Visit our Popular Forums

zydecotoad · 31-01-2011, 08:30

I have made exactly 2 sights at sea. One was a sun+moon sight and the other an evening star sight. I was about 5nm from the GPS position for both of them. I felt this was close enough to find a landfall. Actually taking those sights was very dependent on how much the boat was rolling at the time.

steve_hendry · 31-01-2011, 10:27

also, I have found that using a small pool of water on a black parking lot is easier that a proper artificial horizon. I can't think of a reason not to do it that way.

Too close. The light path to the mirror and the path to the telescope won't be parallel.

**sneuman** · 31-01-2011, 10:39

Quote:

Originally Posted by steve_hendry

also, I have found that using a small pool of water on a black parking lot is easier that a proper artificial horizon. I can't think of a reason not to do it that way.

Too close. The light path to the mirror and the path to the telescope won't be parallel.

Why would it be any different with an artificial horizon, which would be positioned in the same place anyway?

btrayfors · 31-01-2011, 11:21

While stationed in Morocco for four years I used to take sights from the beach periodically just to stay in practice. I also was interested to learn about repeatable accuracy.

I used a Plath sextant which I had bought new in Hong Kong some years before, an accurate timepiece, and stood in exactly the same spot on the beach taking afternoon sun shots over the Atlantic.

I found that with good technique I could pretty regularly get the resultant LOP to within 0.2 to 0.3 nm of my actual position on the beach. This was with top-notch equipment, good youngish eyes, from a known fixed position on the beach.

My conclusion was that this was about the best accuracy one could hope for, and at sea could only be achieved on a very stable platform -- not a small yacht.

On a trip to the Canaries on a 38' ketch, I believe I achieved an accuracy of about 3 nm for noon latitude sights and, interestingly, was able to use the meridian passage technique (equal sextant altitudes before and after LAN to calculate time of meridian passage, and therefore longitude) to obtain noon fixes within about 5 miles. That's just an estimate, though, because we didn't have GPS or satnav then :-)

Bill

steve_hendry · 31-01-2011, 11:26

The Celestaire artificial horizon I have goes in place of the telescope, so after the mirrors where the two light paths are parallel.

I guess what you're doing is equivalent to the Davis artificial horizon, where your sextant reading is double the actual angle of the body.

Page 12 of the Celestaire catalog gives a table of corrections for using the opposite shore of wherever you are as a horizon, so this is another way to practice.

Sailbad · 31-01-2011, 11:31

Doing well!
Now try standing on a beach using the horizon, with someone throwing a cold bucketful of water over you every now and again as well as giving you a push in the back just as you are about to do that final adjustment !

steve_hendry · 31-01-2011, 11:35

Quote:

Originally Posted by Sailbad

Doing well!
Now try standing on a beach using the horizon, with someone throwing a cold bucketful of water over you every now and again as well as giving you a push in the back just as you are about to do that final adjustment !

You forgot the part about tearing up $100 bills while all this is going on.

**sneuman** · 31-01-2011, 11:39

Quote:

Originally Posted by steve_hendry

The Celestaire artificial horizon I have goes in place of the telescope, so after the mirrors where the two light paths are parallel.

I guess what you're doing is equivalent to the Davis artificial horizon, where your sextant reading is double the actual angle of the body.

Page 12 of the Celestaire catalog gives a table of corrections for using the opposite shore of wherever you are as a horizon, so this is another way to practice.

Yes, that's what I'm using. I have a bit of unobtructed shoreline a few nms away - so I could use that for a "dip short" - but it's only about 90d worth, west to north.

barnakiel · 31-01-2011, 12:04

I was making pretty regular sun fixes on our passages.

Conditions: ocean going, a small, rolly 26' doubleender, plain sextant (but not the plastic crapo, standard paper based almanac, no reduction tables just plain calc from the formulas. Later accy checked against the GPS.

Accuracy on a bad day was 20 Nm. Accuracy on a good day was 5 Nm.

Most of the time accy was close to 5 miles, except on a very choppy day when it was always more than 5 and often 10 miles.

Calculating and plotting the fix was never a challenge, getting a clean and reliable sight nearly always was.

