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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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