A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Bris sext. was: Suitable Sextants
From: Gennaro Sammarco
Date: 2005 Oct 17, 20:01 +0200
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. > ___________________________________ Yahoo! Mail: gratis 1GB per i messaggi e allegati da 10MB http://mail.yahoo.it