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.

I assume this is a quiz? You can use a sextant to measure horizontal
angles and take relative shore bearings, other than that I think
you're out of luck. You can not take star sights at night (no
horizon), only at twilight.


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

I've owned this one for years and people have crossed oceans with it:

http://tinyurl.com/6fdkfr

It works but I prefer my GPS chartplotter.

"Wayne.B" wrote in message
...
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.

I assume this is a quiz? You can use a sextant to measure horizontal
angles and take relative shore bearings, other than that I think
you're out of luck. You can not take star sights at night (no
horizon), only at twilight.


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

I've owned this one for years and people have crossed oceans with it:

http://tinyurl.com/6fdkfr

It works but I prefer my GPS chartplotter.



Yes, it's a quiz and it's possible...

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

http://en.wikipedia.org/wiki/Sextant

I'm somewhat familiar with the Davis models... thinking about playing around
with the 25.


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

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

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 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.

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.

"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 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)