On Sun, 20 Apr 2008 12:51:53 -0700, "Capt. JG"
wrote:

Not sure what you mean about not being able to use it at night... What about
the artificial horizon common on modern sextants?

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.

You can get latitude without accurate time by taking a series of noon
sites and plotting the maximum elevation. Longitude to any reasonable
degree of accuracy is very difficult without accurate time.

"Wayne.B" wrote in message
...
On Sun, 20 Apr 2008 12:51:53 -0700, "Capt. JG"
wrote:

Not sure what you mean about not being able to use it at night... What
about
the artificial horizon common on modern sextants?

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.

You can get latitude without accurate time by taking a series of noon
sites and plotting the maximum elevation. Longitude to any reasonable
degree of accuracy is very difficult without accurate time.



Not only possible, but people take star sightings all the time. Besides the
Sun, how many stars are you expecting to see during the day?

Yes, you can get an LOP with the information/equipment I supplied. I didn't
make this up, wish I did, but it works.

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

"Edgar" wrote in message
...

"Capt. JG" wrote in message
news:J8ednTRMl5onF5bVnZ2dnUVZ_gqdnZ2d@bayareasolut ions...
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.

I say that you cannot.
Firstly because at night you probably cannot see exactly where the horizon
is.Even if you got an aeronautical sextant with a bubble level they are
very hard to read on a boat if it is at all rough
Secondly because you cannot derive longitude without an accurate watch.


Does someone own a sextant? What model? I'm thinking about picking one
up.

You do not say what you want it for. For use aboard or just as a talking
point at home?.
You can get various plastic sextants that do a good enough job for use on
a small boat where pinpoint accuracy is hard to achieve.
But a proper sextant is a thing of beauty and mine is a Kelvin Hughes
dating from 1959 which is in pristine condition in its box with all
accessories. I take the odd reading with it from time to time to keep my
hand in in case both my chartplotter and my DR on paper charts fail me.
Mine is a micrometer type but retains the silver engraved scale which is
somewhat overkill for a micrometer since you only need to use the scale to
read whole degrees. But it is very nice to look at an inlaid silver scale.
Vernier sextants are older and are collectable antiques now and
regrettably mostly end up in museums or hanging on someone's wall.
There are a lot of authentic looking but purely decorative ones about
which are not suitable for navigation. Make sure yours comes with an
authentic test certificate.



I never said it would necessarily be a boat that was in rough conditions.
That is always difficult.

The sextant would be a backup, a talking point, and something to mess with.


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

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

"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

On Sun, 20 Apr 2008 16:22:29 -0700, "Capt. JG"
wrote:

Yes, you can get an LOP with the information/equipment I supplied. I didn't
make this up, wish I did, but it works.

Well please enlighten us. Heavenly bodies still hold a certain appeal
in these days of push button navigation.

"Wayne.B" wrote in message
...
On Sun, 20 Apr 2008 16:22:29 -0700, "Capt. JG"
wrote:

Yes, you can get an LOP with the information/equipment I supplied. I
didn't
make this up, wish I did, but it works.

Well please enlighten us. Heavenly bodies still hold a certain appeal
in these days of push button navigation.

They do for me as well, although I'm not familiar with more than the basics.

I'd be happy to, but just to be clear, I didn't come up with this, and I
haven't tried it. It seems to be logically correct, and I'd like to try. If
you have a sextant, and you want to try it, I'd love to hear the result.
When I get one, I'm definitely going to give it a try. Courtesy of a
Celestaire handout at a seminar I attended. No mention was made of accuracy,
which I would think is dependent upon the adjustment of the various
instruments (sextant, compass, watch), as well as the observer.

Most of you are familiar with the stroboscopic effect. That's why the
flashing light makes the dancers seem to be frozen, or at least to have
jerky movements. We check the timing of motors with a flashing strobe, which
makes the rotating timing marks seem to stand still. You can do the same
thing with the stars.

Stars move through the sky so regularly that every 23 hours 56 minutes and 4
seconds, each star is back in the same place as the night before. This means
that if you laid on the ground facing up, and every 23h 56m 4s you opened
your eyes for a moment, the stars would appear to be stationary in space,
always in the same place. If you took a round of sextant readings on any of
the stars you cared to keep track of (you can give them any names you wish),
the readings would be the same night after night, as long as you did it
every 23h 56m 4s instead of every 24 hours. The readings would remain the
same - unless you moved! If you move between the readings, your movement
will be reflected in changed readings - one minute of arc for every mile you
moved.

If a star's reading increases by 23' for example, then you must have moved
23 miles in its direction. If you have a compass with you when taking
readings, you will know what direction to plot the new line of position. You
simply plot a line from your previous position in the direction of the star,
measure 23 miles (the intercept) and plot a line perpendicular at this
point. This is your new LOP. If you do the same for several other stars, you
have a new fix.

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.

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

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

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


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

Did they? Maybe we just don't know how many of them got lost. ;-)

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

"Capt. JG" wrote in message
news:J8ednTRMl5onF5bVnZ2dnUVZ_gqdnZ2d@bayareasolut ions...
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.

I say that you cannot.
Firstly because at night you probably cannot see exactly where the
horizon is.Even if you got an aeronautical sextant with a bubble
level they are very hard to read on a boat if it is at all rough
Secondly because you cannot derive longitude without an accurate
watch.


Does someone own a sextant? What model? I'm thinking about picking
one up.

You do not say what you want it for. For use aboard or just as a
talking point at home?.
You can get various plastic sextants that do a good enough job for
use on a small boat where pinpoint accuracy is hard to achieve.
But a proper sextant is a thing of beauty and mine is a Kelvin Hughes
dating from 1959 which is in pristine condition in its box with all
accessories. I take the odd reading with it from time to time to
keep my hand in in case both my chartplotter and my DR on paper
charts fail me. Mine is a micrometer type but retains the silver
engraved scale which is somewhat overkill for a micrometer since you
only need to use the scale to read whole degrees. But it is very
nice to look at an inlaid silver scale. Vernier sextants are older
and are collectable antiques now and regrettably mostly end up in
museums or hanging on someone's wall.
There are a lot of authentic looking but purely decorative ones about
which are not suitable for navigation. Make sure yours comes with an
authentic test certificate.



I never said it would necessarily be a boat that was in rough
conditions. That is always difficult.

The sextant would be a backup, a talking point, and something to mess
with.

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

Dennis.

ask wilbur



"Capt. JG" wrote in message
news:J8ednTRMl5onF5bVnZ2dnUVZ_gqdnZ2d@bayareasolut ions...
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.

Does someone own a sextant? What model? I'm thinking about picking one up.

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