Sextant

[Pieman]

Hello,

I tried searching for this, but nothing with a clear answer:

1) What make/model of sextant is used by the CF Navy currently? (I assume there is access to one should GPS drop or various emergencies)

2) What make/model of sextant was used in WWII by the CF Navy?

Thanks.

 

[gillean]

This won't be too much use but perhaps it will stimulate some discussion. if memory serves me correctly (from the 1980s) sextants were designated by class. I think it was Class A, B or C.  The major ships had one high end sextant (Class A?) which was very modern with good optics, etc. The Navigator kept that one locked away and a good navigator wouldn't let anyone else touch it. 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. Don't know if or how that has changed in the modern era. "Sky Wrenches" don't get used much anymore and I understand that "Astro" has been dropped from the basic MARS IV syllabus.

 

[Furniture]

I can confirm that heavies carry both a class A and class B sextants... It's not the NavO that keeps them locked up, it's the crusty senior Met Tech that does. Shiny kit has a tendency to go missing, so we keep it all under lock and key because it's on our DA.

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

 

[Pieman]

Thanks for the replies. Best info I can come up with myself:

Americans used to issue a Mark II sextant in the WWII era onwards.  There is a version of a Mark III sextant, which *might* be still used today. Reading comments on some sailing forums, pleasure sailors will often try to get their hands on old Mark II sextants as they are cheap and accurate.

http://sextantbook.com/category/usn-buships-mark-ii-sextant/

British WWII era sextants dominantly came out of a company called  Hughes & Son Ltd., and there seem to be numerous versions that were used.

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.

http://www.ebay.ca/itm/181260393707?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1497.l2649

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

What is interesting to me is that no matter how good the sextant, the calculations for finding your location on the earth are theoretically limited to within 200 meters - you cannot get any more accurate no matter how good the sextant. This is because the equations are calculated through great circles, which assume that the earth is a perfect sphere, and it isn't. You could in theory correct for that, but man the math would become very ugly. I suspect no one bothered as 200 meters prior to GPS days was considered good enough.

 

[Privateer]

As a related follow up, can anyone recommend a good book  - and I include professional / military texts here - for teaching oneself celestial navigation? 

 

[Pieman]

Don't know what is a good book, but there are tons of good youtube videos showing how. Additionally, sailing books will cover how to use them.

There are also a few old sextants around that are for airplanes. The concept is the same, but different optics so pilots could use it. The days before GPS are becoming like the days before the internet. I honestly don't know how I functioned without the internet anymore.

 

[Oldgateboatdriver]

In my days (and considering how long these things last, probably today too) we had the Hughes and Sons.

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.

Anyone who wants to acquire a Sextant these days, again for their pleasure, my recommendation would be get a Japanese made one, plastic, but get the hand held bubble sextant: No need to wait for the horizon to show up, and no height of observer correction to make.

And 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.

 

[Fishbone Jones]

This ebay link shows 400 some odd sextants of various types and prices.

Some look pretty reasonable, including some ex service ones from various nations.

It could temp a guy to buy one just to learn how to use one.

http://www.ebay.ca/sch/Sextants-/37971/i.html

 

[Pieman]

  my recommendation would be get a Japanese made one, plastic, but get the hand held bubble sextant: No need to wait for the horizon to show up, and no height of observer correction to make.

I'm probably splitting hairs here, but I was reading that brass sextants are considered to be the 'best'. Plastic ones are feared to become inaccurate over time because of temperature changes. (I can't verify how true that really is) Aluminum sextants are good, but since the metal is soft they can also become inaccurate over time. The source of this information is a bunch of old guys on a sailing forum, but it seems reasonable enough.

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

  accuracy of celestial navigation is about 0.1 nautical miles (200 m). At sea, results within several nautical miles, well within visual range, are acceptable. A highly skilled and experienced navigator can determine position to an accuracy of about 0.25-nautical-mile (460 m)

Sounds like you are dead on about needing only a few miles accuracy. Pretty amazing that they are as accurate as they are.

 

[ComDvr13]

I'm probably splitting hairs here, but I was reading that brass sextants are considered to be the 'best'. Plastic ones are feared to become inaccurate over time because of temperature changes. (I can't verify how true that really is) Aluminum sextants are good, but since the metal is soft they can also become inaccurate over time. The source of this information is a bunch of old guys on a sailing forum, but it seems reasonable enough.

http://en.wikipedia.org/wiki/SextantSounds like you are dead on about needing only a few miles accuracy. Pretty amazing that they are as accurate as they are.

Incredible stuff. 

I would love to learn how to use a Sextant for celestial navigation when I go sailing.

GPS, while making things easy, takes the wonder out of night time navigation (im talking personal sailing, small boat). Although satellites are upon themselves pretty incredible scientific achievements

 

[SeaKingTacco]

I'm probably splitting hairs here, but I was reading that brass sextants are considered to be the 'best'. Plastic ones are feared to become inaccurate over time because of temperature changes. (I can't verify how true that really is) Aluminum sextants are good, but since the metal is soft they can also become inaccurate over time. The source of this information is a bunch of old guys on a sailing forum, but it seems reasonable enough.

http://en.wikipedia.org/wiki/SextantSounds like you are dead on about needing only a few miles accuracy. Pretty amazing that they are as accurate as they are.

