24' per day, or very nearly 1 knot. You can work
    out the rate of change from the almanac, then use the same formula I gave
    for a moving vessel to obtain the time difference between max. alt and LAN.
    Of course, the correction will be a lot less than it was for the fast
    vessel.
    
    ============
    
    Instead of the Sun, you could use other bodies, stars or planets: this
    would be forced upon you in Winter if you lived a lot further North than
    you do. But you can't use equal-altitudes, because stars and planets are
    visible over only a short interval, around dawn or dusk.
    
    Instead, at dawn or at dusk, measure altitudes of two or more stars or
    planets, in different directions around the sky, work out the intercepts
    from an assumed position, and cross the position lines to give your
    position. This can be very accurate.
    
    George.
    
    
    ================================================================
    contact George Huxtable by email at george@huxtable.u-net.com, by phone at
    01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
    Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    ================================================================
    
    
    

       
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