August 26, 2016


To invent a small cheap functional sextant capable of measuring any angel up to 90 degrees and accurate and rugged enough for life raft use my thinking went like this. I imaged a fixed angled Bris sextant, then one more to the left having a slightly smaller angle a third one to the left of that with an even smaller angle and so on down to zero. To the right I imaged a series of Bris sextant their fixed values kept increasing up to 90 degrees. After that I imagined the steps between them becoming smaller and smaller. After that still in my mind I glued them all together. Now I had a thought model of the finished product, one long sextant with twisted mirrors.

I made the first test by taking two strips of tinted semi reflecting acrylic glass and putting them together, one on top of the other. I joined them in one end, and then I twisted them at the other end to the desired angle and fastened them so together. I added a scale and a cursor to make reading easier because the human eye cannot focus so close. I have the option of adding shades to the cursor.

When taking a sight I hold the instrument close to my eye in a horizontal position, looking through it at the horizon. To move the image of the sun up and down I move my instrument sideways. When the sun is touching the horizon I note its sidewise position in relation to the scale and time it.

The next step is with the help of my position and the time to calculate the suns altitude. That way I find the corresponding altitude of my instrument at that point on the scale. I repeat the procedure for a few different angels.

As there are an infinite number of different angels there is no way I can calibrate them all, but that’s not necessary. It is enough to calibrate a few well spread out spots and make a table of them. To find any other angel I extrapolate or interpolate.

There is two ways to achieve the desired accuracy without a long and cumbersome instrument. One is to make a set of shorter ones, for example a set of three 30 cm long instruments with angels 0 – 30, 30 – 60 and 60 – 90, corresponds to a 90 cm long scale giving 1 centimeter for each latitude.

Eighteen instruments each 30 cm long will give a total length of scale of 5.4 meter long. Still the will require less space than a conventional sextant.

The other way is to make one instrument, like the Bris sextant with several mirrors that crate several images of the sun, spaced to the desired angels. It’s a neater solution but iterations or calculations will be needed to achieve the desired results.

The Bris Sextant and Yrvinds Survival Sextant are both interesting products and they complement each other. The Bris Sextant is extremely small and exact, but like a set of sockets has only fixed values. Yrvinds Survival Sextant is not so exact but can measure all altitudes a bit like an adjustable spanner.

Me, I have leave these experiments for time being to focus on my boat Ex Lex. Boatbuilding is also a lot of fun as is sailing the finished product.

ABOUVE. Long version of Yrvinds survival sextant. To take a reading, slide the sextant sidevays, that will move the sun upp and or down. When the sun touches the horizon time it and note how far  sidevise the sun is on the scale. No moving parts.

To be continued…

Regards Yrvind.

August 16, 2016


Same years ago I invented the Bris Sextant as a backup. It’s a small cheap functional instrument. Its extremely accurate, much more so than the ordinary sextant because it uses a number of fixed angels instead of a continues scale, but like the ordinary sextant it needs accurate time to give you a fix.

Photo above Bris Sextant in my right hand the ordinary in the left.

Time can be lost due to lightning, if there is a fire onboard and you most abandon ship, and for many more reasons such as the unlikely event of our civilization braking down and the time signals stops.

Fortunately latitude can be found without time by observing the suns altitude at noon.

The chronometer was invented only after most of the world had been explored so most discoveries were done without synchronized time. Therefore the explorers could not find their longitude. The going was though. They overestimated how far they had gone. Their latitude they could check, but not their longitude, hence maps from that time show the world much fatter. See illustration below.

To find ones destination without synchronized, one has to sail north or south to the latitude of the desired destination, then sailing east or west, constantly sounding and with a sharp lockout until one arrived at the desired destination.

Of course I could get an ordinary sextant but they are complicated and prone to errors, so I thought if I could invent a small cheap functional sextant with a continues scale, but without moving parts it would be a good thing.

An other picture

Photo above the survival sextant with its continues scale and its cursor.

Me with the first experimental model. I used it to testi if the new principel worked. It did and does but its accuracy is good enough for survival use.

I will soon publish an explanation of its principle and how to you use her.

To be continued …

Regards Yrvind.

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