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    Re: Plastic vs Metal Sextants
    From: George Huxtable
    Date: 1999 Aug 27, 11:17 AM

    I'm glad the question has been raised, once again, about the appropriate
    sextant to use on small craft. There was earlier discussion, in January, in
    the previous incarnation of this mailing list, in which plastic sextants
    were given some dismissive treatment.
    
    I should start off by admitting that I'm not an ocean navigator. I start to
    feel lonely when I get more than 50 miles from land. So my life has never
    depended on astro navigation. But I enjoy using astro to check my position,
    and most astro sights have been taken when I know approximately (and, since
    I succumbed to electronic navigation, exactly) where I am.  Most of my
    observations have been of the Sun or the daytime Moon or a bright planet:
    I've never done a round of dusk star-altitudes, so I defer to the
    experience of those that have. In really rough weather, I'm too busy
    hanging on to my 26-footer to be taking astro sights. I don't have any
    direct experience of Davis or Astra to make a comparison with them.
    Although I've used metal sextants I've never owned one. I would love to
    have one to display on the mantelpiece.
    
    My experience, over the last 30 years, has been with Ebbco plastic
    sextants, and my arguments will relate to those instruments. Many readers
    will be unfamiliar with the Ebbco, so I will start by describing it.
    
    Ebbco sextants are plastic, with a 7-inch index arm. They have a micrometer
    scale and a tangent screw with a 60-minute drum (though some early models
    were vernier). The rack for the screw is machined, rather than moulded,
    into the plastic base-plate, and Ebbco claim a precision of 1 minute for
    this calibration, a claim I have found no reason to doubt. They do not
    provide any calibration certificate, however. The horizon mirror is fully
    silvered over part of its width, and the silvering for both mirrors is on
    the rear face. The silvering on my oldest Ebbco is starting to degrade now,
    within 1 mm or so of its edge, but would still be quite usable. Ebbco
    states that the mirrors can be resilvered. The shades are glass; on early
    models, however, they were made of plastic film, and showed an unacceptable
    ripple. Recent models have three sun shades and two horizon shades. All the
    usual mirror adjustments are provided, but I have not needed to use them.
    The telescope is something like x2.5, with an aperture of about 28mm, and
    is all that I have needed.
    
    I would criticise the design of the handle-grip which, if you squeeze it
    really hard, can flex the base-plate slightly, and shift the Sun image
    noticeably against the horizon. However, a light grip is all that's needed,
    and in that case there's no problem.
    
    My first Ebbco succumbed to being trodden on after it fell to the cockpit
    sole. A drop does no harm at all, usually; it just bounces, being so light.
    However, I suspect that the plastic of the earlier models became more
    brittle as it aged. Recent production is polycarbonate, and should stay
    strong. My second sextant was an improved model which I have had for ten
    years or so. It has had some rough treatment, but is undamaged so far.
    
    My purposes, which I guess are similar to the requirements of many
    small-boat sailors are met admirably by my Ebbco sextant. If I wanted to
    set off round the World (which, thank the Lord, I don't) I would be quite
    happy to rely on it. With some electronic back-up, I hasten to add.
    
     Although Ebbco and I are both in the Thames Valley of the UK, I have no
    connection with the firm. Their address is-
       East Berks Boat Co., Ltd., Station Road, Henley-on-Thames, Oxon.RG9 1AJ, UK.
       Phone number from within UK is 01491 573390, from outside +44 1491 573390.
    
    Current price of the standard model from EBBCO in pounds sterling is �50.70
    + (in UK) �5.90 postage +17.5% tax. The special model is �69.88 with the
    same additions. The special model has the scales inlaid white-on-black for
    easy reading, rather than just moulded into the plastic, and uses ground
    optical flats for the mirrors, and glass lenses in the telescope. It's
    worth the price difference, in my view.
    
                                    ----------------
    
    Now for some comments on the question, which is best asked as "What's the
    most appropriate type of sextant for a small craft?", not "What's the best
    sextant?".
    
    The main consideration is that of the level of accuracy attainable in an
    observation of an altitude from a small vessel. First, the unsteadiness of
    the platform under our feet, which requires some skill as the image dances
    around the sextant mirror: something the navigator of a big ship, with its
    slower roll, is much less aware of. Second, the horizon itself, which from
    a small boat in rough conditions is a collection of wave-peaks, not the
    nice straight line as is usually seen from the lofty viewpoint of a bridge.
    For us, estimating the effective horizon from the wave-tops becomes a
    matter of guesswork. So, except under really calm conditions, there's no
    way we can approach the navigational precision of a big ship, no matter how
    good our sextant may be. In which case, I ask, do we really need the same
    expensive and accurate sextant that he has? Even if a sextant is calibrated
    to 0.2 minutes, how accurate do we think our observations are?
    
    I should add another factor which sometimes affects altitude measurements
    from all vessels, large and small, no matter what sextant is used. This is
    abnormal refraction at the horizon, a common phenomenon we have all noticed
    when we see mirage effects on distant ships, or a distorted image of the
    setting sun. Standard refraction corrections, even after allowing for
    temperature and pressure, can't allow for these effects of different
    air-layers in the light path on the dip of the horizon. No sextant, no
    matter how expensive, can reduce these unpredictable effects of abnormal
    refraction.
    
