Sextant.. Navigation tool or ornament?

[Nostrodamus]

I have no problem with people using astro nav because they enjoy the problem solving aspect. It's a bit like sudoku. But it's a bit rich claiming astro is a necessary safety precaution for any ocean crossing. Some backup. You need the tables, separate book for each 10 deg lat, almanac, or laptop, accurate time and sextant. Each of these (except the watch) is less robust than a hand held GPS. Drop the sextant and the mirror will likely break. Let the tables get wet and the pages you need will be stuck together.

I used astro in pre GPS days and it was useless in the tropics for 8 months of the year because of the heavy overcast. The idea that you could use an astro to help navigate between 2 coral reefs is ridiculous. You can't rely on a chart plotter for reef navigation unless you have been to the same reef previously and verified its position on the plotter.

By all means enjoy astro as a hobby but don't claim it is a necessary back up. I enjoy the engineering in steam trains but I would expect a present day train driver to pass a test to operate a steam train to get a train drivers licence.

At last, a voice of reason from the other side.
Time and technology moves on from the sextant, Decca and GPS.
In the scale of man sailing electronics is pretty new and has been accepted very quickly. With the advent of AIS as well most boats now have more electronic equipment than an old Russian spy trawler.
Because the new "Aids" are so new there are those (like me) who sailed when all we had was paper charts and a radio. There are those that still sail the same way.
There are equally those people who being new to sailing expect all boats to have all the modern aids and no doubt rely to heavily upon them.
There has to be a balance between the old and the new ways of navigation and it is important to know both.
Astro navigation is a skill or an art which has to be used often to be any good. Now it is used to prove that it still can be done rather than being used for any practicle purpose.
If the reading and the GPS were vastly different would you think that you had made a mistake in your reading and take another or think the GPS was malfunctioning? I think it would be the former.

 

[timbartlett]

At last, a voice of reason from the other side.

Hmmm.
I'm not sure that it's "at last": I've been following this thread from the outset, and I don't think anyone has suggested that astro is an absolutely essential skill.
Certainly, for a recreational yottie, it is entirely voluntary -- if you don't enjoy it, don't do it.
It's a bit like sailing itself: modern technology has given us plenty of "better" ways of achieving the objective, but we do it because we enjoy it.
And (like sailing onto a mooring) if the modern technology does ever fail, we will have the satisfaction of having been able to get ourselves out of difficulty rather than relying on others.

 

[maxi77]

I am struggling to understand the meaning or significance of Third Wireless Office. What happened in the other two? Or was it a class thing and the First Office had antimacassars and reading lights? Can't have been anything to do with the Third Programme, they wouldn't have needed DF to tell them where they were...

It is the old RN term for the Electronic Warfare Office. In the old days RN ships had a Bridge Wireless Office and Main Wireless Office, hence Third.

It was where we had the HFDF and the radar intercept equipment, things I understand are a trifle more complex these days

 

[smartcom]

The first Glonass/GPS combined phones are just coming on the market now, and nav devices will follow soon - Glonass will be fully operational by the end of this year, whereas it is still a long haul for Galileo.
But Glonass, GPS and Galileo all share the same weaknesses, so if it is jamming or solar storms or interference you will lose all at once.
It is interesting to note that the Gulf states have held on to LORAN, and many are now updating to eLORAN, to guard against GPS being switched off by the USA.

 

[bbg]

I have no problem with people using astro nav because they enjoy the problem solving aspect. It's a bit like sudoku. But it's a bit rich claiming astro is a necessary safety precaution for any ocean crossing.

Thank you for saying it.

With any luck I'll be doing my first solo ocean crossing later this year. Primary navigation will be by GPS. Backup will be the GPS built into the AIS unit. Backup to that will be a handheld GPS and a lot of AA batteries. Backup to that will be another handheld GPS if I have the funds to buy one (it is a lower priority than a new furler for the Code 0) and more AA batteries. The batteries don't last forever, but taking a fix once or twice a day mid-ocean is surely more than enough, and a bit more frequently until landfall is made.

Backup to ALL of that is a fixed VHF. Backup to that is a handheld VHF or two.

Finally I'll have a plastic Davis sextant and tables to use it, but I firmly expect it to remain in the bottom of a plastic canister. Even if I have total ship's battery failure. And lightning strike. And the US declares war on ___ (choose your destination).

I can understand those who see the challenge of using a sextant, or enjoy it as a mental exercise, or find some other reason that - for them - it is a skill they want to retain. I have to say I had a real sense of accomplishment when I fixed my position to within 5 miles last year.

But it ain't essential.

