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Old 05-04-2014, 14:51   #1
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The Importance Of The Noon Sight?

I've often read about the religious consistency of early mariners in catching the noon sight and never really thought too much about why because i've never thought of trying celestial nav without an almanac – but, seeing as a pretty good fix on latitude is relatively easy just by measuring to the pole position using the southern cross or the polestar, having a simple way to get a rough longitude fix using the noon sight and a watch ought also to be reasonably simple.
If you have a sextant and a watch set to GMT you should be able to calculate your longitude from a midday sunsight - although given the difficulty of determining the precise time of the highest angle, it might be hard to be very accurate. But it ought to be possible to within a few miles which would be better than nothing.
Is that why the noon sight was particularly important?
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Old 05-04-2014, 15:39   #2
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Re: The Importance Of The Noon Sight?

Because the British Navy used it to determine the new day which determined when the rum or spirits were issued?

It's also the easiest sight to take.

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Old 05-04-2014, 15:51   #3
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Re: The Importance Of The Noon Sight?

The noon sight is the only one that lets you determine your position (both Lat and Lon) with a single sight.
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Old 05-04-2014, 15:55   #4
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Re: The Importance Of The Noon Sight?

Finding latitude from a noon sight is the simplest celestial nav calculation there is and yields very accurate results. No timepiece required, only a nautical almanac. Latitude is much trickier from a noon sight needing an accurate timepiece and a steady hand. Hardest for me is to determine actual "high noon" The sun always seems to hang at its highest (or meridian) altitude each day for a couple of minutes or so, and to time noon precisely you need to figure out exactly when the middle of that “hang time” is. Have to stay in practice with the sextant every day.

After a week or so, I can usually get within a couple miles of latitude and within 10 to 12 miles of longitude. Not accurate in these days of GPS, but in a pinch I can always "run down my latitude" and even if I don't make the harbor entrance, at least I shouldn't miss the island.
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Old 05-04-2014, 16:22   #5
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Re: The Importance Of The Noon Sight?

The calculation for lat is simple but diddling around waiting for the sun to reach its zenith then start to fall...the value isn't there for me. I don't have the patience. A standard sight reduction at approx local apparent noon will create a nearly east-west lop in less time.
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Old 05-04-2014, 16:28   #6
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Re: The Importance Of The Noon Sight?

Though if you want, take a series of shots at 2 - 3 min intervals 10 min before your precalculated time of meridian passage continuing for 10 min past. Chart a bell curve of the sights and that will create your true LAN.
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Old 05-04-2014, 18:58   #7
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Re: The Importance Of The Noon Sight?

Just learned to plot the "bell curve" last week in a celestial navigation course. Never heard of it before this introduction. Using the sextant allows you to fully appreciate our early navigators. They were very skilled and intelligent seafarers.
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Old 05-04-2014, 20:23   #8
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Re: The Importance Of The Noon Sight?

what randy says. take a series of shots before and after lan and then extrapolate lan.

another method i've heard of is to take a shot a few minutes before lan. don't change the settings on the sextant. then begin taking shots after lan - without changing the settings on the sextant - until the sun returns to the sextant angle. lan is then exactly (?) between the two shots.

lot's easier than a running fix but requires the sun to be visible for a few minutes around lan and being on deck in the hot sun.

as for the ancient mariners being so careful to take the noon sight, back in those days that's all they had was the noon sight. without a timepiece they couldn't determine longitude. but the noon sight did give them latitude. and because of that, to this very day, a ships daily progress is always calculated from noon to noon, and not calendar days.
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Old 05-04-2014, 20:26   #9
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Re: The Importance Of The Noon Sight?

