Sextant...

[RBEmerson]

And if you really want a lot of fun, look into a computer language called forth - RPN all the way!

[blue merlin]

I'm delighted to see all this continued interest in astro - it is the logical progression of the sailor's general aim to be self-sufficient if at all possible. In trying to learn the subject in europe, it is difficult to find a text that takes the novice from the ealy stages through to the complexities of understanding the maths etc. Most books drop you right in the middle with complex terminology, learning by rote and not by understanding. Any ideas out there? - is there anything recently published?

[James S]

Blue Merlin
I remember clearly sailing out of Hong Kong in 1978, the captain (my dad) came up on deck with a sextant and a paper back called "10 easy steps to navigation"

I'll never forget what he said "OK...lets se if we can figure this out...

I said "you don’t know how to navigate"

He said "come on...how hard can it be"

With in 3 days we had it figured out with no experience at all, at least we thought we did.

It was a long time ago but I think we were doing noon shots only.

I highly recommend this book if it can still be found.

[RBEmerson]

Blue Merlin, if you read German, try Bobby Schenk's Astronavigation ohne Formeln -praxisnah or Wolf Nebe's Praxis der Astronavigation (look for them in the Palstek web site: Startseite - PALSTEK Verlag GmbH -Technisches Magazin von Seglern für Segler) The British authors Conrad Dixon (Basic Astronavigation) and Jeff Toghill (Celestial Navigation) may be easy to find and their books don't have long treatises on astronomy and maths (although some of that is inevitable). Not knowing where in Europe you are, it's hard to make many other specific recommendations.

[alan2]

[quote=blue merlin;241572]I'm delighted to see all this continued interest in astro - it is the logical progression of the sailor's general aim to be self-sufficient if at all possible. In trying to learn the subject in europe, it is difficult to find a text that takes the novice from the ealy stages through to the complexities of understanding the maths etc. Most books drop you right in the middle with complex terminology, learning by rote and not by understanding. Any ideas out there? - is there anything recently published?

You might try some/any of the following:

Celestial Navigation in the GPS Age, John Carl copywrite 2007
Primer of Celestial Navigation, John Favill, MD you might find this in a used book shop.

You might also check bookshops in general, depending where in Europe you are. Surprising what you can sometimes come up with just by asking.

[RBEmerson]

Minor adjustment, that's Celestial Navigation in the GPS Age by John Karl. It should be available through Amazon (Favill is listed there, too).

+1 on bookstore browsing! I've come up with some good additions to my collection that way. And oddly, some of the best finds have come from places over 1000 miles from any ocean.

[alan2]

blue merlin:

You might look for A Primer of Navigation by George W. Mixter.
Sorry about my mistyping of John Carl, as noted by RBEmerson, it's Karl. He mentioned amazon.com, which could be a source, also you might try ebay.com, looking under navigation of celestial navigation.

One more boopk comes to mind, which you might look at. Sky and Sextant by John P. Budlong.

Good luck.

[skookum]

Found a great book titled "One Day Celestial Navigation" by a gentleman named Otis S. Brown.


5th Edition printed in 1994

ISBN #1-56790-021-6

He starts out with a simple exercise where the student shoots a meridian passage to determine latitude. Then he explains how to get longitude with by using an almanac and the data from the previous exercise. He moves on to plotting a LOP through the previously determined latitude to get a fix, and then on to plotting and advancing multiple LOPs.

His method and instruction progresses slowly and keeps the terminology simple. He introduces the concepts logically and builds on them in consecutive chapters. Plenty of diagrams and worksheets provide ample material to help the student visualize the the geometry of navigating by the heavens.

Though 129 short pages in length, the book has you ready to shoot and determine your latitude by page 14. The reader has the essential skills necessary for computing complete fixes by page 66.

It's a good fast read. Contrary to the title, it took me three days before I accurately plotted my first two body fix. But I was trying with frustration to use the concise sight reduction tables included in the nautical almanac. After two days of plotting fixes more than 40 nm from my actual position, and very nearly pulling out the last of what little hair I have left, I went to my local used book dealer and put money down on volumes 2 and 3 of HO 249 that were printed in 1975. They are pretty dusty but my first fix using these tables put me within in 1800 yards of my actual position. I swear I almost fell out of my chair as I watched the line I was plotting nearly run over the center of the cross I had fixed on the plotting sheet over my real location. I re-calculated the fix three times with the same result.

