Watch-chronometers

[Sandy]

I am cr^p with a sextant. I usually find myself on the far side of the moon if the truth be know; I know I really should learn one day! But bought a watch that takes a radio time check from Rugby every day and its very, very accurate.

 

[Sans Bateau]

You wouldn't think the latency was a result of the processing power of the unit's chip?

I've two DAB radios. The big one puts out the time pips about a second before the small one (and both are behind an analogue radio). I assumed the big one is 'fast' and the small one is 'slow' because of differences in the speed of the components.

Confucius say, "Man with watch very lucky, man with two watchs, he never sure what the right time is".

 

[JayBee]

Try this?

A. Take your 7 or 8 sun shots and times, plot them on a sheet of graph paper, discard the one ( or teo ) that are evidently 'well out', then pick one on the line.

B. Next, take the same group of sun shots, discard the same one or two as before, then sum and average the Hs's and times.

Plot the result of both A and B. Consider the difference in intercepts....

Discuss.

What you should do is first calculate the slope of the line and fit that to the plotted points. Much better results guaranteed.

 

[ffiill]

Problem is that my casio seapathfinder looses but I suppose if you calibrate as someone said.
The thing is where do you begin and end-I can shoot a sight and just about use concise reduction tables and the Nautical Almanac;but then do I use a simple astro nav programme and why not the clock on my GPS-just might as well use the GPS.
Actually I do enjoy the maths and get a great thrill if I get it right!

 

[maxi77]

Problem is that my casio seapathfinder looses but I suppose if you calibrate as someone said.
The thing is where do you begin and end-I can shoot a sight and just about use concise reduction tables and the Nautical Almanac;but then do I use a simple astro nav programme and why not the clock on my GPS-just might as well use the GPS.
Actually I do enjoy the maths and get a great thrill if I get it right!

Even the best chronometer is likely to gain or lose, the key thing about them is the rate will be constant, much like your Casio.

 

[prv]

I thought all you old fashioned navigators didnt like GPS because of the electronics / potential for failure / dud batteries etc. And whats in a digi watch but electronics and a battery.

Can't speak for others, but as far as I'm concerned the only eventuality for which astro is a rational backup is the GPS satellites being switched off. The robustness of modern electronics surpasses that of brass instruments and paper tables, so there's no realistic on-board scenario for which the sextant is a backup.

A waterproof handheld GPS and a pack of AA batteries in a waterproof metal box stowed in a secure locker seems like pretty effective backup to anything except, as mentioned, Uncle Sam turning it all off.

Doesn't mean astro is not worth learning - if we always want to do everything in the most efficient way possible, why are we using sails?

Pete

 

[JayBee]

A navigator relying solely on GPS is dependent on an external system over which he has absolutely no control.

Some words on the subject of GPS vulnerabilities from the RIN here

From prv

The robustness of modern electronics surpasses that of brass instruments and paper tables, so there's no realistic on-board scenario for which the sextant is a backup.

Where's the evidence for that ridiculous assertion?

 

[oldbilbo]

What you should do is first calculate the slope of the line and fit that to the plotted points. Much better results guaranteed.

Oh, really? Now, would that be a straight-line slope? Or part of the curve of a sinusoid? And which part?

Akshully, IMHO the point of doing astro at sea in a small boat is the pleasure and satisfaction one derives from doing it well. Er, that's it.

There. I've said it....


Oh, and for Bosun Higgs, can you not conceive of any practical astro procedure(s) where time, even electronic time, is not required? Let me then introduce you to Dr David Lewis and his 'Daughters of the Wind'.....

 

[JayBee]

Oh, really? Now, would that be a straight-line slope? Or part of the curve of a sinusoid? And which part?

Akshully, IMHO the point of doing astro at sea in a small boat is the pleasure and satisfaction one derives from doing it well. Er, that's it.

There. I've said it....

Agree with the pleasure and satisfaction bit.

A straight line slope will do over the short time it takes to average several sights, when the azimuth is not changing rapidly.

The slope is easy to calculate :

Altitude change in one minute of time = 15' cos Lat sin Az

- or use the table provided in AP3270/HO249.

 

[Kukri]

Bilbo, I bet you have a copy of Lecky's Wrinkles?

 

[prv]

The robustness of modern electronics surpasses that of brass instruments and paper tables

Where's the evidence for that ridiculous assertion?

May I take your sextant and drop it ten feet onto a concrete floor? And then put your almanac, tables, and plotting sheets into a large bucket of seawater and stir them around for a couple of days?

I don't actually own a handheld GPS, but if you take up the challenge I'll gladly go and buy one to which you may apply the same treatment.

Most accurate position obtained afterwards wins.

Pete

 

[JayBee]

May I take your sextant and drop it ten feet onto a concrete floor? And then put your almanac, tables, and plotting sheets into a large bucket of seawater and stir them around for a couple of days?

I don't actually own a handheld GPS, but if you take up the challenge I'll gladly go and buy one to which you may apply the same treatment.

