Sextant

[Monsoon]

My experience has only been fairly recent, but I've only seen laser range finders used once inside the radar and Stuart distance meter.

If that's the case, it's a long overdue transition. The sextant was a two-person ass-pain.

 

[Pusser]

I understand how a sextant could be used for distance calculations, but I don't see why anyone would do it that way.  In my experience, we always used a Stuart's distance meter or event the dreaded Weymouth-Ross distance meter!

Yes, a sextant can be used for many things, but in itself, it only does one thing - measure angles.

 

[Sailorwest]

I understand how a sextant could be used for distance calculations, but I don't see why anyone would do it that way.  In my experience, we always used a Stuart's distance meter or event the dreaded Weymouth-Ross distance meter!

Yes, a sextant can be used for many things, but in itself, it only does one thing - measure angles.

The only time I used a sextant for distance was during an exchange with the RNR in 1991. We were doing mechanical mine sweeping and closed to about 20 x. Stuarts distance meter is impractical at that distance. It did take two people though but it was highly accurate as they kept tables of angles to distance for quick reference.

 

[Oldgateboatdriver]

Been there: An EDATS (Extreme Depth Armed Team Sweep) on the River class sweepers, right?

That has been one of the most stressful things I have done: three ships at 25 yards distance passing gear aft. I was in the middle ship -no escape route if anything happened. But we didn't use the sextant 'cause the other two ships had to keep station on US.
 

 

[Pieman]

There is a an excellent book called "Shoreline and Sextant" That teaches you to use a sextant in more ways than you can think of!

Thanks for the suggestion. Will try to track this book down.

 

[tabernac]

and I understand that "Astro" has been dropped from the basic MARS IV syllabus.

While this is true, any good MARS III/IV CTO will break out the sextant for the students to use. Our lead CTO on MARS IV was a navigator, he would have us use the sextant for VSA as a means of judging our 5-3-1 prior to course alterations, as well as judging range to large structures when their height was already known. Admittedly we only ever used it transiting under the Lionsgate bridge, or in the Fraser river. 

 

[Svanen]

The other sextants, which were kept in the chart table for general use by watch keepers, were a lower class. It seems to me that they were all British manufacture but by various manufacturers and with some minor differences in design and function. They all looked to date back to at least WW II and perhaps earlier.

That accords with my memory. They were typically good quality Hughes & Son (aka 'Husun') sextants, but elderly - not necessarily a bad thing - and 'well-used' (more of an issue). In other words, the navigational equivalent of the old Browning pistols.

What seems to be the most dominant sextant today came from a Japanese company, called the Tamaya sextant with various versions in WWII era. It became popular and became known as the 'merchants' sextant since it was used on cargo ships. Over the years the company switched names/owners, and the modern version of the 'merchant sextant' is the Astra IIIB.

Not quite.

Tamaya sextants are still made (in Japan), and both their standard "Jupiter" and premium "Spica" models have good reputations for fit and finish, including the optics. They are not cheap: expect to pay ~US$2,000 for a new one. Many are still found on merchant ships, but due to cost factors they have been largely superseded by the Freiberger trommelsextant, made in Germany.

The Astra IIIB is made in China by the Changzhou Celestaire Instrument Co.: which has no affiliation with Tamaya, and indeed is one of its competitors. The Astra is a favourite of yachtsmen and backyard celnav enthusiasts because it is of reasonable quality and relatively inexpensive (~US700). Although not a top-grade instrument, it is widely available, perfectly useable and very good value. The same comments apply to Russian SNO-T / CHO-T sextants.

There are some fancy ones out there costing $3k and up. Costly optics and glow in the dark readings for siting faint stars.

The sextant manufacturer generally - although not invariably - acknowledged as the best currently available is Cassens & Plath, of Germany. They don't cost >$3,000 though: their top-of-the-line model, the Horizon Ultra with polarized shades (see http://www.wired.com/gadgets/gadgetreviews/magazine/15-07/st_tool), costs no more than US$2,000.

