A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Bris sext. was: Suitable Sextants
From: Alexandre Eremenko
Date: 2005 Oct 11, 12:51 -0500
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.