Ships' Clock: Astro

alant said:
"Subtract 1203 from GMT to get 1 hour 44 minutes. Convert this time difference to arc and get 26 degrees (west)."

I'm always happy to learn, but I do not think you can 'assume' longitude this easily, since 'local time' is not exact.

As you progress, say E-W, every 15 degrees will put you in a different local hour, adjusting 'ships time accordingly', so you have at least this error to account for.

As previously mentioned, if it was that easy, ships would have been plotting exact fixes since John Harrison's Chronometer was invented.

I'm no expert, just teach the RYA course, so enlighten me please.
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I'm shocked that you teach the RYA course.

You are confusing local time with zone time, amongst other things.
 
When I started ocean sailing in the 1970s there was no choice, you had to use astro. I bought a quartz kitchen clock and mounted it where it could be seen from the cockpit. I checked it against radio time signals about once as week and applied a correction.

I got good enough results for all my landfalls.

All my fixes were done using sun-run-sun: morning, noon & afternoon.

I have also successfully used sun position lines crossed with other types for a fix in sight of land.
 
JayBee said:
I'm shocked that you teach the RYA course.

You are confusing local time with zone time, amongst other things.
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I don't think so, but as I said, always willing to learn, particularly from a Master Mariner!

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

"A time zone is a region of the earth that has uniform standard time, usually referred to as the local time. By convention, time zones compute their local time as an offset from UTC (see also Greenwich Mean Time). Local time is UTC plus the current time zone offset for the considered location."

A 'noon' sight, will be at 'local time' - the same 'local time' as "Mer Pass". If the above Wikipedia is correct, an observer could be anywhere in a particular time zone at 'noon local time'.
Hence my argument that it is inaccurate for Longitude.

A GMT timepiece, will only show time at Greenwich, which may be unrelated to how much 'westing' or 'easting' has occured.

'Ships local timepiece' remaining uncorrected until it changes at the next time zone. On yachts, conveniently done at about midday, when watches are least affected, by a +/- 1 hour.
 
For Alant

Alant - forget about zone time for the moment. Although precisely defined, it has no navigational significance and is just an approximation of local time. It only serves to locate lunchtime somewhere near the middle of the day.

When the sun is at maximum altitude, as it crosses the observer's meridian, the Local Apparent Time is 1200. This is the definition of Local Apparent Noon.

Applying the Equation of Time, from the Nautical Almanac, to Local Apparent Time yields Local Mean Time. The difference between Local Mean Time and Greenwich Mean Time completely determines the observer's longitude.

Don't take my word for it. Consult the relevant Admiralty Manual of Navigation, or any other tome written for professional navigators.
:)
 
I'm very glad I had a sextant on a trip back in 2001 thiugh, I had three gps units fail, or becone completely unreliable, so it was back to the old fashioned methods and not really a problem. I would say that accuracy comes with use of the sextant and not your nethod.

It was worse on the way back after days of dense cloud cover our DR from last fix was way and we were getting close to the Azores waiting for a sunny day to find out exactly where we were.
 
alant said:
"A time zone is a region of the earth that has uniform standard time, usually referred to as the local time.
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A good example of why you shouldn't take Wikipedia as gospel. The statement is fine as far as landsmen are concerned but is of no help in navigation terms.
 
The wealth of practical advice given here so freely by you guys is very much appreciated. More than any single book can provide and second only to rubbing shoulders against an old sea dog on deck.

I have Sir Tom's text on Ocean sailing, as well as several other of his masterpieces, but (forgive the heresy) I don't think it's his finest work. I did the RYA course one-to-one with a retired Master Mariner but that was 4 years ago and my life subsequently changed direction from the master plan of that year :D

My next thought is about plotting fixes mid ocean. Duly noting the earlier comment that using the planning chart has some merit. Point well made and taken.

However, what do the panel of experts use? A planning chart (Imray 100 for example), Plotting Sheets, self-generated using DR latitude for scale or something else?