Personally, I consider accy of 5 to 10 Nm acceptable but 5 or below desirable.

For real life data google out Ioannis Rosseau and ask him - he goes by sextant alone and by DR in between - so his take on what is or not achievable will probably count.

b.

Lodesman · 01-02-2011, 05:16

Quote:

Originally Posted by sneuman

I am in fact using a GPS pos as my AP. This is just for practice to see whether my sights are "in the ballpark", so to speak.

I'm not sure what inherent inaccuracies are involved with the artificial horizon, but standing on land, I think you should be able to whittle down that range of accuracy with practice.
I seem to recall that with one of the earlier celestial programs I used, you could input a DR position from GPS down to 3 decimal places, but it would do its calculations and provide you with another Assumed Position with the intercept - this AP wouldn't be as far out as one used with HO229, but more like it was rounded up to the nearest whole minute, or nearest even minute (memory is faded), probably to make it easier to plot. All to say, you should make sure your program is not doing something like that, if you're measuring your accuracy based on your GPS position.

svHyLyte · 01-02-2011, 06:09

Acceptable accuracy varies given prospective hazards. Offshore, where hazards are limited, acceptale accuracy might be 5-10 miles. Near shore, acceptable accuracy would be within ones circle of horizon. From the deck of our yacht, that is a bit over 3 miles. With practice one can usually achieve that with decent equipment. A morning or evening 3 star "fix" should give one a "cocked hat" of which one assumes one is in the center. If the "hat" is smaller than ones visable horizon, it should be a good fix. Running fixes are less reliable due to uncertain set and drift. As we sail on the southwest coast of Florida, an early morning and early evening LOP is most valuable as it gives us a good measure of distance off which, generally, is all I'm concerned abou while we're traveling.

FWIW...

barnakiel · 01-02-2011, 06:39

Except that when we get what we get we have to accept just what we got keeping in mind whatever the likely circle of error is.

Acceptable.

Acceptable in a navigation class? Yes.

On the passage maybe it was the only clear sky moment we got in a fortnight and 5 or 10 miles will be acceptable or maybe 20 or 50? Even 50 would be better than whatever we get from DR.

And (IMHO) using GPS fix for our Assumed Pos (while talkin accuracy) is cheating. Accuracy in Astro Pos Finding begins with our take of port lat / long then we DR for a time and then we take sights and make the Astro Pos. Then we take off from this point (which already has some of the DR error in it and some of the Astro Pos error in it ... then we continue DR ... Off course, to check such a position against a GPS fix is another matter, but if we take GPS fix as our Assumed Pos then the error obtained in our Astro fix will be smaller than if we based it all on our cumulative DR / Astro positions.

b.

**sneuman** · 01-02-2011, 06:48

Quote:

Originally Posted by barnakiel

Except that when we get what we get we have to accept just what we got keeping in mind whatever the likely circle of error is.

Acceptable.

Acceptable in a navigation class? Yes.

On the passage maybe it was the only clear sky moment we got in a fortnight and 5 or 10 miles will be acceptable or maybe 20 or 50? Even 50 would be better than whatever we get from DR.

And (IMHO) using GPS fix for our Assumed Pos (while talkin accuracy) is cheating. Accuracy in Astro Pos Finding begins with our take of port lat / long then we DR for a time and then we take sights and make the Astro Pos. Then we take off from this point (which already has some of the DR error in it and some of the Astro Pos error in it ... then we continue DR ... Off course, to check such a position against a GPS fix is another matter, but if we take GPS fix as our Assumed Pos then the error obtained in our Astro fix will be smaller than if we based it all on our cumulative DR / Astro positions.

b.

Well, it may be cheating, but barring an offshore passage in January, it's the only way I know of to determine if my sights are looking ok.

boasun · 01-02-2011, 06:51

practicing with a sextant and after awhile you will develop a sense of your personal error. Usually found from seeing what it takes to reduce you 2 to 5 mile error down to nada or as close as you can come. Then from that moment on you apply that personal error to your sextant reduction. But it can change as your eyes change when that dreaded desease called Aging or getting older, over comes your youth.