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  #1  
Old 18th June 2012, 20:16
Genevieve Genevieve is offline
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Navigation from scratch

Dear friends,
Would it be possible to impose upon you to ask a rudimentary navigation question please?
It struck me that with the use of GPS today, we are sometimes
spoilt with navigation. But if you were "plonked down" at some
random location with a sextant, a book of relevant astronomical
tables and log tables, would it be possible to ascertain a reasonably
good location based on stellar sightings alone? I think a latitude could be obtained as long as, say, Polaris, could be seen above the horizon....?

Having read up on the importance of the knowledge of local time
when taking sightings, I'd like to ask the above question above with
an extra condition.
Say you were on a boat, whose position was known at time "T"
and whose clocks would be correct at noon for the longitude that
would be reached at time "T + t". If your boat's engines were to break down at some point between "T" and "T+t" so that you were at the mercy of the wind and tides alone, would it be possible to find your position without the recourse of utilising DR calculations? Would the lack of accurate local time be a huge hinderance, especially as time increased?

I feel embarrassed asking such knowledgeable people as yourselves
as I only have a rudimentary grasp of sea faring.
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  #2  
Old 18th June 2012, 23:28
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re your first question.. with a reasonable dr ,then 4 good star sights at evening/morning twilight will give an accurate position,or one star just before sunrise and a shot of the sun before it got above 10 degrees would give another good position.
Would be interesting to know whether sights are taken nowadays.my grandson is joining maersk in sept,do I offer him my sextant/

jim
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  #3  
Old 19th June 2012, 05:14
Rogerfrench Rogerfrench is offline  
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Re: question 1 - it would be tough without a chronometer or some timepiece. You could get a good latitude at noon, if you were patient enough to find noon, that is! Polaris will give you a rough latitude, but if you could be literally anywhere, it would be approximate.
Your second question is a bit of a catch. If the clock was right for the longitude, then you'd know your longitude by looking at the clock, wouldn't you? That assumes that the clock is constantly adjusted, which of course, it never is. And you'd get your latitude at noon, presumably.
Celestial navigation gives a series of position lines, or arcs on the surface of the earth, one for each body observed. There is a point on the earth's surface where a line is drawn from the centre of the earth to the body, and the tables give you that position at the time of the sight. The altitude of the body is used as a variable in the trigonometric formula to calculate your distance from that position so you can draw an arc.
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  #4  
Old 19th June 2012, 07:02
Genevieve Genevieve is offline
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Hello,
Thank you for your replies, they are very interesting.

I admit that I am a bit confused by the situation which popped into my head
yesterday. To answer your question Roger, yes I'd assume that the chronometers
were adjusted once per day, so that the time at noon the next day would be right.

I think I was curious to know what would happen if the ship was unable to
make the noon position when the clocks would be right but started to drift off course. For instance, say at midnight on July 1st, the clocks are reset to be "right" at noon. But the ship never makes that position at noon due to problems between midnight and noon (say, 3am). By noon, she is still some
216 miles off her anticipated location. By noon the next day, she has drifted
another 36 miles or so further away. Without resetting the chronometer,
the current local time and the real local time would be getting further and further divergent.

I thought after typing all the above up that its fairly obvious that the time
of observation of stellar objects would have an impact on the deduced position, as astronomical tables would give the altitude of "guide" stars for certain times of the day, so that you can work backwards to give your own location. If you don't know the current local time, then you can't say for certain where you are?

If I have grasped the incorrect end of the stick please let me know!
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  #5  
Old 19th June 2012, 07:30
Genevieve Genevieve is offline
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Quote:
Originally Posted by Rogerfrench View Post
You could get a good latitude at noon, if you were patient enough to find noon, that is! Polaris will give you a rough latitude, but if you could be literally anywhere, it would be approximate.
Hi Roger,
Could you get local noon-time by waiting till the sun reached its highest point in the sky?

Forgive me as I am new to all this, but if you had a clock that was working but not set correctly, and could find out when the sun was at its highest, could not comparison with a hypothetical GMT clock give you the longitude?