    The original octant was a brilliantly clever concept which measured the
    angle between the Sun and the horizon, without the instrument itself having
    to be aligned precisely with either. It was generally made of ebony and
    ivory, with a vernier scale, and with peep-holes in place of a telescope.
    This rather crude construction sufficed for the rough-and-ready latitude
    navigation of its day.
    
    Why, one might ask, does the modern sextant need to be so accurate? Partly,
    I'm sure, it's a historical thing. The sextant was developed from the
    octant in order to measure "lunar distances", the angle-in-the-sky between
    the Moon and the Sun, or the Moon and certain stars, angles which often
    exceeded 90 degrees. This measurement allowed the navigator to obtain
    Greenwich time at sea, when he was unable to afford a chronometer: a common
    state of affairs right through the 19th century. (Joshua Slocum was one of
    the last exponents of the "lunar"). Note that these lunar observations were
    not affected by the horizon difficulties referred to above. It was quite a
    tour-de-force to achieve a good "lunar". The difficulty with lunars is
    this: every minute of error in the Moon-angle becomes an error of about 30
    minutes in the resulting longitude. So navigators sought extreme accuracy
    in their lunars. A sextant became the ultimate expression of the precision
    engineering of the day. Official arrangements for calibration were set up,
    and the instruments were calibrated to a small fraction of a minute.
    
    In my view, all this was largely redundant after the chronometer became
    available, affordable and trusted, backed up later by radio time signals.
    Lunars were no longer measured, the 30 times multiplier disappeared, and
    from then on, an error of one minute in altitude became an error of one
    mile in the position line (for both lat and long). The requirement for
    extreme accuracy had gone, but sextants didn't change.
    
    So, all those beautiful sextants remained, the navigator's badge of office,
    a totem and symbol of his craft. Some have come down to us small-boat
    sailors, to make our crude observations with. And why not, you may ask?
    Even though their accuracy is excessive for our needs, that does us no
    harm. And there is pride and pleasure in owning and cherishing an
    instrument that was at the time a high point in technology. I couldn't
    argue with that. But perhaps there's some instrumental snobbery in it, too,
    the idea that a modern, cheap-and-cheerful, plastic sextant just couldn't
    possibly be good enough to do our job, undemanding though it may be. But it
    must depend, to some extent, on the quality of the design.
    
    The January correspondence showed much concern about problems with mirror
    adjustment, and I do wonder why. The simple optical principles of a sextant
    make it rather immune to misadjustments until they become grossly obvious.
    Perpendicularity error and side error can be tolerated without readjustment
    to a limited extent, as the measured altitude varies only as the cosine of
    angles which are near 90 degrees. Unless the mirrors are loose or insecure,
    or have been knocked, users would generally be better to leave them alone.
    I would never use the adjustment screws to minimise index error (except
    after a great trauma). Instead, I simply note what the index error is, from
    a measured altitude of the horizon, or a view of the Sun in both mirrors,
    and allow for it. This is so quick and easy that I usually bracket each
    observation between two index-checks, though I never observe a shift
    between them. I think the urge to tinker arises from the fact that every
    publication about the sextant devotes so much space to adjustments of the
    mirrors, that users expect mirror adjustment to be something they must do.
    
    I have a comment to add to the discussion on index error resulting from
    differential heating. I would expect that when a sextant is used, or set
    down, if sunlight then falls on sensitive components (for example, on just
    some but not all three of a mirror's adjustment pads), the resultant
    radiant heating may well cause expansion which could shift the effective
    zero on the index scale. Because plastics have in general a greater
    temperature coefficient than metals, a plastic sextant might be expected to
    suffer worse; especially as its poorer thermal conductivity restricts heat
    flow which would otherwise tend to equalise these temperature differences.
    The effects would be worst in still air: any breeze would reduce such solar
    heating effects, by bringing the temperatures back toward the local
    temperature of the air. As described above, measurement techniques can be
    adopted which make you virtually immune to such shifts.
    
    There's an interesting note in the Journal of Navigation (from the Royal
    Institure of Navigation, London), vol 52 No 1, Jan 99, page 138, by
    W.W.Huybrecht, entitled "Vertical sextants give good sights". The author
    draws on what is clearly a long experience of astro-navigation in
    commercial shipping. He has many interesting views on the disappearing art
    of astro-navigation. One of these views particurly intrigued me, in his
    statement that "A marine sextant should be heavy (Plath) ....", and later
    "Wind can be a problem, but that is why a marine sextant should be heavy,
    having more inertia." I wonder if other sextant users agree with this, my
    own view being exactly opposite. After all, makers of metal sextants go to
    some trouble to skeletonize the frames in order to minimise weight. And if
    sheer weight was itself such an advantage in using a sextant, it would be
    simple for a user to improve an insufficiently weighty instrument by
    clamping on some lead ballast in an appropriate place. Yet I've never heard
    of this being done; has anyone else? But Huybrecht speaks with some
    authority and experience, which gives weight to what he says, although my
    own view is " the lighter, the better".
    
    My aim has been to stimulate some argument and discussion. Let's see if it does.
    
    George Huxtable.
    
    ------------------------------
    george---.u-net.com
    George Huxtable, 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    Tel, or fax, to 01865 820222 or (int.) +44 1865 820222.
    ------------------------------
    

       
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