 

[JayBee]

.......But it's a bit rich claiming astro is a necessary safety precaution for any ocean crossing. Some backup. You need the tables, separate book for each 10 deg lat, almanac, or laptop......

Necessary or not, you don't need all this stuff. There are perfectly good sight reduction tables (or a basic scientific calculator) and long term almanacs which occupy a lot less space - about the same size as a couple of paperbacks.

I used astro in pre GPS days and it was useless in the tropics for 8 months of the year because of the heavy overcast.

Nonsense. How do you think ships on ocean passages within the tropics were navigated, when there was no alternative to astro?

The idea that you could use an astro to help navigate between 2 coral reefs is ridiculous. You can't rely on a chart plotter for reef navigation unless you have been to the same reef previously and verified its position on the plotter.

It rather depends on the distance between the reefs, doesn't it?

 

[timbartlett]

It rather depends on the distance between the reefs, doesn't it?

Depends on whether anyone knows what horizontal datum the chart was drawn on, too.
If you're using astro, you're like to be using the same "unknown" datum as the bloke who drew the chart in the first place -- who almost certainly wasn't using WGS84!

 

[JayBee]

Yes, the horizontal data were all over the place, particularly in the Pacific, pre-WGS84.

The question of navigating between two Pacific Atolls using a single LOP was a theoretical scenario set to illustrate one of the many uses of a single LOP and to test the ability of the examinee to work out what time to take his sight to get this LOP, as opposed to just mechanically doing sight reductions by rote.

As Tim and others will know, not only was the single astro LOP useful in all sorts of situations, getting a good one was an absolute matter of survival in Francis Chichester's epic flight across the Tasman Sea to Lord Howe Island, (as referred to earlier, by her Ladyship).

However, in case anyone assumes that all astro-nav enthusiasts belong to a sub-set of Luddites, I own up to being a heavy user of technology. I like GPS, use it all the time and have a chartplotter, one other fixed receiver and two handheld sets on my boat.

I just like traditional navigational methods and those other old skills associated with harnessing wind energy to drive a boat - a method of voyaging that might also be said to have had its day.

 

[Nostrodamus]

In these days of energy conservation, wind and solar engergy, and leaving a carbon footprint sailing is the way forward. It is the boats with big engines and no sails which are the problem.

 

[JayBee]

In these days of energy conservation, wind and solar engergy, and leaving a carbon footprint sailing is the way forward. It is the boats with big engines and no sails which are the problem.

Hah! Carbon footprints in an astro-nav thread.

Consider the carbon footprint of the GPS satellite constellation and all those electronics and plastics technologies.

Compared with, say, that of the sextant manufacturers and almanac compilers.

 

[Neil_Y]

I haven't read all the postings but I can't seem to find mention of the fact that a sextant has more uses than astral sights.

I've been there when GPS 1 failed (pushpit mounted aerial connection failed) GPS 2 back up failed (moisture in screen lost lines until unusable) GPS 3 failed again moisture behind screen and lost image.

We were approaching the North Spanish coast in November with low cloud, poor vis, and were unsure of our position (24hrs since last gps plot) We could see one lighthouse that we could identify and needed a quick accurate distance off. Only a sextant could have done this with enough accuracy to give us confidence to continue. Without it we would have kept a safe distance off until we had multiple fixes to know exactly where we were.

After this experience I see the Sextant as useful as a compass.

The big problem with electronics is they will fail in the worst possible conditions as they do not like moisture, so any weakness in the waterproof box will only show up when waves have been crashing over the deck for a day or two, or the hand held has been sat on after you put it in the pocket of your waterproofs.

In a simple answer a very useful navigation tool, not an ornament.

 

[scotty123]

OK, here we go...

You take a sun sight that gives you a sun position line. You know that you are somewhere on that position line. Draw a parallel line through the intended landfall, i.e. the transferred position line.

Choose an arbitrary point A on the sun position line. Sail South (green track) until your plot shows you have reached the transferred position line (point B). You now know you are somewhere on the TPL. Turn and sail along the bearing of the TPL and you will hit the landfall.

Note that I have omitted the calculation of tidal set & drift (and leeway) for clarity.

Thanks for that, now much clearer.

 

[scotty123]

Yes, the horizontal data were all over the place, particularly in the Pacific, pre-WGS84.

The question of navigating between two Pacific Atolls using a single LOP was a theoretical scenario set to illustrate one of the many uses of a single LOP and to test the ability of the examinee to work out what time to take his sight to get this LOP, as opposed to just mechanically doing sight reductions by rote.

