Sextant Use

[carstenb]

I only do enough to remember how to do it. anything under 10nm is good for me. When I circumnavigate, I'll do it every day and I hope it will get better

[gjordan]

A tid bit of information (trick) to make evening/morning sights easier. Sometimes it is hard to pick up the star/planet in the mirror while you are trying to keep the horizon visible. Bringing the star down to the horizon is the goal, but it is easy to loose sight of the star in the mirror, or it can even be the wrong star that you picked up. When I was a young pollywog, an old greybeard told me "Hell, thats easy, just turn your sextant upside down and find the star in the scope, and then bring the horizon up to it. Then turn the sextant back to normal, and the star will be in the mirror, and you are close enough for fine adjustments." It worked well on a 26 foot boat. I didnt use it much but if having trouble picking up a star, I would revert to that. Give it a try sometime. It is just something else for your bag of tricks. _____Grant.

[goboatingnow]

to find stars, I typically preset the sextant from the almanac info, Generally avoids me pointing at random bits of the sky.

dave

[Jim Cate]

Back when I did celestial as a sole means of navigating at sea, I found a way to improve the accuracy of noon sights. It involved using a programmable calculator (HP-41) to do a least mean squares fit to a parabola... took altitudes every two minutes for around 10 minutes before and after local noon, entered the data and out would pop a very decent time and altitude for the series of sights. Made a reasonably accurate longitude as well as latitude. This replaced the old method of graphically estimating the parabola, and was much easier to do as well as being more accurate.

I still carry the old HP-41, and if required, I hope that I could resurrect the technique in my old age!

Cheers,

Jim

[gjordan]

Dave, yes the preset is a valid method, but I never got into that, since I never knew what I would be shooting when I reached the cockpit. It might be stars, planets or moon, or more likely a combination of those, depending on what was available. Then I would go below and do all the calcs. I agree that on a long passage you often use the same stars every night, so precalcs or preset would work, but if trade wind clouds are blowing, you may have to shoot whatever is available. Both methods will work, but I like the SHOOT FIRST AND FIGURE IT ALL OUT LATER oops, that is not a political statement. ____Good Sailing ______Grant.

[svinshallah]

When I'm out for more than a day-sail, I try to do a couple of fixes every day just to keep in practice. I use a 40-year-old Tamaya sextant, a $25 Timex watch corrected to WWV about once a week. For sight reduction, I'll normally use the Starpath program on a TI-89 calculator, but if I was buying it today, I'd use the Starpath program on an iPod since it is much cheaper than the older, dedicated calculator. I do have, and at least once a trip will do the reduction the hard way, either HO249 or fighting my way through the sight reduction tables in the Nautical Almanac, just in case everything fails.

The Starpath program lets you easily average a number of sights made over a 10-15 minute period, which for me makes a huge difference in the accuracy of my positions. Under good conditions the resulting averaged fix is seldom out more than a mile from my 45' boat.

[Sam Plan B]

Get the DVD: Celestial Navigation Simplified by William F. Buckley (yes, that Buckley). He tell yous all you need to know about shooting a sun sight and plotting a running fix.

[kefroeschner]

Just finished tricking out an Astra III and have some left over parts: the original 3.5x telescope, Owner's Manual (original chinese), handle with battery compartment, wire and connectors, spare bulbs(2), misc. screws, original case with black flock interior, etc.

Free to anyone who can use the parts to restore or replace missing parts of an Astra III.

I replaced the scope with a 10x50 prism monocular and re-did the scale lighting with LEDs and a Li coin cell. New grip is hand-carved walnut.

I found the sighting with the 10x scope to be as easy as the original, although most say that big power is a no-no. However I found that the field of view is the same as the original 3.5x scope. And for not losing an object, that seems to me to be what matters. With the old scope I see a little bitty moon in a field of view of about six moon diameters, like looking through a TP tube. With the 10x scope I see a big moon in a big field of view, still about six moon diameters, but the TP tube is gone and the image is brighter and sharper to boot. And just as steady.

Anyhow, if you want the parts, let me know where to ship them. First call gets it all.

kef, Orion

[kefroeschner]

The parts have been committed to S/V Alchemy.

Also, the field of view is more like ten moon diameters, not six, for either scope.

[S/V Alchemy]

Originally Posted by kefroeschner

The parts have been committed to S/V Alchemy.

Also, the field of view is more like ten moon diameters, not six, for either scope.

And thank you very much!

