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    Re: Winter Sextant Sight Accuracy?
    From: George Huxtable
    Date: 2002 Jan 10, 5:58 PM

    Jared Sherman asks-
    >How tight a fix are other folks getting with a sextant in winter conditions?
    >I ask because even on a fixed beach, I notice it is harder for me to get a
    >good position in the winter. The low altitude sun sight makes atmospheric
    >effects (eye height, temperature, barometer all affecting refraction) way
    >more significant and I'm wondering how much this affects everyone /vs/
    >just how good or bad I'm doing.
    >My last cluster of sights were within 1 mile (+-.5m) of each other, but
    >still off by nearly two miles in absolute position. On the brighter side
    >the bearing was within one degree of the same on all of them. I think I
    >found some of my errors but was wondering how much better anyone else is
    My comment follows-
    Jared doesn't tell us what latitude he is observing from, or what altitudes
    he is measuring. Is he making a noon-Sun observation for latitude? Or is he
    perhaps trying for a morning or evening observation for longitude, in which
    case the altitudes might be so low as to give rise to refraction problems.
    Even in my latitude in the UK of 51 degrees-odd, we get a rather pale noon
    sun at about 16 degrees altitude in midwinter. Even at that altitude, the
    refraction correction is only 3 minute 30 sec and is rather well-known and
    predictable. At that altitude, it's inconceivable for temperature or
    pressure variation to be enough to account for the errors that Jared refers
    to. The refraction is rather insensitive to overall pressure or temperature
    variations. A change of 14 degrees Celsius in the temperature of the air
    changes the refraction by only 0.2 minutes of arc. see Norie's tables, for
    So unless he is very far North or is measuring at very low altitudes, I
    doubt that the effects of winter are causing the problems that Jared
    However, in Winter or in Summer, anomalous refraction near the sea-surface
    can be a more significant effect. This depends, not on the temperature so
    much as on the temperature gradient in the lower levels of the atmosphere,
    bending light from the Sun (and also light from the image of the horizon)
    as it skims over the waves on its way to the observer's eye.
    These effects can easily cause variations of a minute or two in the
    measurement of altitude, and occasionally (but fortunately, rarely) special
    atmospheric conditions can cause errors of several minutes. It's the same
    effects that in extreme cases give rise to mirages of distant ships, and
    sometimes inversions, at the horizon.
    In the absence of ships on the horizon to show it up, an observer will be
    quite unaware that anomalous refraction conditions are affecting his
    observations. When other errors are small, because conditions are
    favourable, such as a smooth sea and clear visibility, anomalous refraction
    can become the limiting factor in the accuracy of a sextant observation.
    Such effects appear to be more common near coasts, when air which was over
    the land and at a different temprature, blows over the sea. Most often they
    have been reported under hot-Sun conditions rather than in winter. However,
    polar explorers navigating over sea-ice have reported very high values of
    anomalous refraction.
    Other possibilities:
    1.  I presume that Jared is checking and subtracting his index-error on
    each occasion.
    2. It might be that the scale of his sextant has been badly divided, in a
    non-uniform way, so that errors occur at low values of altitude that are
    not so bad at the higher-altitude part of the scale.
    3. If the sextant has been taken out of a warm car onto a cold beach, has
    enough time been allowed for all the metalwork to reach a uniform
    All in all, I doubt whether the discrepancies Jared reports can be blamed
    on the winter.
    George Huxtable.
    George Huxtable, 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    Tel. 01865 820222 or (int.) +44 1865 820222.

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