As Tim and others will know, not only was the single astro LOP useful in all sorts of situations, getting a good one was an absolute matter of survival in Francis Chichester's epic flight across the Tasman Sea to Lord Howe Island, (as referred to earlier, by her Ladyship).

However, in case anyone assumes that all astro-nav enthusiasts belong to a sub-set of Luddites, I own up to being a heavy user of technology. I like GPS, use it all the time and have a chartplotter, one other fixed receiver and two handheld sets on my boat.

I just like traditional navigational methods and those other old skills associated with harnessing wind energy to drive a boat - a method of voyaging that might also be said to have had its day.

"As Tim and others will know, not only was the single astro LOP useful in all sorts of situations, getting a good one was an absolute matter of survival in Francis Chichester's epic flight across the Tasman Sea to Lord Howe Island, (as referred to earlier, by her Ladyship). "

Thought he did this by selecting a destination at the same latitude, but East or West of it.
When latitude reached, turned East or West & run down this line until destination reached. Didn't know this was known as a single LOP method though.

 

[snowleopard]

Thought he did this by selecting a destination at the same latitude, but East or West of it.
When latitude reached, turned East or West & run down this line until destination reached. Didn't know this was known as a single LOP method though.

Here is a chart of his crossing of the Tasman sea via Norfolk and Lord Howe islands. His solution was to aim well off to one side of the target and fly on until a single sun position line through his position also passed through the island then turned onto the bearing knowing it would be ahead. The chart shows the position lines he took.

 

[scotty123]

Here is a chart of his crossing of the Tasman sea via Norfolk and Lord Howe islands. His solution was to aim well off to one side of the target and fly on until a single sun position line through his position also passed through the island then turned onto the bearing knowing it would be ahead. The chart shows the position lines he took.

Notice he didn't bother with a LOP to find Australia!

 

[JayBee]

"As Tim and others will know, not only was the single astro LOP useful in all sorts of situations, getting a good one was an absolute matter of survival in Francis Chichester's epic flight across the Tasman Sea to Lord Howe Island, (as referred to earlier, by her Ladyship). "

Thought he did this by selecting a destination at the same latitude, but East or West of it.
When latitude reached, turned East or West & run down this line until destination reached. Didn't know this was known as a single LOP method though.

Thanks Snowleopard. I haven't seen that chart for a long time.

Scotty 123
What you are describing is a navigational technique known as "running the easting down", which again uses a single LOP, in this case a parallel of latitude. Before chronometers became widely available at sea it was very difficult to determine longitude with any accuracy, if at all, but a simple matter to find the latitude. So,the prudent navigator sailed towards the latitude of his destination, making sure, by dead reckoning or other means that his ship was sufficiently to the east or west of his destination to remove any doubt as to the direction (090 or 270) in which to finally alter course.

 

[scotty123]

Thanks Snowleopard. I haven't seen that chart for a long time.

Scotty 123
What you are describing is a navigational technique known as "running the easting down", which again uses a single LOP, in this case a parallel of latitude. Before chronometers became widely available at sea it was very difficult to determine longitude with any accuracy, if at all, but a simple matter to find the latitude. So,the prudent navigator sailed towards the latitude of his destination, making sure, by dead reckoning or other means that his ship was sufficiently to the east or west of his destination to remove any doubt as to the direction (090 or 270) in which to finally alter course.

So why "running the easting down", when being perhaps pedantic, presumably it worked when running west?

Found this on google -
"run the easting down, to, an expression which refers to the long easterly passage from the Cape of Good Hope to Australia between the latitudes of 40° and 50° S., in the area known as the Roaring Forties. It originally referred to the big square-riggers on the Australian run which used the prevailing westerly gales which blew in this area to help them on their way, but yachts engaged in round-the-world races (see yachting: round-the-world competitions) also run their easting down in this area of the Indian Ocean."

 

[JayBee]

Good question. The answer is lost in the mists of time, but probably originated in voyages from England to Lisbon in the wine trade.

Just seen your earlier post. I am pretty sure the expression and the technique originated long before the days of the big square riggers in the Roaring Forties.

 

[maxi77]

Before longitude could easily be determin ed the normal procedure was to sail to your destinations latitude then go east or west as appropriate till you got there. Because of the timekeeping problems the only practical sight for backstaffs, octants etc was Mer Alt which gives you latitude and off course the time of local noon.

 

[VO5]

Before longitude could easily be determin ed the normal procedure was to sail to your destinations latitude then go east or west as appropriate till you got there. Because of the timekeeping problems the only practical sight for backstaffs, octants etc was Mer Alt which gives you latitude and off course the time of local noon.

And Lat by Polaris in N.Lat.