That was over a week ago. I've been out every day since to take a morning and afternoon shot and have yet to plot a fix more than three nautical miles from my actual location (not counting errors due to my own occasional arithmetical incompetence). It's hard to describe the satisfaction I've gained by acquiring this new skill.

I highly recommend "One Day Celestial Navigation."

Cheers

[RBEmerson]

I just flipped through my copy of Brown and it's certainly a good starting point. The title is, in one sense, a bit misleading in that, as you note, it takes more than a day to feel at home with sight reduction. However, in many regards, I think the hardest part of celestial navigation is really the sights themselves. Once you have good data - that is, accurate angles or altitudes and precise timing, reducing the data is, literally, cookbook stuff. Follow the recipe faithfully and you have to come up with a usable result if the sight is good. With a bad sight, no amount of number twiddling is going to give a really trustworthy result, period.

There are different recipes (worksheets) and some seem to fit better than others. Making the choice of which one to use is simply a matter of experimentation and practice.

One possible route to consider is investing in Louis Soltero's StarPilot sight reduction package, either for a TI calculator or PC. I admit to some bias toward StarPilot, having had a small role in beta testing it some time back, but even without that experience, I'd still recommend StarPilot. With this package, the job is "take the sights, plug in the numbers, and plot the results". It gives immediate feedback on the quality of the sights by eliminating "are the sights bad, or can't I add 2 and 2?" factor.

With sights under control and sight reduction being handled by magic , there's time to work on doing good, clear plots, a skill that takes time to develop. Just doing "X marks the spot" isn't plotting, there's more to it and getting the right is just as important as working up sight reductions.

With sights and plotting nailed down, go back and do hand reductions, comparing them with StarPilot, and there's a ready "answer at the back of the book" to compare results with.

DISCLAIMER: As stated above, I did some beta testing of StarPilot but have no other involvement with StarPilot LLC or related companies.

[roverhi]

I found taking the sight really quite easy. Used a Tamiya sextant without the scope. Keep both eyes open. Bring the sun down to the horizon, rock the sextant to be sure you've got the sight right and 'Bob's your uncle. This is the system that Moitessier used. By not using a scope, you have a much wider field of view that makes capturing the object very easy. Probably not as accurate as using a sextant with a scope on a super tanker but way easier with the low height and quick motion of a sailboat.

It's been a long time since I've taken a sight so I need to brush up on my paper skills. Took a course at San Pedro JC using the teachers ring binder bound book. The book had a worksheet that made calculating the sight very simple. Just plug the numbers from the sextant, HO 249 tables and almanac in the right spot and do a little simple math and you ended up with a fix. Managed to navigate to Tahiti and back. We tried to use the HO 249 tables for star shots but didn't have much success. With HO 249, you have to preplot the specific stars that you will be shooting before hand. Makes locating the celestial body and getting the shot really rushed. Just couldn't get the hang of it. I'd think about using the ship navigation tables (HO229/HO211) if your going where the sun doesn't reliably shine.

Aloha
Peter Ogilvie
Pa'akai O'o, Pearson 35 #108
Too many Landrovers, some even run.

[RBEmerson]

I'm not sure what "without a scope" means here. Are you referring to a (typically) 6x or 7x monocular versus a (typically) 3x simple telescope or the 3x telescope versus a sight tube? Agreed that using a 6x monocular on a moving deck isn't a lot of fun.

While Sun sights are easier than star sights (the choice of possible targets is somewhat smaller ), there's still a bit of skill in getting the sextant set to make finding even the Sun simple. "Rocking" is one of those things that, once learned, seems easier than fall off a log, but for those who don't quite get it, not so easy.

Precomputing the expected azimuth and altitude makes locating an object easier, but I don't recall that it's, per se, required. All that's needed is the time and altitude and the name of the object (Sun, Moon, planet, or star) to go to the right places in the tables.

[alan2]

Seems to me, possibly incorrectly, that the plotting part is essentially an exercise in "mechanical drawing". Since once upon a time I was a draftsman, while my lettering these days looks like something a cat with questionable taste dragged in, I still remember how to use a couple of triangles.

