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    Re: Sextant Accuracy and anomalous dip.
    From: George Huxtable
    Date: 2003 Mar 18, 17:17 +0000

    Gary Harkins said-
    >In a message dated 3/17/2003 2:45:15 PM Eastern Standard Time,
    >george@HUXTABLE.U-NET.COM writes:
    >> My general objective is to get these observations as accurate as
    >> possible, say to within 0.1 or 0.2 miles.  I'm not sure why I have this
    >> objective, but it persists.
    That message didn't come from George Huxtable, but from Fred Hebard.
    Gary went on to say-
    >Is your house at sea level?  If not you will have error caused by your
    >elevation.  Usually these can be ignored at moderate elevations, but in your
    >case, attempting to get ultimate accuracy, they may be a factor.  The error
    >will be greater when the sight is a smaller Hs.
    In response to a question from Peter Fogg, asking if he was referring to
    dip, the answer was-
    >Imagine yourself on top of a mountain taking a sight.  Imagine that the sight
    >measures 45 degrees using a bubble horizon or artificial horizon.  At sea
    >level and the same geographic location (if that were possible) the sight
    >would measure higher.  If you measured the same 45 degrees at sea level you
    >would have to be further away from the sun's GP.  I hope that's clear, it's
    >easier to show using a sketch.
    The effects Gary refers to (caused by parallax) are completely negligible.
    The worst-case would be that of the Moon, because it's by far the closest
    to us in the sky. Almanacs provide the direction of the Moon in Dec and GHA
    as seen by an imaginary observer at the centre of a transparent Earth. It
    has to be so, because the compiler of the almanac has no idea where on the
    surface of the Earth his customers are going to be. When observing the
    Moon,  it's essential to correct its almanac positions to correspond with
    the geometry from the viewpoint of a real observer, wherever he happens to
    be on the surface. This is the correction for parallax.
    The worst-case is when observing an object near the horizon, and this
    maximum value is called the Horizontal Parallax or HP. The HP of the Moon
    varies slightly in its out-of-circle orbit, but is always somewhere near 1
    degree, and depresses the Moon, toward the horizon, by that amount. That is
    the parallax caused by the displacement of our imaginary observer from the
    centre of the Earth to a real position on its surface, a shift equal to the
    radius of the Earth or about 4000 land-miles. An elevated observer would
    change that parallax only in (inverse) proportion to the change in his
    distance from the Earth's centre.
    The effect Gary is referring to would require his house to be at an
    altitude of about 1/60 of 4000, or 66 land miles above the Earth's surface,
    to shift the Moon's position by an extra minute of arc. For other objects
    than the Moon, the effect of parallax is even smaller, by a factor of at
    least 100.
    For these reasons, the variation of parallax with altitude above the
    Earth's surface can be neglected by navigators, though precise mountain-top
    astronomers may need to take it into account.
    Peter Fogg and Fred Hebard have responded correctly to this topic, but
    perhaps the rough numbers provided here will give a bit of extra insight.
    George Huxtable.
    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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