Quote:
Originally Posted by Jim Walsh View Post
Just learned to plot the "bell curve" last week in a celestial navigation course. Never heard of it before this introduction. Using the sextant allows you to fully appreciate our early navigators. They were very skilled and intelligent seafarers.
You betcha! Pretty interesting reading about the evolution of navigation. Until there was an accurate time piece onboard it was pretty rough trying to determine where your longitude was. More or less dead reckoning. Determining latitude wasn't too hard but still required a bit of skill so folks would sail down or up to their latitude then head for their destinatione along that latitude. Called "Sailing the Latitudes."
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Old 05-04-2014, 21:39   #10
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Re: The Importance Of The Noon Sight?

Art Birney wrote a terrific and brief and simple book on Noon Sight Navigation and Dava Sobel's "Longitude" details the historical account of John Harrison's clocks which finally won him the Admiralty Prize and can be seen in Greenwich--a worthwhile trip I was lucky to do. With the bell curve as described above, I found it helpful to wear my watch face up (opposite side of "normal" wear) as I make adjustments on the sextant. I did all this years ago before cellphones and gps and all on a plastic Davis sextant...and with help from an rdf to fix lines and get WWV. Nice to know folks are still doing this. The beauty of sextants in this high tech age is it requires you to keep a log. And all you really need for the noonsight.....is the sun!
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Old 11-04-2014, 21:31   #11
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Re: The Importance Of The Noon Sight?

yeah, thats how i learned to do sights - usually try to get at least 6 and then average them, only difference is in calculating the bell curve.
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Old 13-04-2014, 02:26   #12
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Re: The Importance Of The Noon Sight?

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Is that why the noon sight was particularly important?
Was a (long ) time when that was all they had.

I've just been reading Basil Ringrose's 'South Sea Waggoner' (again).

A bit of background - Bartholemew Sharp and some likely lads including young Bazza crossed the Isthmus in 1680, nicked a couple of Spanish ships and wrought havoc on the Pacific coast of south America for a couple of years.

That done they left an anchorage on Duke of York Island in 50*37'S... pottered off down and around the Horn ( going down to 58*23'S and becoming the first englishmen to go eastabout round the Horn).

About 2 months later they reached the latitude of Barbados... hung a left.. and made landfall ten days later.... 3 months and 9000 miles without sight of land.

Not a bad bit of work with no clock, a couple of astrolabes and possibly a cross-staff, and just their reckoning of their 'eastings' and 'westings'.
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Old 13-04-2014, 02:36   #13
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Re: The Importance Of The Noon Sight?

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Originally Posted by Randy View Post
The calculation for lat is simple but diddling around waiting for the sun to reach its zenith then start to fall...the value isn't there for me. I don't have the patience. A standard sight reduction at approx local apparent noon will create a nearly east-west lop in less time.

Absolutely. With the motion of a small boat. Bell curves to calculate zenith are a search for precision not accuracy.

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Old 13-04-2014, 02:53   #14
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Re: The Importance Of The Noon Sight?

Three other probs.... if you are in high lats the sun is going to 'hang' for a long time .... if your lat and the sun's dec are about equal its going to go from an easterly to a westerly azimuth very very quickly and its not easy on a big ship to get a mer alt when the alt is greater than 85* never mind a yacht.
If you are keeping time well removed from actuality the sun may be on the meridian well into your afternoon. Chile f'rinstance keeps double summer time for much of the year... sun not on merid until about 1400 local. Cuts into Piscosour time and the afternoon nap...

Best daily navigation solution as far as I am concerned is a sight when the sun is either right ahead or right astern... gives you an accurate run.... and another when the sun is on the beam.. lets you know if you are left or right of track.

Back to OP... yes mainly tradition even 50 years ago.... running down to Capetown ex UK we would stay on GMT ( or BST) until a few nights before Capetown. Midday position ( ie 1200 ship's time) was simply a morning sight run up to another sight at 1200.

The 2nd mate would take a mer-alt in the middle of his watch just to fill in his afternoon....
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Old 13-04-2014, 02:55   #15
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Re: The Importance Of The Noon Sight?

Personally I prefer star sights., proper fix

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