Air Navigation by Bubble Sextant

[Tom]

Dear All,
I am a new member here, I have been intrested in Celestial Nav for years. I have a bit of free time at the moment, so i thought I would put more effort in.

I have purchased 2 Mark IXa bubble sextants and I am in the process of trying to get one servisable one at the end. I fly across the Atlantic quite often, usually westbound during the day and eastbound at night, I plan to check the INS/GPS?, and develop my skills.

As this august body has great knowledge, I intend to ask a lot of questions.

In the early day of Astro in aircraft, a dome was used, latter on periscope sextant were stuck though a hole in the roof, neither of these are avaliable to me. I just have a limited field of view through the cockpit windows.

This leads to a couple of problems, celestial body selection, I can’t do much about that, and refraction which probably exists and maybe with some expert help I can correct for refraction.
I know domes had refraction corrections.

Any thoughts would be appreciated.

All the best

Tom

[EdCa]

Tom,

I don't know if I can be of any help but will and address refraction, dip and body selection.

For your Westbound trip, since it's during the day the obvious choice is the Sun made to the starboard side of the aircraft or, if you can, "right on the nose" looking forward.

For your Eastbound trip, at night, depending upon the time of year, find the brightest stars you can and shoot them. Vega, Deneb and Altair would be a good choice- they're not too difficult to find, and so are the stars in Orion.

Take a look at this procedure and chart for star identification-

Navigational Star chart- how to use it.

If that seems confusing, just look at the bottom of that star chart and notice the months.  Example, on your local meridian for January 21 (approximate, of course) Mirfak and the stars in it's vicinity...including all those in Orion are good ones. Capella is noticeably bright. Of course much of star selection depends upon your shooting position.  This time of year you'll see the Planets Jupiter, Saturn an Venus quite well (But Jupiter is really high in the sky at night, presently). 

Refraction:  Since you'll be at high altitude you'll need to correct for refraction.  The 2025 Air Almanac page 908 lists the correction for Refraction based on altitude. Page 909 list correction for the Dome (just provided that for curiosity for you)

As to Dip...that has me somewhat uncertain if a Dip correction is required considering you're using a bubble sextant.

Get it here- 2025 Air Almanac

For starters, you might make your life easy and just try shooting Polaris for latitude and see how close  you get to what the GPS reads.

How to Determine Latitude using Polaris is found on this page (scroll down and you'll see it)
Determine Latitude using Polaris

I learned CN by "reverse engineering" so to speak- took shots that weren't well done and then reduced the sight. Plotted the LOP and then figured out why there were large errors. If...if you record the correct time in GMT and get a "close enough" altitude measurement of the celestial object, you'll then have enough information to slog through many of hours (God forbid) trying to figure it all out.

I hope that assists you in some way and isn't too terribly confusing.

Ed

[Tom]

Ed,

Thanks very much for the detailed reply, I like the reverse engineering idea, as you say many hours of fun to be had here.

I will be using a bubble sextant, if I can get it working, so I believe I can ignore dip, please will someone correct me if I am barking up the wrong tree here.

All the best

Tom

---

I was not clear about the refraction.

The aircraft windows will cause an error similar to the astrodome.

The front windows are flat, the four side windows have a bit of curve in them. All are made of multiple sheets of glass laminated with something rubbery in the middle. Guess I will find out if this is a problem.

All the best

Tom

[Tom]

Update on progress.

I bought two Mark IXA bubble sextants, and with help of Bill Morris’s excellent guide I have got one working save for the averager. I have not checked the index error yet.

However yesterday the sun appeared briefly and I got a sight.

I plotted the ensuing LOP and the closest I was to that position line was 70ish nm. 

Could I just check a couple of things.

I used the air almanac, corrected for atmospheric refraction, but did not use LL or UL as I put the sun in the bubble. Is this correct?

Can anyone explain how the averager on a Mark IXA works, i.e. the principle?

Thanks

Tom

[Tom]

Brief update,

I went to Jamaica at the weekend. On the outbound the sun was almost due south for the whole cruise, so I produced a lot of parallel LOPs, I did not bother to transfer them as the crossing error would have been very big.
The return at night was much better, and I got to within about 30 miles.  Using the above mentioned stars The main problem at the moment is shooting technique. As I have other duties, I do not have long to acquire night vision, the mirrors are not perfect and my rubber eye cap needs some work.
However I will keep trying, I have also just come across Astron which is brilliant.

