# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

**Re: Navy MK 5 Octant Using Natural Horizon**

**From:**Alexandre Eremenko

**Date:**2012 May 2, 16:40 -0400

Greg, Here is my series: MkIXA, bubble, 19:50-20:03 GMT, Sun altitude errors (no refraction correction made, just compare what I read with the true altitude): -1'4, +5, -0'5, 0'5, -0'9, -4'5, -0'5, -1'4. And then the Sun disappeared: The average error of these is -0.46, and refraction correction is -0'6. So if I do take refraction into account I was 1' off. And the spread is 9'5 :-( In your longer series the spread is 7'2, if I read your numbers correctly. The question of standartization of terminology was raised. I am curious how aviators called the altitude (angle) to distinguish from the altitude which is in feet:-) Is this a standard practice in US Cel Nav to write "A" and "T" AFTER the number instead of + or - BEFORE the number, as most of the rest of the world does ? And to use this weird abbreviation 3' moa instead of just writing 3' ? What else 3' can mean in Cel Nav context? Three feet? It took me some time to figure out what this "moa" in your messages stands for:-) OK, Sun is comng back:-) Alex. On Wed, 2 May 2012, Greg Rudzinski wrote: > > Alex, > > I thought you might be interested in the last 32 bubble horizon observations (without averager) using the Navy MK 5 in perfect conditions from the stability of my boat dock. I'll let you crunch the numbers. Various bodies Sun, Moon, Planet, Stars observed over the last few weeks. > > 0.6T 0.8A 2.1A 0.1T 0.8A 4.6A 1.6A 0.1A 0.5A 1.7T 1.2A 0.5T 0.1T 0.5A 2.7T 2.9A 0.2A 0.3T 2.2A > 2.1A 2.0T 2.7A 1.7T 2.6T 3.4A 2.4T 1.7T 2.3T 1.0T 2.0T 3.2A 3.7A > > Did I use the correct index correction ;-) +2 moa I.C. for the bubble horizon. > > Greg Rudzinski > > NavList] Re: Navy MK 5 Octant Using Natural Horizon > From: Alexandre Eremenko > Date: 1 May 2012 18:17 > Greg, > >> The results are not extraordinary in that any data set from >> any sextant can be made to look good by adjusting the index error. > > Does your octant have adjustment for the index error? > I understand that with ordinary sextants the index error can be > adjusted. The problem is to determine it with 0.2 accuracy:-) > > Besides, we know that even good ordinary sextants can have substantial > arc error, etc. Which is again hard to find. > > Now, the 0.2 average error in your 30 observations shows that the > index error as you determined it is very close to the truth. > Did you adjust it? I supose not, (otherwise why would you adjust > it to 6') > >> is why I look at the maximum and minimum values of a set >> of data to judge a sextant and or navigator performance. > > The maximum spread of 2.7 shows that the errors are all less then > 1.2 or 1.4 in magnitude. > > But you did not answer my question: how did you choose the preset values? > And why they were such as we see them. > > Alex. > >> Keep an eye open on ebay for U.S. Navy MK 5's which can be >> had for under $100 but use caution on the buy. > > I don't know how can one "use caution" with a sextant on e-bay, > especially with an air sextant:-) > You can only take the risk. > 99.9% of e-bay sellers would say in advance that they know nothing about > sextants, which is true, about half of them photograph a normal sextant > upside down. Once I asked a seller what is the radius of the arc, and his > reply was that he is not sure what the "radius" is :-) > So I do not expect them to answer such questions as whether the bubble is > present or not. I simply thought that $100 or $140 that I payed is a sum > I can afford to risk:-) > > I was lucky: the only thing which did not work was the light, > and I found how to fix it and fixed in few hours. > But repairing a bubble is definitely beyond my possibilities:-( > > Alex. > ---------------------------------------------------------------- > NavList message boards and member settings: www.fer3.com/NavList > Members may optionally receive posts by email. > To cancel email delivery, send a message to NoMail[at]fer3.com > ---------------------------------------------------------------- > > > > > View and reply to this message: http://fer3.com/arc/m2.aspx?i=119346 > > >