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    Bris sext. was: Suitable Sextants
    From: Alexandre Eremenko
    Date: 2005 Oct 11, 12:51 -0500

    > Hi, I would be very pleased to see how to build and to use one.
    > Gennaro Sammarco
    
    Let me explain first what is this. It consists of 3 little
    pieces of glass (less than 1/2 sq inch each) glued together.
    That's all. Before I go into details, let me briefly state
    its advantages and disadvantages in comparison with ordinary sextants.
    
    It does not really measure the angles.
    It only permits you to TIME the moment when the Sun
    altitude equals to one of the several pre-assigned values.
    In my Bris, the number of these pre-assigned values is 7
    and they range from about 7 degrees to 46 degrees.
    
    So the disadvantages are the following:
    1. You can only do Sun altitudes, usually few hours in the
    morning and few hours in the evening. Practically enough for
    2 pairs of position lines per day, if the weather is ideal.
    (You cannot do LAN, Moon, stars, art horizon,
    cannot measure angles between
    objects on the shore, not speaking of the Lunars).
    2. The weather should be very good. If the Sun or the horison
    is obscured for few seconds when needed, you missed the observation.
    3. You cannot average your observations to increase the precision.
    
    The advantages I listed in my previous message.
    
    How to make it. You need three rectangular pieces of glass,
    in my Bris they are of size approx 1 times 1/2 inch,
    somewhat less. One of the three pieces has to be of shaded
    (dark) glass, like an ordinary sextant shade.
    It should be dense enough so you could harmlessly look at the
    Sun, but not to dense so that you see the horizon through it.
    
    As I understand, no special "optical quality" of glass is needed.
    
    You glue the three pieces together to a configuration that looks
    like a slightly open book. The spine of the book is where three
    short sides of the three rectangles meet in one line. It is probably
    helpful to grind the glass at this edge so that they fit together
    nicely.
    The angles between the three pieces are about 10 degrees, precise
    angles are not important. In my sample two little rods of glass
    are inserted near the other short edges (opposite to those
    where the rectangles are glued together) ko keep "the
    book" in slightly
    open position. The inner glass is transparent, one of the outer glasses
    is shaded. That's the whole device. You want it to be as rigid as
    possible,
    so use a good glue.
    
    My one has one more important element: a rope going through the space
    between two pieces of glass, to wear the thing on the neck.
    Otherwise it is easy to loose or misplace it.
    
    One attractive feature is that
    there are no moving parts and no precision work is required.
    The quality of glass is irrelevant (I mean it will work better if
    the glass is smooth, transparent and
    polished, but it is not important
    that the
    surface is perfectly flat or that two surfaces of a piece are perfectly
    parallel).
    The angle between the pieces does not have to be made with
    any precision.
    
    After the device is ready, the hardest part comes: It has to
    be calibrated. You need at least one sunny morning/evening
    on a beach to do this, but better 3-5 evenings/mornings.
    Look at the Sun through the sextant. The wide side up.
    (The "spine of the book" down. This is the position in which
    it naturally hangs on the rope). You will
    see 5-7 "Suns"
    of various brightness, and the horizon.
    These  "Suns" are created by multiple reflection of the ray
    in the surfaces of the glass panes. One of the images,
    the brightest one, is the "real Sun" (non-reflected ray)
    you don't use it. Each ray that
    goes
    through
    to your eye makes some fixed angle (depending on your device) with the
    true direction
    to the sun. The purpose of calibration procedure is to measure
    these angles for your particular device.
    
    For this you time the moments when 1-st, 2-nd, 3-d etc. "Sun" touches
    the horizon with its upper and lower limb. Then compute the Sun altitudes
    for these moments and your known position, as you do in the ordinary sight
    reduction. Correct the results for dip and refraction (and wave height
    if there are waves). Then make a little table showing
    1-st, 2-nd etc. "Suns" altitudess, for each limb.
    Make few copies of this
    table and keep
    them in a safe place. All your future observations will depend on this
    table, so try to make it as carefully as possible.
    Averaging of 3-5 days of observations will help.
    (And also will give you an idea of precision of these observations.
    Another good way of control is comparison of the lower and upper limb
    altitudes with the 2SD given in the Almanac. This gives you an idea
    of how reliable your calibration is).
    
    Rocking. When you slightly rotate the devise about VERTICAL axis,
    you will see that the reflected "Suns" move up and down slightly.
    You want to measure the altitudes when the reflected Sun is in the LOWEST
    position. This happens when the horizontal lines in the planes of your
    glasses are perpendicular to the line of your sight.
    This rotation plays the role of rocking the usual sextant.
    
    The use of the Bris sextant is simple.
    You wait till one of the reflected Suns comes close to the horizon.
    Then look through the sextant, slightly rocking it and wait
    until a limb touches the horizon. And time the moment.
    Then look to your table, and it gives you the Sun altitude.
    Then reduce the sight by the usual rules,
    correcting for dip and refraction.
    
    Actually you can save on refraction correction.
    Under the normal conditions, refraction will be always
    approximately the same for the given altitude,
    and you can just take it into account in your table.
    
    In a next message I will publish my calibration results,
    and discuss the precision of observations. I will also
    ask Bill to make a good photo of my Bris, and will post
    it on the web.
    I have to say that the weather in the North sea in August was
    not good enough. So I could not fully and reliably calibrate
    the device in 2 weeks that I sailed.
    
    While with a usual sextant, on several days I could catch the Sun
    in the holes between the clouds. I consider this the main advantage
    of the usual sextant design. But probably in other seas you have more
    sunny days:-)
    
    Alex.
    
    
    

       
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