There are so many of you to thank but Snowleopard hit the nail on the head with the succinct sun-run-sun advice.

Please keep contributing with the great tips on becoming a competent small boat celestial navigator :cool:

David
 
Plotting Sheets

You can buy plotting sheets but it is very easy to make your own on the back of an Admiralty chart. This gives you a lot more space to work with and at a suitable scale will cope with a day’s work on a yacht. It can be used and re-used until it falls apart.

In ink, draw a horizontal line across the paper to represent a parallel of latitude. Near the middle of the sheet draw two meridians at 90 degrees to, and across the horizontal line, with 60 subdivisions between them. This takes care of the longitude scale. The meridians and the parallel can be labelled, in pencil, so that your estimated position lies somewhere near them.

Down in one corner of your new plotting sheet, draw some lines, again in ink, spaced apart on the same scale as your chosen longitude scale. To get your latitude scale, simply pencil in another line which cuts this grid at an angle to the horizontal, equal to your estimated latitude. Using your dividers, minutes of latitude can be measured along this pencilled line.

It is a helluva lot easier to make than to describe!
 
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As an aside, I note that most posts refer to the sun, meridian passage et al. How about twilight three point fix from the stars. Is this "common practice" on a recreational yacht? Perhaps start a new thread on the practicalities, common use and techniques for astro...any takers?
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It is little known now that the finest astro navigators were the trans-oceanic 'aviation bods' who pioneered the sharpest techniques, taking the best of maritime practice then improving on that. Like Sir Francis Chichester, for example, who was one of the greatest aviation nav pioneers, doing sun astro in an open cockpit, stick between knees, at 50 feet over the Tasman Sea, wrestling with a paper chart, a cylindrical slide rule, a ruler and pencil.....

Maritime practice, on a steadyish and civilised ship, was to have the Navigator's Yeoman and several Deck Officers prepare and take a 'round of star sights at twilight'. They'd then retire to the Chart Room for cocoa and biscuits. Three, four, five or more would be precomputed, 'shot', resolved or 'reduced', discussed, and then one or more Lines of Position ( LOPS ) would be plotted as a fix.

On aircraft using astro as part of the navigation routine - as in RAF Air Support Command or Strike Command ( VC10s, Vulcans, Victors.... ) - there was a requirement to produce and use 3, sometimes 4, night astro fixes each and every hour. Think about that.

It became common to use the AP 3270 'Selected Stars' to pick just two bright, fat stars - whose azimuths were close to 'parallel to and normal to' the track ( so that the resultant position lines would give distance ALONG track and distance OFF track ). Sequentially, those would give good indicators of 'speed along track', hence ETA, and 'tracking error' or leeway/drift angle, so that correction back to track could be made.

Consider, then, the merits of a Two-Star Sandwich Fix, the azimuths being at about 90 degrees to each other - and not one shot on each, but alternating several ( i.e. A,B,A,B,A,B,A ), averaged and resolved around a central time. Far less precomputation, the data taken from AP3270 by inspection, and the fix on the chart and used far faster than a 3-, a 5-, or a 7- star 'round of sights'.

With the Sun available during the day, one can take a morning Longitude shot when it's bearing is 090, a mid-morning shot at about 135, a noon latitude, an afternoon shot on around 225 degrees, and a late afternoon shot for Longitude. Additionally, also when the Sun's azimuth is 'normal' to the vessel's track, and also parallel to, should the above not quite suit.

There is plenty there, if the Heavenly Bodies and a clear horizon are visible, to keep one in practice. Far more than the weebly RYA's 'nip out to the Western Approaches, take ONE sun shot, and bring it back to the yacht club for reduction in the warm and dry'. Shameful.....

Finally, do not forget what is perhaps the most important use of 'astro' - that of checking the compass error daily by means of 'sun's amplitude'. That's quick and easy, once you're set up for it. On most Brit yachts, the only compass check that gets done these days is "It's still there....!"
 
