NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Hughes explanation of Chichester's navigation.
From: Bruce J. Pennino
Date: 2014 Dec 5, 10:07 -0500
Bruce
Precision astrolabe, bubble sextant development
http://www.fer3.com/arc/imgx/Bubble-sextants-Precision-Astrolabe.pdf
http://fer3.com/arc/imgx/sextants.pdf
http://fer3.com/arc/img/111880.img_4663.jpg
http://fer3.com/arc/img/111880.img_4664.jpg
http://fer3.com/arc/img/111884.img_4666.jpg
http://fer3.com/arc/img/111884.img_4665.jpg
From: David Pike <NoReply_DavidPike@fer3.com>
To: garylapook---.net
Sent: Friday, December 5, 2014 1:21 AM
Subject: [NavList] 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
From: Gary LaPook
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:
Precision astrolabe, bubble sextant development
http://www.fer3.com/arc/imgx/Bubble-sextants-Precision-Astrolabe.pdf
http://fer3.com/arc/imgx/sextants.pdf
http://fer3.com/arc/img/111880.img_4663.jpg
http://fer3.com/arc/img/111880.img_4664.jpg
http://fer3.com/arc/img/111884.img_4666.jpg
http://fer3.com/arc/img/111884.img_4665.jpg
gl
From: David Pike <NoReply_DavidPike@fer3.com>
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