I feel stupid for asking this, but if you could find the altitude of Polaris, would this not given you your latitude, irrespective of where you actually are? You'd have to wait till you got a clear horizon, so the sun would had to have gone down.
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  #6  
Old 19th June 2012, 07:38
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Regarding ships "clocks", or chronometers, if memory serves me correctly they are always kept on Zulu time (used to be Greenwich mean time - GMT) A good chronometer usually has a predictable daily error (+ or -) in seconds.
The error is applied to the chronometer reading at the time of the sight as nautical almanacs are (or at least used to be) based on GMT. The chronometer was not normally altered for any reason and to maintain the predictable error it was wound at the same time daily.
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  #7  
Old 19th June 2012, 15:37
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Quote:
Originally Posted by Genevieve View Post
Hi Roger,
Could you get local noon-time by waiting till the sun reached its highest point in the sky?
You would get local time within a few seconds but that's no use unless you also know the GMT.

There is a method called lunar distance which was used in the (very)old days. Not sure how it worked or even if the required information is still in the Almanac. Joshua Slocum used it during his circumnavigation.
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Old 19th June 2012, 15:41
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Lunars

Good explanation of lunars here
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  #9  
Old 21st June 2012, 12:34
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The simplest way would be to use a few solar obs (sun) rather than stellar (stars)
Most basic kit required would be a radio, a watch, a stick and a tapemeasure ( if ashore) or a sextant (if afloat)....and an almanac.
Finding latitude is simple enough..... altitude of the sun when it is at its highest....local apparent noon.... if you are ashore poke your stick in the ground... measure length of shadow when it is at its shortest.... opp ( length of stick)/adjacent( length of shadow) = Tan Alt.
Then you just have to do a simple Meridian Altitude calculation using the altitude and the sun's declination at the time.... about a 4 line calculation...give or take.
For longitude you want to know the time.... use BBC time signal for that...set your watch to GMT ...
Now the sun will be on the Greenwich meridian at 1200 GMT. ( * more on this in a mo).
So if it is on your meridian at 1320 GMT ( shortest shadow from stick or greatest altitude on your sextant) you are 1 hour 20 minutes west of Greenwich.
Sun does 360 degrees in 24 hours, 15 degrees in one hour, 1 degree in for minutes....
Therefore you are in Longitude 20 West.

* things get a bit complicated as the earth's orbit around the sun is an ellipse. As a result while the mean sun is on the greenwich meridian at 1200 GMT the actual sun may be on the greenwich meridian up to 15 minutes before or after 1200. This is called the 'equation of time' and is shown on the daily pages of your almanac.
So lets say the sun is on the greenwich meridian at 1210 and on your meridian at 1320.... 1 hour and 10 minutes means you are 17 degrees 30 minutes West.

Hope this helps.
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  #10  
Old 21st June 2012, 13:54
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Just a small point but do not use a conventional digital radio (or TV) for the reception of time signals i.e. DAB or DRM type because the time signal received may be in error.

Added Note The reason is that the encoding and decoding of the digital signal causes a delay, of usually between 2 and 8 seconds. In the case of satellite broadcasting, the travel time of the signal to and from the satellite adds about another 0.25 seconds.

Last edited by mikeg; 21st June 2012 at 18:42.. Reason: Clarification
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  #11  
Old 21st June 2012, 15:17
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I find the BBC on HF is accurate enough... within a second or so of GPS time but yes, some commercial stations just send the 'pips' when they feel like it.

I'm suprised no one has picked me up on the fact that length of shadow/ length of stick will give the Co-tangent of the Zenith Distance....... then the meralt calculation is very simple.