Of interest, the Royal Navy now issues Cassens & Plath sextants, identical to its "Professional" model with two exceptions: (i) they fitted with Chinese-made Luger MD 6x30 lightweight monoculars rather than C&P's standard Japanese-made 6x30 prismatic monocular - a questionable choice, in my opinion; and (ii) although made in Germany by C&P, they are rebranded on the arc as being made by the British distributor Lilley and Gillie.

 

[Svanen]

I have no idea who makes them though, and unless the GPS is down or the NavO approves nobody will be taking mine out to check either

When you have a moment, please check the Class A yourself and post the manufacturer here … would be interesting to know.

There's no need to take the sextant out of its case: just open the lid and the manufacturer's name will likely be prominently displayed on the sextant's arc. 

I have shot to within a mile of my DR position on several occasions.

Unless your DR position is geographically correct, its proximity to celestial LOPs is largely irrelevant, no?

No offence intended, and I don't mean to imply any lack of skill on your part. I'm just saying that a DR position may or (more likely) may not be entirely accurate, and so it isn't really useful as a reference for demonstrating the accuracy of celnav.

 

[Svanen]

BTW, in the middle of the ocean (where we do astronav.), 200m. (probably more like yards) is about ten times more precise than you need to be, even if using electronic navigation systems. As long as you make landfall on the other side within 5 miles of where you intended to be, you'll be ok. Besides, think about the largest map you have ever seen showing the whole Atlantic ocean: take a pencil and draw a fine line of position on it and to scale, it is probably ten miles across in width.

Exactly.

A good book for learning astro, at least for pleasure or merchant ships: "A Yachtman's Guide to Astronavigation, can't remember the US publisher's name, and it is probably up to the 7th edition by now.

There are a lot of books available, and everyone has his favourite … but mine is certainly not Mary Bewitt's Celestial Navigation for Yachtsmen (presumably the book you are referencing). Although it's been around since 1950 and is widely regarded as a classic, the presentation is rather dry and the short length (72 pages in my copy, the 12th edition) means that many topics are necessarily rather abbreviated. On the plus side, it is certainly compact.

I second the recommendation for John Budlong's books, Shoreline and Sextant (1977) and Sky and Sextant (1978) are both excellent. Well-written, well-illustrated and quite comprehensive, and by a Canadian author too!

 

[SeaKingTacco]

When you have a moment, please check the Class A yourself and post the manufacturer here … would be interesting to know.

There's no need to take the sextant out of its case: just open the lid and the manufacturer's name will likely be prominently displayed on the sextant's arc. 
Unless your DR position is geographically correct, its proximity to celestial LOPs is largely irrelevant, no?

No offence intended, and I don't mean to imply any lack of skill on your part. I'm just saying that a DR position may or (more likely) may not be entirely accurate, and so it isn't really useful as a reference for demonstrating the accuracy of celnav.

A fair point.  I was sloppy with terminology. I have, on numerous occasions, shot to within a mile of my actual position (as derived by a GPS fed INS ), on an aircraft doing 200 kts.  In short- celestial navigation works.

 

[Svanen]

I have, on numerous occasions, shot to within a mile of my actual position (as derived by a GPS fed INS ), on an aircraft doing 200 kts.

Impressive … BZ!

 

[Colin Parkinson]

A fair point.  I was sloppy with terminology. I have, on numerous occasions, shot to within a mile of my actual position (as derived by a GPS fed INS ), on an aircraft doing 200 kts.  In short- celestial navigation works.

I read a good story of a Coastal Command navigator in WWII that had been transferred to Bomber Command, his old CO told him "Take a sextant with you" He said the Bombers crews laughed at him practicing star sights on the sorties because they all flew on OBE for direction finding, when the bomber got shot up and all the electronics were gone and the DR position was totally screwed up, he was able to bring them home, said everyone encouraged his practice after that. 

 

[SeaKingTacco]

Do you mean Obo?

 

[Colin Parkinson]

I believe so, dragging this all from the memory banks

 

[Svanen]

"Oboe".