Theoretical Navigation ?

NigelFortune

NigelFortune

Well-Known Member
Joined
12 Feb 2008
Messages
72
Location
North Wales
Visit site
Imagine waking up on a palm tree lined beach gentle breeze on a small island with no previous knowledge of how you got there.
How do you find out where you are ie North or South Hemisphere, what Lat and Long approx. What continent you are on or near, what the time is. What methods would you use to acertain where you are. Come on all you No.6's do your stuff or the big bubble will come and get you.

Please show all working out !!

(was hell of an Xmas do !)

Nigel
 
OK - here goes for a start:

Sounds like you are in the tropics, or maybe on the W Coast of Scotland. The temperature might give you a clue. :)

If the sun moves across the sky from left to right you are in the N hemisphere, or if from right to left, in the S hemisphere.

If you can identify Polaris in the night sky, the vertical angle between the horizon and the star is roughly your N latitude. Not so easy in S latitudes because no convenient star. I assume you haven't woken up to find a nautical almanac at your side.

When the sun is at its highest point in the sky, your local apparent time is 12 noon.

Longitude and nearest continent a little more tricky....

Next!
 
Last edited:
If you know the date it should be pretty simple to approximate latitude from a meridian altitude.

Longitude is pretty much impossible without knowing the time.
 
bedouin said:
If you know the date it should be pretty simple to approximate latitude from a meridian altitude.
Click to expand...

How do you get from the date to the expected altitude, though? Is there some simple rule, or does it require tables, which you don't have?

I'd go with JayBee's observation of the pole star - I believe that gets you to within a degree by simply measuring the angle, and the measurement you make with improvised equipment is unlikely to be that good anyway.

So how do you make an angle-measuring device from scratch?

And what's the point of it all anyway if you don't have a chart to plot your hard-won position on? :)

Pete
 
nigelmercier said:
Doesn't moss grow on the North side of trees? :D

Next step is to walk to the hotel at the end of the beach, buy a beer and ask the bartender what the time is. Go into the lobby and pick up a brochure.
Click to expand...

No in southern hemisphere of course moss grows on south side of trees. In the tropics however it doesn't grow on any particular side. Which might indicate that you are between tropic of Cancer and Capricorn. (or less than about 27degrees of latitude.
If knew roughly what the date was you could infer from the path of the sun if you were in N or S hemisphere. You will find west from the setting sun so what sort of angle was the sun at local noon. How did that relate to the date. (month at least). Bearing in mind at the equator the sun would be immediately overhead at March 23 and Sept 23. Fully south at Dec 23 and fully north at June 23rd.
if the sun is overhead March 23 then you are at about 27 degrees south. Any further south then the sun would never be truly overhead. You would need to mount a tall stick vertically in the sun to make the observations.
Proximity of a continent is not easy but may be guessed by flight habits of birds. The proximity of reef or more islands particularly to the east might be indicated by the nature of waves arriving. ie big ocean swell, no reef or island. Muted wind waves indicating island or reef. You mentioned palm trees presumably coconut palms indicates in the NE or SE trades or in between.
Just knowing the approximate date and the weather might also indicate latitude. ie warm and dry in June means south latitudes. Humid warm in Dec means south latitudes opposite for N hemisphere. (note here my natural bias to S hemisphere as opposed to most forum folk. Around the equator not so easy.
If the island is so good why are we worried about where we are and why go home. You would not be the first to go native. olewill
 
prv said:
How do you get from the date to the expected altitude, though? Is there some simple rule, or does it require tables, which you don't have?
Click to expand...
I would approximate the movement of the sun to be a sine wave between the two tropics, and depending on the accuracy you want use the "rule of twelfths" or a power series expansion.

Of course it doesn't work too well near the equinox so I might need to hang around a bit.

Polaris works fine in higher northern lattitudes but not much use near, or south of the equator.

For angle measurement I would use shadows for the sun sight. (which is another reason the sun is easier than Polaris) and a lot of the "calculations" can be done by construction rather than calculation
 
Last edited:
bedouin said:
If you know the date it should be pretty simple to approximate latitude from a meridian altitude.

Longitude is pretty much impossible without knowing the time.
Click to expand...

You can get time from a lunar distance measurement to within about 15-30 seconds if you know the approximate date (to within a week or so). 15 second precision needs the measurement of the lunar distance to within 0'.1 and a very accurate almanac. Both of these requirements are tricky, but not impossible, to satisfy.