If plonked down with absolutely no idea where on the planet you are my method would give a reasonable DR which could be used to reduce some decent sights.
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  #12  
Old 21st June 2012, 16:00
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Thank goodness I was an engineer always knew where I am = down below.
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  #13  
Old 21st June 2012, 16:37
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Nice explanation Cisco,
Being Retired and bored Yesterday I worked out my Latitude using the garden fence (Shadow of) and a tape measure! Sun is at 23 1/2 Deg (Solctice) so no need for tables there....Answer was within 12 miles of correct!
A good book to look at is the facsimile copy of Cap't Blyth's log book. The mutineer's left him the sextant but kept the Chronometer .......... His working shows simple Meridian Alts 'crossed' with the calculation of the boats course and speed (based on hourly observations. A very nice bit of seamanship that got him across the Pacific.
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Old 21st June 2012, 17:38
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I recall reading that the British prisoners on "Altmark" kept track of their course by the shadow method, using a pencil to cast the shadow. Noon was ascertained by watching the German Officers take their sights. A quick internet trawl hasn't thrown anything up so maybe I'm mistaken?
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  #15  
Old 21st June 2012, 18:04
Rogerfrench Rogerfrench is offline  
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Quote:
Originally Posted by Duncan112 View Post
I recall reading that the British prisoners on "Altmark" kept track of their course by the shadow method, using a pencil to cast the shadow. Noon was ascertained by watching the German Officers take their sights. A quick internet trawl hasn't thrown anything up so maybe I'm mistaken?
I have read that, too. You were not mistaken!
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  #16  
Old 21st June 2012, 18:16
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Quote:
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Thank goodness I was an engineer always knew where I am = down below.
Navigation is an approximate science, or is it the science of approximation?
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  #17  
Old 21st June 2012, 19:12
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Quote:
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Navigation is an approximate science, or is it the science of approximation?
As a mere Engineer I was always told that navigation was an art, not a science
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Old 21st June 2012, 20:11
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As a mere Engineer I was always told that navigation was an art, not a science
Yes, you are right. It is certainly not a science but it seems to be more difficult than I found it to be,

Last edited by Split; 21st June 2012 at 20:41..
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Old 22nd June 2012, 07:14
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Quote:
"..............a sextant, a book of relevant astronomical
tables and log tables........."
No mention here of a radio for time signals.
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Old 23rd June 2012, 02:31
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Quote:
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No mention here of a radio for time signals.
That was mentioned at #9...'Most basic kit required would be a radio, a watch, a stick and a tapemeasure ( if ashore) or a sextant (if afloat)....and an almanac.'
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  #21  
Old 23rd June 2012, 02:51
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Quote:
Originally Posted by Genevieve View Post
Having read up on the importance of the knowledge of local time
when taking sightings, I'd like to ask the above question above with
an extra condition.
Local time isn't required..... what you need is a reasonably accurate knowledge of GMT/UTC..... the rest is but detail.....

I say 'reasonable' because an error of one minute in time will put your longitude out by 15 minutes of arc.... 15 miles at the equator.... less and less the further north or south you go..... not a big enough error to have you sailing past the New World without spotting it.....
Cheers
Frank
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Old 23rd June 2012, 08:29
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[QUOTE=Cisco;603289]
Finding latitude is simple enough..... altitude of the sun when it is at its highest....local apparent noon.... if you are ashore poke your stick in the ground... measure length of shadow when it is at its shortest.... opp ( length of stick)/adjacent( length of shadow) = Tan Alt.
Then you just have to do a simple Meridian Altitude calculation using the altitude and the sun's declination at the time.... about a 4 line calculation...give or take.

All this assuming the ground is not only level, but horizontal and that the stick is perfectly plumb.
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Old 23rd June 2012, 09:01
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OK... add a spirit level to the list.....
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Old 23rd June 2012, 09:21
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Quote:
Originally Posted by Genevieve View Post
"............But if you were "plonked down" at some
random location with a sextant, a book of relevant astronomical
tables and log tables............."
The rules of this game were laid down at the outset. You can't just add useful items when you feel like it. That would be no fun.
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Old 23rd June 2012, 09:43
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OK....so if not allowed anything not included in the first post... therefore without a clock, a copy of 'Lecky's Wrinkles', a spirit level, an admiral or wheelbarrow, and no rabbit or hat, its 'south until the butter melts' and then hang a right....
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