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