Excellent stuff!
Also note Chichester's technique for finding Lord Howe Island by deliberately aiming off until he could get a position line which would intersect his destination and allow him to fly down it. He only had one chance of getting it right IIRC.
 
JayBee said:
Excellent stuff!
Also note Chichester's technique for finding Lord Howe Island by deliberately aiming off until he could get a position line which would intersect his destination and allow him to fly down it. He only had one chance of getting it right IIRC.
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'Aiming off to one side' is equally valid for most landfalls so, when your DR ETA is up, you know which way to turn when you can't see anything you recognise.

I usually use that whenever I'm trying to find France - or other chunks of Europe - from here. It usually works, and I haven't needed to come back instead, for quite a few years now.

:D
 
JayBee said:
Alant - forget about zone time for the moment. Although precisely defined, it has no navigational significance and is just an approximation of local time. It only serves to locate lunchtime somewhere near the middle of the day.

When the sun is at maximum altitude, as it crosses the observer's meridian, the Local Apparent Time is 1200. This is the definition of Local Apparent Noon.

Applying the Equation of Time, from the Nautical Almanac, to Local Apparent Time yields Local Mean Time. The difference between Local Mean Time and Greenwich Mean Time completely determines the observer's longitude.

Don't take my word for it. Consult the relevant Admiralty Manual of Navigation, or any other tome written for professional navigators.
:)
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Back to my earlier remark, are you saying that you can determine longitude accurately from a noon sight?
 
snowleopard said:
When I started ocean sailing in the 1970s there was no choice, you had to use astro. I bought a quartz kitchen clock and mounted it where it could be seen from the cockpit. I checked it against radio time signals about once as week and applied a correction.

I got good enough results for all my landfalls.

All my fixes were done using sun-run-sun: morning, noon & afternoon.

I have also successfully used sun position lines crossed with other types for a fix in sight of land.
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So, you determined longitude How?
 
Re: Lady Campanula

Yes.

BTW, in the ships I sailed in there was no chance of a group of deck officers "taking a round of star sights at twilight", no Navigator's Yeoman and certainly no cocoa and biscuits in the chartroom while this was going on. :D

I think you must be referring to the Grey Funnel Line, or perhaps the RFA.

Your summary of astro nav practice in the air was very interesting. I suspect that with astrodomes and bubble sextants or periscope sextants fitted with mechanical averaging devices, accuracy was sufficient for purpose, but not that great.
 
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alant said:
Back to my earlier remark, are you saying that you can determine longitude accurately from a noon sight?
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Yes. It helps to be stationary and necessary to know GMT at meridian passage.

There are some observational difficulties in a moving craft, relating to the actual time of transit, which is why the method is not commonly used at sea. The transits of the sun and other celestial bodies were, until very recently, used to establish the longitude of points of land during hydrographic surveys.

Have you sorted out your confusion of zone time with local time yet?
:D
 
JayBee said:
Yes.

BTW, in the ships I sailed in there was no chance of a group of deck officers "taking a round of star sights at twilight", no Navigator's Yeoman and certainly no cocoa and biscuits in the chartroom while this was going on. :D

I think you must be referring to the Grey Funnel Line, or perhaps the RFA.

Your summary of astro nav practice in the air was very interesting. I suspect that with astrodomes and bubble sextants or periscope sextants fitted with mechanical averaging devices, accuracy was sufficient for purpose, but not that great.
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Aw, bless..... :D

At the risk of getting just a little geeky about a 'trade skill' which had a brief flowering, then passed, let me just outline some of what the RAF - er, Number One Bomber Group - was capable of during the 70s. One must understand that the Vulcans still had a central role in our deterrent posture ( Avro Vulcan wiki ) and the operational requirement involved 'penetration' while radiating nothing - so Internal Aids Navigation was the norm, and what all the crews trained for week-in, week-out. It was expected that all ground-based navigation beacons - NDBs, Consol, Decca, LORAN, TACAN, VOR, area radar - would be unavailable, so astro took a prominent role in updating the rudimentary airborne DR computers of the time.