I have done both airborne celestial and maritime celestial navigation. I have shot to within a mile of my DR position on several occasions. And less than 5 NM on almost all occasions (excepting the occasions when I have made some monumental math error... )

IMHO, maritime celestial nav is way easier- there are fewer correction factors; the pace is far more sedate; you can see the whole sky to pick an object, vice a 2 deg FOV (better hope  that your DR position was a good one in airborne cel nav) and you can do the computations after you shot, vice before hand.

As

 

[Pieman]

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

Pretty darn good.  -- 'DR' meaning Down Range?

This ebay link shows 400 some odd sextants of various types and prices. It could temp a guy to buy one just to learn how to use one.

I've got one coming in the mail...I got my hands on a sailboat, so there is a bit more motivation to learn now.

I did something similar to this before years ago in an astronomy course. Prof. gave us a year long project where we had to put a stick in the ground somewhere on campus. Twice a week we would have to measure the position of the shadow on the ground at noon. After a few months we were able to identify the declination of the sun through an equinox point (I think it was spring equinox), and even calculate our position on the earth after that. Can't recall how precise it was, but it was pretty good.

 

[CombatDoc]

DR = Dead Reckoning.

 

[Pusser]

And Dead Reckoning is to calculate your approximate current position by starting from a known point and then plotting every course and speed change in the interim - basic geometry.

It's important to note that a sextant is only one of the tools needed in celestial navigation (albeit an important one) and it really only does one thing - measure angles (usually the angle subtended by the gap between the celestial body and the horizon*).  You will also need an accurate time piece, a nautical almanac, a means to identify the celestial body you're using (e.g. a star globe, unless you restrict your sights to the sun and the moon - I assume you know those ones) and either the appropriate sight tables (e.g. H.O. 229) or a programmable calculator with a navigation module (chip).  If you don't have an appropriate calculator, you will also need to have an understanding of the math involved.

*A sextant can also be used with a Battenburg to plot your position using horizontal sextant angles, but that's pilotage or terrestrial navigation, which is different from celestial navigation.

 

[Colin Parkinson]

You can use a sextant for much more than celestial navigation. The CCG still uses them for horizontal angles to place buoys(although many use GPS now). You can determine your distance off of any object you know the height of by taking a vertical angle. You could determine how far away you are from a measured lighthouse and determine how far away from that lighthouse another vessel is that is between you and the lighthouse. 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!
I once used a Sextant, 3 arm protractor and Chart to resolve a squabble between 2 individuals that were arguing about waterlots and who was anchored in who's. I did my angles in front of them and then proceeded to do Horizontal angles by construction. They stood over my shoulder watching the "black magic". finally I offered them a solution based on my findings. They sorted things out and the problem went away. I could have done the same with a GPS, but it was the "magic show" that convinced them that we knew what we were talking about. That's the thing about being able to use a sextant nowadays. It gives you much creditability to those that don't know much about them.
Volunteering at the Maritime museum and looking at charts drawn by Capt. Vancouver, using overlapping triangulation I am in awe of their abilities and what they were able to accomplish, often in a open launch. 

 

[Monsoon]

You can use a sextant for much more than celestial navigation.

True. On ships these days, you generally only see the sextant used to calculate the distance to the other vessel using masthead (or radar) height angle when doing replenishments at sea at ranges too close for the radar to help you. Not to contradict Weatherdog, but I'm certain there must be a bridge sextant available to the watchkeepers on his ship for this purpose; the one he has locked up is likely the high accuracy navigation sextant.

 

[SeaKingTacco]

True. On ships these days, you generally only see the sextant used to calculate the distance to the other vessel using masthead (or radar) height angle when doing replenishments at sea at ranges too close for the radar to help you. Not to contradict Weatherdog, but I'm certain there must be a bridge sextant available to the watchkeepers on his ship for this purpose; the one he has locked up is likely the high accuracy navigation sextant.

From what I have just observed, laser range finders seem to be in vogue during RAS evolutions.

 

[Furniture]

True. On ships these days, you generally only see the sextant used to calculate the distance to the other vessel using masthead (or radar) height angle when doing replenishments at sea at ranges too close for the radar to help you. Not to contradict Weatherdog, but I'm certain there must be a bridge sextant available to the watchkeepers on his ship for this purpose; the one he has locked up is likely the high accuracy navigation sextant.

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

 

[cupper]

There are several Apps for iPhone / iPad (and presumed Android based platforms as well) for celestial navigation.

 

[Pieman]

And Dead Reckoning is to calculate your approximate current position by starting from a known point and then plotting every course and speed change in the interim - basic geometry.

Got it. Got to have something to compare results to otherwise your confidence level is going to be low.