With approximate time you can get latitude from two (simultaneous) altitudes (this gives two solutions for latitude, but a later repeat of the process will resolve it to one)

With latitude you can get longitude from one time sight (same comment re: two solutions).
 
John Barry said:
You can get time from a lunar distance measurement to within about 15-30 seconds if you know the approximate date (to within a week or so). 15 second precision needs the measurement of the lunar distance to within 0'.1 and a very accurate almanac. Both of these requirements are tricky, but not impossible, to satisfy.

With approximate time you can get latitude from two (simultaneous) altitudes (this gives two solutions for latitude, but a later repeat of the process will resolve it to one)

With latitude you can get longitude from one time sight (same comment re: two solutions).
Click to expand...
But both assume both an accurate sextant and tables neither of which were stipulated in the OP :)

Might as well just switch on the GPS and then call up a passing ship on the satelite phone :)
 
You would be at the exact centre of your universe.
The question is, where is everybody else?

It all depends on what prior knowledge you are allowed to assume.
With no prior knowledge other than a belief the earth was a sphere that goes around the sun, you could measure your latitude, but it might take a year.
Longitude is an arbitrary convention anyway.
 
bedouin said:
But both assume both an accurate sextant and tables neither of which were stipulated in the OP :)

Might as well just switch on the GPS and then call up a passing ship on the satelite phone :)
Click to expand...

Well, you have nothing else to do, so constructing a sextant should be possible (I'll leave the details as an excercise for the class).

If you have a good memory you may recall the full Chalpront-Touze '84 lunar theory (it's only a couple of thousand numbers) and of course remembering the VSOP87 data will allow you to construct planetary ephemerides. Stars should be simple once you've done these.

Can't really see how not having a sextant and almanac should slow you down much :D

There's a book by David Burch called Emergency Navigation (in my grab bag on long trips) that gives a lot of good techniques for navigating and determining positions without standard instruments.
There's a section at the back (IIRC) that gives advice for various missing items: what if you don't have an almanac, what if you don't have a sextant, what if you don't have accurate time, etc.

Well worth a look: there's one in the CA library, or perhaps the Santa list.
 
Other people have noted various astronomical ways of determining latitude; longitude is tricky, without a time-source. If you've got a magnetic compass, the variation might give a clue, but that would be pretty vague. You CAN manufacture a magnetic needle from scratch - heat a piece of iron to red-heat and then beat it while it is approximately aligned N-S and it will develop a weak magnetism. Float a magnetized needle on a bit of cork in a puddle of water and you've got a compass. And of course, true north (or south) can be found by determining the midpoint between sunrise and sunset directions; tracking the shadow of a long stick through a few days will give a good indication of true north.

You can calibrate angles with geometric constructions for 90, 60, 45, 30 and 15 degrees. You can go further by using bisections of the angles, but these are the starting points. These constructions don't need a calibrated ruler, just compass (a bit of string) and a straight-edge (the advantages of an ancient education that included Euclidean Geometry!).

One thought I've got is that the fauna and flora are reasonably distinctive in different parts of the world. I'm not knowledgeable enough to make many confident predictions without a text-book in my hands, but there are plenty of species that indicate very clearly where you are. So, for example, the presence of marsupials other than opossums would be a strong clue you were in Australasia; the South American avifauna are pretty distinctive and so on. Of course, marine birds such as albatrosses and penguins won't help - they can cross oceans.

Incidentally, if for some reason you couldn't readily observe the sky, penguins only inhabit the southern hemisphere (OK, they do just make it to the Galapagos, but no further) and birds of the auk family (guillemots etc) inhabit the north.

Another clue would be the number of land-birds; the fewer, the more isolated.

I should say that I am no twitcher, but I am sure that someone with good knowledge of birds and a good reference book could probably pinpoint their location by observing and listing the local bird-life.

Of course, there are other clues: Butter melts = tropics, Ice doesn't melt = Arctic or Antarctic!

"The Mysterious Island" by Jules Verne suggests ways in which a group of ill-equipped castaways might find their location and develop a comfortable living on an unknown island! Sadly, it depends on a highly unlikely geology for the island :)
 
I vaguely remember a TV programme years ago, of the "enthuse people about science and technology" genre, where they were supposedly marooned on a tropical island and built various gadgets from scratch to show the principles involved. One episode, the challenge was to find out where they were. I don't think I watched the full episode - at least, I don't remember it - but one of the things they built was a crystal radio to receive a time signal to establish longitude.

