Noon Sites

Petronella

Petronella

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29 May 2004
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Scarborough Marina, Australia
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I've been teaching myself Celestial Navigation from Tom Cunliffe's book Ocean Sailing but I seem to have a fundamental gap in my understanding.

It seems that a Noon Site can give you your latitude but not your longitude. Why is this? Surely when the sun is at its highest (and you are taking your latitude reading) you know the precise time and hence should be able to look up the GHA of the sun at that time and thereby work out your longitude too. What am I missing?
 
simple really, if you take a single sun/moon/star, you will only get one line. That is why you do a sun/run/sun to improve your positional accuracy.
 
You're right, I've tried it...but the problem is getting the exact time that the sun is at its highest to better than 15seconds error. On a moving boat your sites 2 or 3 mins either side of mid day will look pretty similar, and as the sun is moving relative to you at 15 mins of long/min you could easily be 20 miles out.

What I found worked, was to plot a good curve taking sites say every 30 mins 2/3 hours before and after noon. Plot the curve and your estimate of the time of local noon is much better. I also tried getting accurate sun rise and sun set times, allow for distance travelled between these and half them...bingo you get local mid day pretty accurately, as its much easier to time the sun touching the horizon to the nearest 2/3 seconds.
 
It's good to see someone thinking about this stuff, John. What you're trying to do puzzled navigators for centuries - and led directly to the invention of the Marine Chronometer. The problem is, when taking a noon sight, there is no obvious 'instant' when the sum is at its highest - it seems to hang at about the same height for a minute or more...... which is no good for the precision needed to find a channel in the Caribbean!

Some navigators then measured the sun's height about half-an-hour before local noon, then left that setting fixed on the sextant. They simultaneously took the time shown on their inaccurate clock.

When the sun later 'descended' to that fixed setting again, they took the clock's time again - and the mid-point of elapsed time gave them the time of Local Noon - and therefore Longitude - with better accuracy.

Read 'Bowditch' for a more detailed explanation....

Doesn't work well at all latitudes, but is an interesting 'other way' for you to try.
 
Don't want to be a pain.. but it is sights and not sites....thought you were posting about a site where we all converge at noon everyday for simultaneous postings.... /forums/images/graemlins/smile.gif
 
It is good to see someone trying proper navigation and thinking about it. I must admit that I've always been shakey on the theory but on my first ocean passage I was chuffed to plot a landfall to within 20 miles.

That was when the art of navigation was viewed as an inacurate science and GPS was something of science fiction.
 
I am sorry but whoever taught you to do a merpas by going for the exact time is a plonker.

The correct method is to work out the time as close as you can, and then start taking sights befre that time. as soon as you stop increasing the elevation, and just before starting to reduce, you are at the correct meridian passage. you then have a max elevation and the time of that max elevation which you quickly turn into a lattitude using the air tables.

If you are 20 miles out in your EP, you should have done a sun/run/sun earlier to refine your position.
 
No body taught me, but this was an example of experimenting with one sight and a clock, I also tried a protractor weight and string!

I know how to get an accurate fix, but had fun seeing how accurate I could be on mid day to give me a position that was just time and mid day altitude related. Like the original post was suggesting.

The 20 miles is what you might get if you just try to get a mid day time taking a few sights near mid day. Using sun rise and sun set I could get 5 miles, probably similar to setting the sextant and seeing when the sun passes this point on its way up and down as someone else suggested.
 
You say you know the time? All you know is that it is noon at the point you are on the earths surface. The reading you get will be the same all the way round the world on the lattitude circle. You need to know GMT at the time of the sighting to calculate the longtitide. The difference between GMT at the time you take the sight gives you the hours difference. Then it is 15 degrees west for every hour of difference to give longtiitude.
 
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