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    Re: Bris sext. was: Suitable Sextants
    From: Gennaro Sammarco
    Date: 2005 Oct 17, 20:01 +0200

    thanks a lot, winter is coming and it's always nice to have some nautical
    work to do...
    fair winds
    Gennaro Sammarco
    
    ----- Original Message -----
    From: "Alexandre E Eremenko" 
    To: 
    Sent: Tuesday, October 11, 2005 7:51 PM
    Subject: Bris sext. was: Suitable Sextants
    
    
    > > 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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