[Delfin]

Originally Posted by gjordan

I would suggest that you NOT learn to do a noon sight. I know that sounds like blasphamy to the start up navigators . I saw too many people that once they learned to do a noon sight , they never went any farther. A noon sight is one of the more difficult sights to get any accuracy on, and if you have a few minutes of cloud cover at the wrong time, you are out of luck for another 24 hours. Once you know how to do a proper LOP, you have the basic for the rest of the joy of celestial nav. Star sights only take a couple of more steps than sun sights, and are not any more difficult (if you have a scope on your sextant) and planets and moon are just a small variation of stars. One of the nice things is to do a sunsight and a moon sight for an actual fix during daylight hours. Yes, the moon is available during the daytime for about 1/3 of the month. I tried star sights with a Davis lifeboat sextant, and could not get any sort of clarity of the horizon. I used an English EBBCO plastic sextant for about 12 thousand miles, and it worked fine with its 3 power scope. As far as time went, I used a Radio Shack Time Cube, and a stop watch. With the wife taking times and recording my readings, I could take a round of 3 stars, 3 shots each during evening twilight. I know that was overkill, but it (Mexico to Marquesas) was my first long passage. A year later, going from the Tuamotus to Hawaii, I took a shot every other day until we got close. I eventually baught a nice 3/4 size Tamaya at about the same time as GPS was becoming affordable, and I have only used it a few times. I need to relearn it all, but I will be up to speed before I go off shore again. I like electronics, but I have no faith in it. ____My 2 cents worth. ______Grant.

Interesting. I never used noon sights, preferring star sights at dusk and dawn. Using a small hand held calculator with a Merlin II program I could take 4 sights pretty quickly, reduce them and assume I was somewhere near the locus of the crossing lines. I know, the 4th sight would be considered by most to be useless, but I always thought it helpful. On landfall, we never seemed to be off by more than 1 nm. Stars are nice because they're so small, they seemed a more accurate target.

It's a wonderful skill to have and highly recommended even if you have other means of determining position.

[kefroeschner]

Astrid showed me hers, so I guess I should reciprocate. (Hers is a British Admirality, solid brass, with a venerable history.)

I have been practicing on land with the sun, moon and Venus. The average of a dozen or so sights puts me within 150 feet of what GPS on my Android says. Any particular sight is generally less than 2 n.mi. -- except for the few that put me in Kansas City. Standard Deviation, 'sigma' is 0.8 n.mi.

I am sure it won't be this good at sea, but it's nice to know it works. I am doing the reduction with pencil, using Ageton's H.O.211. (Using electronics to do the math seems kind of missing the point -- if electronics works we probably have GPS.)

[gjordan]

Kerfroeschner, using HO 211 is a bit more accurate than HO 249, but it is also a bit harder to learn. HO 249 is plenty accurate for small boat navigation, but the book for HO 211 is much smaller than 249, which can matter on smaller boats. Learning HO 211 is great if you are going to go for your Oceans endorsement on a USCG Masters license. The upgrade to ALL OCEANS is the only reason I learned HO 211, and I went back to using 249. Learning any method of celestial is a wonderful addition to your sailing skills. Good Luck with it all. _____Grant.

[kefroeschner]

gjordan: Thanks, I did not realize 211 was more accurate. I like it because it seems close to the actual geometry and math in method. It is a bit tedious, and I find it is woefully easy to make mistakes, looking up the wrong column, swapping A for B and so on. It is, however, usually obvious when you have screwed up -- Kansas City?? I should look for a copy of 249 on e-bay to use when we actually go to sea as it is probably a more idiot-proof method, which would be worthwhile when the foam is flying. Probably too old for a license of any sort, but never too old to learn. And as Charlie says, "You are only as old as the girls you feel."

[Adelie]

Originally Posted by kefroeschner

Astrid showed me hers, so I guess I should reciprocate. (Hers is a British Admirality, solid brass, with a venerable history.)

I have been practicing on land with the sun, moon and Venus. The average of a dozen or so sights puts me within 150 feet of what GPS on my Android says. Any particular sight is generally less than 2 n.mi. -- except for the few that put me in Kansas City. Standard Deviation, 'sigma' is 0.8 n.mi.

I am sure it won't be this good at sea, but it's nice to know it works. I am doing the reduction with pencil, using Ageton's H.O.211. (Using electronics to do the math seems kind of missing the point -- if electronics works we probably have GPS.)Attachment 68876

Attachment 68877

Attachment 68878

Happy to find someone else that likes 211. Have you looked at the S Tables, a modification of 211 that I believe is smaller and gives more accurate results when certain values are near 0 or 90 degrees.

Actually I want to learn the method that is in Kolb's long term almanac, everything you need rolled into 50pg. I only wish someone could could come up with a long term almanac for the moon. With that you can survive a lightning strike that wipes out all the electronic. Wind up a mechanical watch and set it with time derived from a lunar per the Letcher method.