NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Bris Sextant
From: Alexandre Eremenko
Date: 2005 Nov 6, 20:42 -0500
From: Alexandre Eremenko
Date: 2005 Nov 6, 20:42 -0500
John, Thank you for your interesting observations. (I infer that you live on a sea shore, so I envy you:-) When you have time and good weather to to graduate the scale of your Bris, please inform us, I am interested in the precision you obtain. > I then held the apparatus in front of one side of a 7 x 35 binocular. As I said before, this is dangerous! > But the Bris > Sextant has to be rotated in the "pitch" direction and moved vertically > up or down to bring the successive sun images into the field of view. Yes, this is a problem. No telescope has the field of view wide enough to show all 8 Suns:-) >the binocular adapter would have to be adjustable in two directions, >a tricky design problem. (It might turn out that only tilt is required >if only one sun image, that near the horizon, was needed.) One direction should be enough, "pitch", if the axis of rotation is chosen properly. > Pity the poor navigator in the past who used a cross-staff! I believe those guys were not that stupid and used a BACKstaff. In a backstaff you align the Sun shade with the horizon, rather than the Sun iteslf. I saw two marvelous devises of this sort, genuine ones, perfectly preserved since XVII century, in the Schiffahrtsmuseum in Bremerhaven, the best miseum of this sort I've seen so far. Also has two Pistor and Martins reflection circles. So I suppose a cross staff could be only used safely for star observations. Unless the observer wore dark eyeglasses. > I have trouble calling this test apparatus a "Bris-Sextant" . Me too:-) As you can see from my messages I prefer to call it "Bris device". Alex. P.S. I posted several new pictures on my web site (also made by Bill) showing the angles he measured from the pictures of the device. In addition I posted some of my observations and computation of the angles between the glass panes from these observations. One interesting remark. The observations fit the theory only VERY approximately, when you round to 1/2 degree. I mean the angles I obtained do not fit exactly the theoretically derived pattern A, B, 2A, A+B, 2B, 2A+B, A+2B, 2A+2B. The disagreement with this pattern is much larger than the errors of these observations. (The error of observations was controlled by Sun SD and by repeated observations, and it is about 2'-4'). This indicateds that the glass panes in my Bris are actually of very poor quality:-) I mean the surfaces of one pane are not parallel. However, this does NOT affect the preformance of the device, and this is the main beauty of the inventor's idea. Alex.