Every 'combat-ready' crew had to fly and submit an assessed exercise at frequent intervals, to demonstrate their ability to navigate to the target(s) and deliver a weapon, using only Internal Aids, to a satisfactory accuracy. This was typically a 2000nm night flight around the Atlantic, using an astro fixing technique called 'Fix-Monitored Azimuth' ( FMA ) and ending with a simulated weapon delivery over a ground radar unit called a Radar Bomb Plot, which measured the aircraft's position in space when simulated weapon release was indicated, and from that calculating with precision the aircraft's cumulative Terminal Error - or how far out the navigation was at the end.

The permitted 'corridor in the sky' was 5 miles wide, and the 'time-slot' was within 30 seconds of pre-briefing. Just about all squadron navigators could, and did, routinely track down a corridor 2 miles wide, and within 15 seconds of planned time. Competition was fierce, there were annual 'NATO bombing competitions', and the very best crews - constantly tweaking every aspect - were able to demonstrate very considerably sharper performance than that above. The best Command Select crews regularly beat the USAF SAC in their B-52s, with 'money-no-object' star trackers and procedure simulators.

The 'FMA' technique may be of interest to some. At the heart of the Vulcan's nav-kit fit was a superb compass system and an analogue DR computer, which took inputs of heading and airspeed, and several secondary systems, to compute where the aircraft was in space. This was monitored and fed 'error corrections' from a variety of sources, such as the Attack Radar - this only during routine transit flights, and during the final few seconds of a 'real' bombing run - and from the results of continuous astro.

One of the two navigators would take 1- or 2-minute averaged shots on a star right ahead, or right behind. The Height Observed, as averaged by the 'Pendulous Reference Sextant', would be passed to the Nav-Plotter, who would apply a series of tweaks and corrections, then plot the Position Line. That would give an Across Track 'distance-run' since the last one, hence Ground Speed, hence a revised ETA, hence a speed adjustment if required.

The first navigator would then set up for and shoot a star out at 90 degrees to the beam ( AP 3270 'Selected Stars' ), producing a Height Observed and in the same way a Position Line, which indicated how far to one side of track the aircraft had drifted. This permitted an input to the compass system to make the aircraft track correctly right down the planned track line. It also permitted the experienced nav to see whether the winds at height were close to forecast or whether something else was going on way up there above the tropopause, and make adjustments to accommodate those changes. That alternating 'Along-Across' astro routine would continue as briefed, and/or necessary.

The co-pilot would do the precision timing, and the aircraft captain would fly the thing by hand, to damp out the Vulcan's slow cyclic oscillations.

This 'Along-Across' FMA technique was continuous. It amounted to an astro fix every 15 minutes; sometimes more frequently. With a lot of experience in choosing just the right amount of 'tweaks' or corrections to the 'Ho', a good crew could regularly put the 'Iron Triangle' within 300 metres or so of where they wanted it, and within 10 seconds of when wanted.

That might surprise some, but it's a matter of record, within the 'Giant Voice' and other NATO Bombing Competitions.

It's worth bearing in mind that the Yanks AND the Russians knew about and practised similar procedures - but our crews were far and away the best.

All our yesterdays..... :D
 
I'm also enjoying this thread, as a navigator trained in the pre GPS days.

I appreciate, and use, all the benefits of GPS but am still proud of the navigational theory I learned and the practical use to which it was put all those years ago.

Indeed, I served as second officer (the navigating officer) on a 17,000 tonne vessel running between UK-Australasia with only a magnetic compass, echo sounder, RDF. No radar or gyro. It was no problem at all.

I admire all those who used these skills in their differing ways (RN, RAF etc.) It is sad that such skills are no longer required but the technology replacing it is so much more accurate that it is inevitable that the old ways will be replaced.

The thread has remained interesting throughout, without descending into the ridiculous levels of minutiae that many others do.

My thanks to all the contributors.

Tom
 
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