On Sun, 20 Apr 2008 18:08:02 -0700, Gordon wrote:

Dutton wrote:
On Sun, 20 Apr 2008 11:26:00 -0700, "Capt. JG"
wrote:

Can you navigate (lat and long) at night with a sextant and a compass, but
without a nautical almanac, sight reduction tables, the time of day, and
without knowing the names of the stars? The sextant has an error, but you
don't know what it is, just that it's off. You can keep your modern watch,
but you just replaced the battery and the time is wrong.



Why do you need all that stuff? The south pacific islander types
followed the appropriate star in the appropriate constellation and got
along just fine!
Gordon


Well, possible. Of course those times it didn't quite work we didn't
hear about it...

Bruce-in-Bangkok
(correct email address for reply)

On Sun, 20 Apr 2008 20:25:09 -0600, "Roger Helio"
wrote:


"Wayne.B" wrote in message
.. .

It might be possible but I doubt it. There used to be a sextant
designed for aircraft navigation in WW II that used an artificial
horizon of sorts but those guys were happy to know where they were
within 20 miles or so.

That was not a sextant. It was called an octant.

Here's one:

http://www.icarusbooks.com/images/1458.jpg

I've got no idea what that thing you referenced is but I can assure
you that the devices carried aboard aircraft, at least until the
1970's was called a "Bubble Sextant" and didn't look a thing like your
picture.


Bruce-in-Bangkok
(correct email address for reply)

On Sun, 20 Apr 2008 22:57:42 -0700, "Capt. JG"
wrote:

The advantage of this method is that it is incredibly easy to do. You don't
even have to correct for refraction or dip. Even if your sextant has an
error, it won't matter.

Conceptually it works but has big issues in real life. As mentioned
previously accurate star sights are only available for a few minutes
at sunrise and sunset every day. The times of sunrise and sunset are
changing every day from both seasonal variation and also from your own
travel. And of course there are many days when no sights are possible
because of overcast weather.

On Mon, 21 Apr 2008 18:55:22 +0700, Bruce in Bangkok
wrote:

That was not a sextant. It was called an octant.

Here's one:

http://www.icarusbooks.com/images/1458.jpg

I've got no idea what that thing you referenced is but I can assure
you that the devices carried aboard aircraft, at least until the
1970's was called a "Bubble Sextant" and didn't look a thing like your
picture.

Yes.

http://www.warbirdsite.com/museumnavigation.html

http://www.nautical-products.com/bubble-sextants.html

http://www.physics.uq.edu.au/physics...ant_manual.pdf

http://www.qmss.com/seastories/seastory05.html

Capt. JG wrote:
....

The disadvantage is that each position depends upon the previous one, so
that small errors may build up to be large ones.

No, you can go a number of days referencing the last fix.

If you are on a long
voyage, you may find that the original stars are no longer visible at a time
when they are useful.

Yes, this is a problem.

Before this happens, you will need to pick some new
stars, and take a double round of sights - one for the fix, and one to
establish a basis with the new stars.

The other problem is that getting the compass bearing of a star is not
that easy.

I would recommend that anyone interested in emergency celestial
navigation get a copy of "Particularized Navigation: How to Prevent
Navigational Emergencies" by Francis Wright, which shows how to do "good
enough" navigation with paper sextants, etc. Unfortunately, its long
out of print, so a bit of work would be needed to adjust some of the
tables to current dates.

BTW, I own three sextants (two WWII vintage, one antique) plus two
"lifeboat" sextants, but I haven't taken a sight on board in a dozen years.

Bruce in Bangkok wrote:
On Sun, 20 Apr 2008 18:08:02 -0700, Gordon wrote:

Dutton wrote:
On Sun, 20 Apr 2008 11:26:00 -0700, "Capt. JG"
wrote:

Can you navigate (lat and long) at night with a sextant and a compass, but
without a nautical almanac, sight reduction tables, the time of day, and
without knowing the names of the stars? The sextant has an error, but you
don't know what it is, just that it's off. You can keep your modern watch,
but you just replaced the battery and the time is wrong.