Most accurate position obtained afterwards wins.

Pete

Nice try

My sextant is as good as the day it left Carl Plath's works in Hamburg, 50 years ago. How do you think your GPS receiver will be functioning in 2062?

Let's get real. There is no concrete floor in my boat with a ten foot drop.
I can keep my almanac, tables and plotting sheets (don't actually need the latter) in your hypothetical tin box.

Even without a sextant, almanac and tables, my interest in astro-nav has given me some tools to cope with emergency navigation, in a way probably not available to those spoon fed by a mysterious little black box.

 

[oldbilbo]

Bilbo, I bet you have a copy of Lecky's Wrinkles?

D'you mean the good 'Squire Thornton Stratford'....? "Mais certainment", as M. Poirot would exclaim. And I have a fair few wrinkles of my own.....



This 'bear of little brain' finds quite enough to keep him intrigued in his copy of the said 1881 'Wrinkles', his 'Dutton's 12th Ed', his 1962 'Bowditch, his copy of 'Sheet Anchor', and his two copies of 'Burtons Tables' - one of which containing a slip 'With Mr Burton's Compliments'.

Then there's Harland's 'Seamanship In The Age Of Sail', which frequently provides the bones of an answer to a practical problem in a current small vessel of today.

Sometimes I suspect, over the past 40+ years, I have outgrown the RYA's little comic-strip booklets, excellent though they are, but I am very far from being an exemplar of seamanship, or even a deep thinker on abstruse navigational issues, having quite enough trubl following the reasoning and judgements of acknowledged masters such as Richey and Chichester.

I need all the help I can get.....

 

[JayBee]

This 'bear of little brain' ......

I need all the help I can get.....

I am sure you are being unduly modest.

 

[Kukri]

My sextant is as good as the day it left Carl Plath's works in Hamburg, 50 years ago.

Mine is not; the switch for the electric light has packed up. Apparently this is not uncommon with C Plath sextants of this vintage.

 

[prv]

How do you think your GPS receiver will be functioning in 2062?

Does it matter? I'm not planning to stay at sea for 50 continuous years. Doesn't volume 1 of the tables only run for ten years at a time anyway?

Not that that has anything to do with physical robustness, which is the specific assertion you said was "ridiculous".

I'm not trying to put astro-nav down; I think it's a wonderful thing and one I would like to learn properly. I bought a book on it last year, and a few years before that I was walked through the process of reducing a sight in mid-Atlantic by the Second Officer on Stavros. I keep finding myself tempted to go out and buy a sextant even though I sail a small semi-weekender that rarely goes out of sight of land.

It just narks me every time I read that a sextant is needed "in case the GPS at the chart table breaks down, or a leak floods your batteries". In either of those cases, surely the rational backup plan is not a sextant, a shelf of books, and a carefully acquired skill, but rather a £50 Garmin and a packet of AAs. It's not romantic, but it is practical.

Failure of the GPS system as a whole (satellites and ground stations) is what a sextant is backup for.

Pete

 

[Kukri]

...
Then there's Harland's 'Seamanship In The Age Of Sail', which frequently provides the bones of an answer to a practical problem in a current small vessel of today....

I have never yet managed to find the passage in Lecky, cited approvingly by Tilman, "The navigator knows of no sensation more distressing than that of running ashore, unless it be accompanied by a doubt as to which continent the shore belongs to!"

But I have taken a midnight meridian (reverse the declination) and got a horizon in a fog by getting as low as possible.

May I commend, in addition to the admirable Harland, Todd and Whall's "Practical Seamanship for the Merchant Service"?

And, of course, Falconer:

"For he who strives, the tempest to disarm
Must never first embrail the lee yard arm!

 

[JayBee]

Mine is not; the switch for the electric light has packed up. Apparently this is not uncommon with C Plath sextants of this vintage.

 

[prv]

"For he who strives, the tempest to disarm
Must never first embrail the lee yard arm!

And indeed years ago I got shouted at by a bosun for doing exactly that!

Pete

 

[JayBee]

I'm not trying to put astro-nav down; I think it's a wonderful thing and one I would like to learn properly. I bought a book on it last year, and a few years before that I was walked through the process of reducing a sight in mid-Atlantic by the Second Officer on Stavros. I keep finding myself tempted to go out and buy a sextant even though I sail a small semi-weekender that rarely goes out of sight of land.

You should do it. My astro-nav interests originate from my time as a deck officer in the 1960s MN, when it was the only game in town, but a routine skill with a lot of job satisfaction attached to it. It has stayed with me into another life as a yottie.

It just narks me every time I read that a sextant is needed "in case the GPS at the chart table breaks down, or a leak floods your batteries". In either of those cases, surely the rational backup plan is not a sextant, a shelf of books, and a carefully acquired skill, but rather a £50 Garmin and a packet of AAs. It's not romantic, but it is practical...

Pete

Funnily enough, I actually agree with you, but the "carefully acquired skill" has its own rewards, which I would like to think you are going to discover.