Is that something that would be feasible for a person with the relevant knowledge in the situation prescribed by this thread? I think they used coal or charcoal or something for the crystal, but I suppose you'd need a lot of wire which could be tricky. The TV folks had a crashed plane or a boat or a car or something (can't remember exactly what) from which they could salvage useful materials (but nothing too useful!) to be able to build the things that the producers wanted to teach people about.

Pete
 
prv said:
I vaguely remember a TV programme years ago, of the "enthuse people about science and technology" genre, where they were supposedly marooned on a tropical island and built various gadgets from scratch to show the principles involved. One episode, the challenge was to find out where they were. I don't think I watched the full episode - at least, I don't remember it - but one of the things they built was a crystal radio to receive a time signal to establish longitude.

Is that something that would be feasible for a person with the relevant knowledge in the situation prescribed by this thread? I think they used coal or charcoal or something for the crystal, but I suppose you'd need a lot of wire which could be tricky. The TV folks had a crashed plane or a boat or a car or something (can't remember exactly what) from which they could salvage useful materials (but nothing too useful!) to be able to build the things that the producers wanted to teach people about.

Pete
Click to expand...

Sounds like the Rough Science with the delicious Kathy Sykes. An excellent series.
 
prv said:
I vaguely remember a TV programme years ago, of the "enthuse people about science and technology" genre, where they were supposedly marooned on a tropical island and built various gadgets from scratch to show the principles involved. One episode, the challenge was to find out where they were. I don't think I watched the full episode - at least, I don't remember it - but one of the things they built was a crystal radio to receive a time signal to establish longitude.

Is that something that would be feasible for a person with the relevant knowledge in the situation prescribed by this thread? I think they used coal or charcoal or something for the crystal, but I suppose you'd need a lot of wire which could be tricky. The TV folks had a crashed plane or a boat or a car or something (can't remember exactly what) from which they could salvage useful materials (but nothing too useful!) to be able to build the things that the producers wanted to teach people about.

Pete
Click to expand...

You can build a crystal radio using a bit of pyrites and a needle point to give a rectfying junction. However, if they'd got a crashed car or a plane or suchlike, I am sure they could salvage a diode from some of the electronics! I once built a crystal radio using a semi-conductor diode, when I were a lad, using a scavenged diode.

The nice thing about a crystal radio is that it needs no power source. However, it does need a headphone; creating one of those from scratch would be difficult! But of course, you've got one of those in your pocket from with the iPod that got ruined in the salt water, don't you?

Starting from scratch - without the advantage of a derelict vehicle - your main problem would be wire. The number of manufacturing steps to create wire from naturally occuring materials (which would be unlikely to be available anyway) would be immense. First, find a suitable ore (not likely in sterotypical "desert islands"), then smelt it, then create some tools from the first batch, then smelt some more, then use crude tools to make better tools, until you can create a hardened die to draw the wire through. Verne describes the steps in the Mysterious Island! However, he does make one assumption that isn't true - he assumes you'd need a power-source (he uses a water-wheel) to draw the wire. Actually, it can be done using a pendulum action; the artisan sits on a swing, grips the wire and swings back to draw a length, then swings forward to grip the next bit and so on.

You'd have other problems in creating a variable capacitor, but if you can hack making wire, I reckon you'd have no problem with a capacitor!
 
AntarcticPilot said:
You can build a crystal radio using a bit of pyrites and a needle point to give a rectfying junction. However, if they'd got a crashed car or a plane or suchlike, I am sure they could salvage a diode from some of the electronics!
Click to expand...

Since the whole thing was a TV programme aiming to demonstrate specific scientific and technical points, the "crashed plane" proved surprisingly rich in some things but deficient in others :). I'm fairly sure they made their crystal rather than salvaging components.

Pete
 
nigelmercier said:
Doesn't moss grow on the North side of trees? .
Click to expand...

I believe that this isn't the best way of indicating north (in our hemisphere).

Trees commonly grow as a single stemmed sapling. The first major branch tends to be towards the most light, ie south, and the main trunk of the tree will lean slightly the other way. Individual trees won't always show this but you should be able to find south from a grove of them.
 
You must log in or register to reply here.

Other threads that may be of interest

garymalmgren
Tokyo Bay at night (mare)
Replies
3
Views
205
garymalmgren
garymalmgren
ari
What is your lifejacket policy?
3 4 5
Replies
86
Views
3K
Greg2
Greg2
geem
Cat A ocean standard need full revision
15 16 17
Replies
335
Views
7K
WindyWindyWindy
W

Share this page

Top