Why do you need all that stuff? The south pacific islander types
followed the appropriate star in the appropriate constellation and got
along just fine!
Gordon


Well, possible. Of course those times it didn't quite work we didn't
hear about it...

Bruce-in-Bangkok
(correct email address for reply)

Thats how they discovered new islands! They were knowm as the Fugawis!
Like. "Where the Fugawi?"
Gordon

On Mon, 21 Apr 2008 10:10:13 -0400, Wayne.B
wrote:

On Mon, 21 Apr 2008 18:55:22 +0700, Bruce in Bangkok
wrote:

That was not a sextant. It was called an octant.

Here's one:

http://www.icarusbooks.com/images/1458.jpg

I've got no idea what that thing you referenced is but I can assure
you that the devices carried aboard aircraft, at least until the
1970's was called a "Bubble Sextant" and didn't look a thing like your
picture.

Yes.

http://www.warbirdsite.com/museumnavigation.html

http://www.nautical-products.com/bubble-sextants.html

http://www.physics.uq.edu.au/physics...ant_manual.pdf

http://www.qmss.com/seastories/seastory05.html


The device carried aboard USAF aircraft until the 1970's looked most
like the bubble sextant shown as a US Navy bubble sextant Type A-12 at
the top of the page in your reference
http://www.warbirdsite.com/museumnavigation.html

Note that it was a sextant, not an octant.


Bruce-in-Bangkok
(correct email address for reply)

"Wayne.B" wrote in message
news
On Sun, 20 Apr 2008 22:57:42 -0700, "Capt. JG"
wrote:

The advantage of this method is that it is incredibly easy to do. You
don't
even have to correct for refraction or dip. Even if your sextant has an
error, it won't matter.

Conceptually it works but has big issues in real life. As mentioned
previously accurate star sights are only available for a few minutes
at sunrise and sunset every day. The times of sunrise and sunset are
changing every day from both seasonal variation and also from your own
travel. And of course there are many days when no sights are possible
because of overcast weather.



You don't need accurate times for sunrise and sunset to make this work. You
don't need to be dead on accurate if you're doing emergency navigation,
which is what this would be good for. Getting close would be fine. And, you
can use any couple of stars at night.

--
"j" ganz @@
www.sailnow.com

"Jeff" wrote in message ...
Capt. JG wrote:
...

The disadvantage is that each position depends upon the previous one, so
that small errors may build up to be large ones.

No, you can go a number of days referencing the last fix.

Don't understand... yes, you can go a number of days, but after that it
seems to me it would start to get way off.


If you are on a long voyage, you may find that the original stars are no
longer visible at a time when they are useful.

Yes, this is a problem.

Thus, I good idea to take several batches of stars in your observation, so
you can fall back on a second or third set.


Before this happens, you will need to pick some new stars, and take a
double round of sights - one for the fix, and one to establish a basis
with the new stars.

The other problem is that getting the compass bearing of a star is not
that easy.

Well, yes... that hard any time...

I would recommend that anyone interested in emergency celestial navigation
get a copy of "Particularized Navigation: How to Prevent Navigational
Emergencies" by Francis Wright, which shows how to do "good enough"
navigation with paper sextants, etc. Unfortunately, its long out of
print, so a bit of work would be needed to adjust some of the tables to
current dates.

BTW, I own three sextants (two WWII vintage, one antique) plus two
"lifeboat" sextants, but I haven't taken a sight on board in a dozen
years.


--
"j" ganz @@
www.sailnow.com

"Dennis Pogson" wrote in message
...

They look great on the mantleshelf. A wow at cocktail parties. Buy plenty
of Brasso.

Dennis.

I'd like to keep mine aboard... a good talking point for students.

--
"j" ganz @@
www.sailnow.com