if GPS were not available

To add to this

electrosys said:
A LW radio will give you the Greenwich Time Signal, and the watch is used to store GMT, giving you the 'zero' Long reference. The next thing you need is Local Noon.
This can be found by plotting the sun's altitude against time on graph paper as it crosses the sky. Begin (say) an hour, or half an hour before 'top dead centre' so to speak. With a sextant this can be done quite accurately, but a guessing stick can be used - anything by which to estimate the altitude. Keep plotting as the sun 'hangs' for a while and then begins to descend. All you really need is the time when a given rising and decending altitude is equal (i.e. the same angle), but plotting on graph paper enables you to better estimate, should you lose the sun behind a cloud at a critical moment. Then simply bisect the graph to give Local Noon. Greenwich Noon plus/minus Local Noon gives your Longitude.

Latitude can be calculated by recording local sunrise and sunset times and calculating local day length, which is then compared with the day length of a known reference latitude - or even pulled straight from tables (if memory serves).

Obviously this ain't precision position fixing ...

That's about the best explanation I can muster 'off the cuff' - I've got a better (fuller) explanation with some worked examples somewhere ... in the archives ... will dig 'em out if there's any interest.
Click to expand...

I think this is the same approach, but what I have tried is to take about 5 sights an hour to an hour and a half before noon. Record exact time of sight, and altitude.

Then take altitude at noon, which gives you latitude but not a very accurate time because the sun hangs.

Then, set the sextant for the altitudes you recorded before noon, and record the exact times the sun comes down to those altitudes.

For each altitude you'll have a pair of times - one before noon and one after. Split the difference and that should give you the exact time of noon, which gives longitude.

I've found that by doing it this way I typically get several pairs of sights that correlate pretty well, and one that is an aberration. I usually throw that out and average the rest to give longitude.

It may not be the "sun-run-sun" approach taught in books, but for my purposes should be sufficiently accurate. If I'm mid ocean I would be quite happy to have my position to within 50 miles (and I've achieved 5 miles with this approach). I reckon as I approach shore I can use lists of lights to identify my position, or call up a freighter to confirm my position.
 
Morgana said:
Haven't the RDF beacons been switched off? (not sure on this one, as I haven't had an RDF on board for lots of years!)

I sailed well before GPS was even a twinkle in the eye, so have lots of practical experience of trad nav..... but am honest enough to admit that, while I could definitely still do it if needed, it would nowadays be a somewhat slower exercise, and the anxiety levels might be a bit raised upon landfall!

I do occasionally plot a DR when doing a Nth sea passage, but usually use GPS to actually nav... its been a while... :D
Click to expand...

Really interesting and almost certainly true for most people who navigated pre gps in anger. Bit like we've forgotten how to meet up with friends - can't find anyone without a mobile phone. But what about the post gps generation, think about how long they would take to get up to speed.

RDF would be my electronic aid of choice too. Even if the beacons work I have no info about them so they would be useless anyway.

On my last boat the outside plotter was on a bracket so I used to take it home. I often forgot it and would go boating anyway, often to the horror of my friends.

Latest boat has a huge one built in, so it is always there.

As an aside, whilst nav skills may have got rusty, I reckon pilotage skills are even worse. Even more so for mobos where the plotter is more often in front of he helm.
 
Surely all this is irrelevant, as far as I'm aware, from reading a number of sources, if GPS fails all boats will immediately sink as it's impossible to sail without it.:eek:

Just don't be in Studland when it happens you might squash a seahorse, or better still, a diver.:D
 
Needed to move my new(to me) boat from the Swale To Folkestone, my son was over from Plymouth and we decided to take her round,; he'd borrowed a GPS, we had a good compass and charts. Day before we had mooved her to the end of the creek, to get a good start early next am, with the advantage of the double tide. 6 am next day thick fog could not see 40 yards across the creek, I thought 'no go' and I normally would be more likely to take a risk. Son said it had to be that day, otherwise next year, anyway we had GPS. We motored out of the creek, set sails, found the GPS was u/s and decided to buoy hop all the way to N Foreland - buoys(between 1 and 4 miles apart), were coming into sight less than 50 yds ahead on 'the nose'. The tide was in the same direction as we were. After N Foreland visibility increased to 1000 yds. The only vessel we saw at sea was when passing Dover (which we had radioed) and a Seafrance ferry changed course astern of us. However we were not minus GPS on purpose.
 
Before GPS we were always getting called up by yachts midocean - "havent seen the sun for days - can you tell us where we are please?". It was almost expected as soon as we saw a yacht on the horizon. I doubt many would own up to it though.