[neil_s]

Well done for real Air Nav! I note from your posts above that your averager may not be working. This device takes a sample of the sextant's altitude setting once every second over the averaging elapsed time and sums them - displaying the result in the little window. I have two Mk IX BM models that I use in the back garden and the averager makes a big difference to the accuracy of your sights. From a chair on solid ground, I can get LOPs that pass within 3-4 miles of the GPS fix. You might have a look also at the post immediately below yours discussing the results achieved by war-time navigators - maybe 30 miles is not too bad!

Neil_s

[Tom]

Hi Neil,
Thank for the reply, I have looked on the other thread, and I don’t think I can offer anything, I think you more experienced Astro navigators will be more helpful.
I put my Mark IXA together from two sacrificial models. I may have introduced errors that I don’t know about. I didn’t realise that the index error would be different with the 5 degree lever activated, but now you mention it, it’s obvious. Can you tell me how to check this? I have removed the averager due to excess friction, but I think I am going to have to fix this and put it back if I am to improve the accuracy.
All the best
Tom

---

Well done for real Air Nav! I note from your posts above that your averager may not be working. This device takes a sample of the sextant's altitude setting once every second over the averaging elapsed time and sums them - displaying the result in the little window. I have two Mk IX BM models that I use in the back garden and the averager makes a big difference to the accuracy of your sights. From a chair on solid ground, I can get LOPs that pass within 3-4 miles of the GPS fix. You might have a look also at the post immediately below yours discussing the results achieved by war-time navigators - maybe 30 miles is not too bad!

Neil_s

3-4 miles is very good. If I get down to 10 miles I’ll be very pleased.

[neil_s]

Hi Tom

The easiest way to measure your ie is to look at a sea horizon and correct for dip. You can use the 'D' setting on the coarse altitude control to check the 5 degree switch although I confess that I didn't bother! I was, though, very careful during the restoration process not to disturb the settings of the horizon and index mirrors. I average LOPs over my past 10 or so sights and compare to GPS to keep a running check on ie. One of my sextants had suffered damage to it's bubble mechanism, had a massive ie and drifts over time for some weeks after adjusting the index mirror screws although the other is very stable.

You will find your results improve significantly if you can get your averager (called 'automatic attachment', in the literature) to work. I was lucky in this respect - both of mine function without any help. If you have a look on e-bay for 'bubble sextant' you will find a seller called 'mark-ix-sextant' who sells a range of little information booklets that I have found very useful. I have 'Station Level Repairs' -  tells you how to remove and replace the averager. 'Bubble Replacement and other Repairs' includes just about all the info I needed to get my sextants working and there is another called 'Civilian Maintenance Handbook'  that gives details of the BM model averager mechanism - I don't have this yet!

You can always do manual averaging - just take a series of snap sights of your selected body, recording time and altitude, and average them with your calculator. This works well with a Link A12 sextant and a cheap plastic maritime sextant that I keep on my boat. I find 5 snap sights is generally enough.

Neil_s

[Tom]

Dear Neil
Thanks for the very quick reply. I’ll get the pamphlet you suggest. I did my rebuilds using the guide by Bill Morris which was great, but he suggested not to fiddle with the clockwork mechanism of the automatic attachment. May be I should try and get a BM model.
I am doing another North Atlantic flight next Monday, I’ll try and improve on the results.
All the best
Tom

[neil_s]

Hi Tom
Good luck on your flight! I was shamed into checking the ie with and without the 5 degree switch! I took 5 sun sights for each state, from a good solid chair in the back garden with care to get the best size bubble. I got ie for 'with' of 2.2' on arc and 'without', 4.5' on arc. This with my 1944 vintage Mk IX BM. The BM model is helpful for stars, because it has a small telescope that swings in front of the eyepiece. It also has a choice of averager running times, 1 or 2 minutes. although I find 2 minutes is too long to hold the sextant steady.

Cheers! Neil