As for plotting and sight reduction forms, there are several that one can use. I find the ones that USPS uses, they are downloadable from their web site, usps.org, handy enough. Of course my navigation consists of standing on the beach or a dock somewhere, which saves me from the possibility of getting sea sick. Then every now and then some bikini clad ogleable young thing comes by, and asks questions what I'm doing and what that thing I'm holding is.

My wife, who usually does record keeping duty, just sort of smirks/chuckles quietly, while I ogle.

[RBEmerson]

Actually, the plot tells a number of tales. For example, comparing a good fix to a dead reckoning estimated position can say something about possible currents, foul or fair. Of course, the plot also carries a history of the boat's movement, should the batteries fail and the boat be returned to the Age of Sail. And, frankly, a good set of plots that hold together are just plain enjoyable to look at and study when the trip is over.

[chala]

Stars are the good friends of the navigator. I always enjoy seeing them. 40 years ago I learn to use a sextant by reading Reed’s Nautical Almanac and Eric Hiscock book Cruising under sail. Here is a site where you should be able to find a very good free download software, Navigator Light 32, at Navigator Star Finder. The Complete On-Board Celestial Navigator by George G. Bennett is also an excellent book.
For Time, I get it from that nice lady in Hawai, her voice does not seems to have changed much in 40 years.

[skookum]

Now here's a question for someone knowledgeable. First some background.

A couple of nights ago I attended the local antique auction. I don't usually frequent auctions but in this particular auction there were two lots of particular interest.
Lot 453 was an Astra IIIb sextant in excellent condition with a full horizon mirror and all documentation including a certificate of examination dated in the mid 1990's. The estimated selling price was listed at between $150 and $300 Canadian.

Lot 705 was a Tama Sokki Co. sextant in fair condition in its original wood box. The certificate of examination was dated December 12, 1940 and matched the serial number on the instrument. The estimated selling price was $150 to $300.

I was certainly in no financial position to be able to afford either instrment if they were to reach the upper end of their estimated price range. And I expected both of them to do just that.

Nonetheless I attended the auction out of simple curiosity for the local maritime antique market, and boy am I glad I went.

The opening bid for the Astra was $100 and a fierce bidding war ensued. Three enthusiastic bidders topped each other $5 at a time for the shiny new sextant until the final price of $210 brought the hammer down.

Though a steal at the final asking price, I knew from the opening bid that I had no hope of walking home with that instrument. The most I could afford that evening was perhaps $75.

The Tama Sokki came up for bid near the end of the evening and by that time the crowd had thinned out some. Expecting a similar result as the previous auction I was only half listening to the auctioneer's description of the sextant as I leaned wearily against against the far wall, eager to be out of there and away to home where I could pine for a new sextant in the privacy of my own living room.

"We'll start the bidding at tewnty dollars."

Before I had the chance to form a clear concept of the auctioneer's words in my head my right hand, holding a white lot list, whipped out from my side and pointed straight to the ceiling.

"I have twenty dollars. Do I have twenty five?"

The auctioneer glanced to his left, acknowleged the other bidder, and then looked at me again.

"I have twenty five dollars. Thirty?"

I don't recall answering, but my hand stayed in the air until the bidding reached $80. The price was already beyond what I could afford, so grudgingly I lowered my hand and got on with congradulating myself for showing some composure and for not getting caught in a bidding war. But I guess the other bidder and I shared the same opinion of the situation for he shook is head at the auctioneer's request for a bid of $85.

The hammer dropped and I now owned a small piece of maritime history.



Now



Upon more detailed inspection, the instrument appears in fine shape save for areas of minor corrosion on some of the chrome machine screws and fittings which I've since brushed away with a nylon wire wheel. Even the original incandescent scale light still works! The only outstanding deficiencies on the instrument are a tarnished brass scale (which some Brasso should remedy) and a badly corroded horizon mirror (see images below) though the index mirror is nearly perfect. The silvering that remains intact on the horizon mirror still gives a usable image, but I would very much like to replace either the silvering or the entire mirror.

Which brings me to the question?



Where can I acquire a new split horizon mirror of standard size to replace one that has deteriorated beyond its practical utility?

I may be able to source out re-silvering locally but I would prefer to simply replace the mirror.

Any suggestions would be most welcome.