# NavList:

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

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Re: Hughes explanation of Chichester's navigation.
From: Bruce J. Pennino
Date: 2014 Dec 5, 10:07 -0500
Speaking of dip at high elevation.....  A couple of year ago I went to the Blue Hills observatory near Boston, Elevation 700 ft +/- MSL, and measured dip with my theodolite. Remember back then, I was gathering dip data.  I was quite surprised to see that my measured dip was about equal to 0.9-1.0 sqrt HoE ft, and this was the same as at low elevations. Or the refraction does not change much with height ...kinda makes sense.....earth is round.

What is the approximate vale of  dip  coefficient at 5,000 ft and 10,000 ft? Near 0.9?

Regards

Bruce

Sent: Friday, December 05, 2014 5:31 AM
Subject: [NavList] Re: Hughes explanation of Chichester's navigation.

I poosted about the development of bubble sextants before, check out these links:

gl

To: garylapook---.net
Sent: Friday, December 5, 2014 1:21 AM
Subject: [NavList] Re: Hughes explanation of Chichester's navigation.

Dip of the Sea Horizon.  Well doesn’t it make you want to spit?  You look all night on the internet for a set of dip tables which go up to aircraft heights, and then you find one in front of you.  Nories Tables has a set going up to 10,000ft around p149.  Interestingly, it’s only one page deep, because whilst he gives you dip for every foot of height up to 100ft, he only needs to give it every 500ft by the time you get to 10,000ft.

Aircraft Windows.  On large pressurised aircraft pilots windscreens are very thick parallel glass although they do have a gold film inside them and they don’t cover much sky.  Canopies for smaller aircraft and passenger windows are less of a known quantity.  The only solution is to try them and see.  You can do it on the ground.  If you’re happy with the results, then continue to shoot that way.  I practice shooting lunar distances through the double glazed window in my lounge.  It’s warmer than outside, but when I prove UTC is out by several minutes, it makes me wonder if its me or the window.

Bubble Sextants in Aircraft.  As I said earlier, the main reason for developing bubble sextants for aircraft was because you couldn’t guarantee always being able to see the true horizon.  The earliest bubble sextants were just marine sextants with a spirit level attached.  However, the bubble brought fresh problems, and it was best to take more than one shot.  Recording became a problem, so the score mark on a drum or disk system was introduced.  After that came automatic recording of five shots as with the first RAF MkIXs.  Next came automatic shooting, as with the one shot per second for one minute or one shot per two seconds for two minutes clockwork mechanisms.  With later sextants you had the choice of one or two minutes.  (But never try to change gear when the clockwork is running!  Sorry, there’s always one thing they taught you at Nav School that you never forget!)  Later still you had electric drive, so you didn’t have to wind the sextant up.  However, if you’ve got a marine sextant and a good horizon, by all means use it.  I suppose that in theory a two minute shot might be better for some aircraft, but it’s a long time for a pilot to concentrate upon holding the aircraft steady and for the navigator to concentrate on following a body.  Dave

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