Suggest a VHF might be a useful tool - as well of all the other yokes.

CC
 
Capt Cautious said:
Before GPS we were always getting called up by yachts midocean - "havent seen the sun for days - can you tell us where we are please?". It was almost expected as soon as we saw a yacht on the horizon. I doubt many would own up to it though.

Suggest a VHF might be a useful tool - as well of all the other yokes.

CC
Click to expand...

Moitessier communicated by firing a message onto the bridge of a ship in a film canister, using a sling shot.

VHF is, clearly, for wimps.

But for the rest of us it is still greatly appreciated when a ship, far from shore, asks us if we would like to know our position.
 
electrosys said:
A LW radio will give you the Greenwich Time Signal, and the watch is used to store GMT, giving you the 'zero' Long reference. The next thing you need is Local Noon.
This can be found by plotting the sun's altitude against time on graph paper as it crosses the sky. Begin (say) an hour, or half an hour before 'top dead centre' so to speak. With a sextant this can be done quite accurately, but a guessing stick can be used - anything by which to estimate the altitude. Keep plotting as the sun 'hangs' for a while and then begins to descend. All you really need is the time when a given rising and decending altitude is equal (i.e. the same angle), but plotting on graph paper enables you to better estimate, should you lose the sun behind a cloud at a critical moment. Then simply bisect the graph to give Local Noon. Greenwich Noon plus/minus Local Noon gives your Longitude.

Latitude can be calculated by recording local sunrise and sunset times and calculating local day length, which is then compared with the day length of a known reference latitude - or even pulled straight from tables (if memory serves).Obviously this ain't precision position fixing ...

That's about the best explanation I can muster 'off the cuff' - I've got a better (fuller) explanation with some worked examples somewhere ... in the archives ... will dig 'em out if there's any interest.
Click to expand...

You have brought up an idea I have mused over for a very long time.
I absolutely concur that with different latitudes the day shortens or lengthens. What I have never been able to stumble upon is the datum latitude nor the calculations for the difference. I have looked into The Admiralty Manual of Navigation Volumes 2 and 3. I have looked in The Nautiical Almanac. I have looked in Nories Tables and Burtons Tables. All of these but not a clue. I am really interested to hear that a table (of corrections) actually may exist and is hidden somewhere or given an arcane nomenclature.
Whereizzit please and what izzit called and whereizzit to be found ? I am eagerly interested, sincerely. Thanks.
 
VO5 said:
You have brought up an idea I have mused over for a very long time.
I absolutely concur that with different latitudes the day shortens or lengthens. What I have never been able to stumble upon is the datum latitude nor the calculations for the difference. I have looked into The Admiralty Manual of Navigation Volumes 2 and 3. I have looked in The Nautiical Almanac. I have looked in Nories Tables and Burtons Tables. All of these but not a clue. I am really interested to hear that a table (of corrections) actually may exist and is hidden somewhere or given an arcane nomenclature.
Whereizzit please and what izzit called and whereizzit to be found ? I am eagerly interested, sincerely. Thanks.
Click to expand...

I'd be surprised if such a table existed (although I expect to be proved wrong now...). The problem is determining when you are looking at sunset/sunrise. It occurs when the centre of the sun is on the rational horizon, as you probably already know, and due to the effects of refraction the sun actually appears to be approx a semidiameter above the visible horizon at this time. If the sun was simply dropping vertically then the instant could be more or less pinpointed, but in any latitude outside the tropics the sun is dropping at an increasing angle to the horizon - to the degree that the actual instant of sunset / sunrise is somewhat difficult to pinpoint. Chuck in variables of refraction due to you low height of eye, and high latitudes and you have a bit of an impossible task.
The "equal altitudes" method described above is something I train young officers to do. Its an excellent tool to get some confidence with a sextant, and we've had position circles of approx 2 miles diameter if done properly. That is plenty accurate enough for mid-ocean.
CC
 
electrosys said:
Latitude can be calculated by recording local sunrise and sunset times and calculating local day length, which is then compared with the day length of a known reference latitude - or even pulled straight from tables (if memory serves).

Obviously this ain't precision position fixing ...
Click to expand...

Yes, not too precise... :)

At the equinox and ignoring effects of refraction, the sun is above the horizon for 12 hours in all latitudes, so not much help there. At other times of the year, in any part of the world, the difference in sunrise/sunset times is just too imprecise to measure latitude in any practical way. Have a look at tables of True Amplitudes.
 
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