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There seems to be a misunderstanding of the Noon Sight. Most people on this thread seem to think that a Noon Sight will only give Latitude. That is wrong. It will if taken accurately and carefully give a position latitude AND longitude. This is why the Noon Sight was revered and was a Noon ritual on the Square Riggers-

The technique is simple. I used it for years sailing up and down the Atlantic and across and back.

Work out your local noon. Accurate time is good but not essential. The ships clock will suffice. Even better if it is a chronometer.

Briefly take a sun sight just before noon. Note the time. Keep the sextant as is with no further adjustment but use it to mark when the sun has finished ascending and starts to fall again. Now pay close attention. When the sun again kisses the horizon on it's way back down note the time again. The mid point gives your longitude. Plotting the angles on graph paper gives Latitude.

For accuracy, it is necessary to take a line of sextant shots to plot the sun on graph paper as it climbs then falls. Just plot six sextant shot angles with the time. Plot them on graph paper. Read off the angle and time at top of rise. No need for maths or tables or waiting an hour for a second position to cross with the first one. Instant Lat and Long inside ten minutes and you are done and dusted.

The main problem with the noon sight is that the actual height of the climb (forgot the name) is relatively flat and gradual with no apparent movement either way for a minute or two. Hence a quick succession of shots to graph the maximum height accurately.

Wow that took a long time to describe.
 
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BurnitBlue said:
There seems to be a misunderstanding of the Noon Sight. Most people on this thread seem to think that a Noon Sight will only give Latitude. That is wrong. It will if taken accurately and carefully give a position latitude AND longitude. This is why the Noon Sight was revered and was a Noon ritual on the Square Riggers-

The technique is simple. I used it for years sailing up and down the Atlantic and across and back.

Work out your local noon. Accurate time is good but not essential. The ships clock will suffice. Even better if it is a chronometer.

Briefly take a sun sight just before noon. Note the time. Keep the sextant as is with no further adjustment but use it to mark when the sun has finished ascending and starts to fall again. Now pay close attention. When the sun again kisses the horizon on it's way back down note the time again. The mid point gives your longitude. Plotting the angles on graph paper gives Latitude.

For accuracy, it is necessary to take a line of sextant shots to plot the sun on graph paper as it climbs then falls. Just plot six sextant shot angles with the time. Plot them on graph paper. Read off the angle and time at top of rise. No need for maths or tables or waiting an hour for a second position to cross with the first one. Instant Lat and Long inside ten minutes and you are done and dusted.

The main problem with the noon sight is that the actual height of the climb (forgot the name) is relatively flat and gradual with no apparent movement either way for a minute or two. Hence a quick succession of shots to graph the maximum height accurately.

Wow that took a long time to describe.
Click to expand...

This from wiki


" Longitude cannot accurately be measured at noon, when it is very easy to determine the observer's latitude without knowing the exact time. At noon the sun's change of altitude is very slow so determining the exact time that the sun is at its zenith


"Longitude by chronometer" on @Wikipedia: https://en.wikipedia.org/wiki/Longitude_by_chronometer?wprov=sfta1
 
By coincidence and unrelated to this thread, yesterday someone emailed me and asked if it is possible to determine exact noon, and thus determine your position, without a sextant by watching exact sunrise and sunset. Is this something people have used?
 
alant said:
This from wiki


" Longitude cannot accurately be measured at noon, when it is very easy to determine the observer's latitude without knowing the exact time. At noon the sun's change of altitude is very slow so determining the exact time that the sun is at its zenith


"Longitude by chronometer" on @Wikipedia: https://en.wikipedia.org/wiki/Longitude_by_chronometer?wprov=sfta1
Click to expand...

Yeah but you can get close enough for us and I've used it too, no prob. Its also in the Davies sextant manual.

http://www.manualsdir.com/manuals/406143/davis-mark-15-25-sextant.html?page=14

This is not a difficult guide either.
 
Foolish Muse said:
By coincidence and unrelated to this thread, yesterday someone emailed me and asked if it is possible to determine exact noon, and thus determine your position, without a sextant by watching exact sunrise and sunset. Is this something people have used?
Click to expand...
You would need to be stationary to make that calculation easy
 
Foolish Muse said:
By coincidence and unrelated to this thread, yesterday someone emailed me and asked if it is possible to determine exact noon, and thus determine your position, without a sextant by watching exact sunrise and sunset. Is this something people have used?
Click to expand...

It can be used to determine an approximate time of Noon and longitude. If you are stationary. Even If you are Stationary the suns declination changes to much over the course of the day to be accurate. Add in the vessels movement and it becomes impractical.
Even the sight before and after Noon. To find the time and longitude of Noon. While practical looses accuracy due to the change in the suns declination and the vessels movement. Is it accurate enough? Yes. Particularly on slower vessels.
The traditional Running fix is used in preference because it is more accurate.
At one time a set of tables were printed for use in lifeboats to determine a position based on sunrise. I have not seen a set for many years. Not sure who printed them.

Instead of trying to determine noon from sunrise and sunset.

You can determine a rough Longitude and latitude from the time of sun rise. Or sunset. Without a sextant. If you have a compass an almanac and a watch.

It is not something I would routinely do. having a sextant I would take a set of star sight prior to sunrise or after sunset.

I would routinely use sunrise or sunset to check my compass error. It would simply be going backwards to determine my latitude then longitude. The accuracy would be limited. But still a useful check an upgrade from an EP or DR.
John Barrie mentions using this technique in his book sextant. But does not go into detail on how to do it.

If anyone is interested I can post how to check you compass by amplitude. Its quite simple.
 
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Uricanejack said:
It can be used to determine an approximate time of Noon and longitude. If you are stationary. Even If you are Stationary the suns declination changes to much over the course of the day to be accurate. Add in the vessels movement and it becomes impractical.
Even the sight before and after Noon. To find the time and longitude of Noon. While practical looses accuracy due to the change in the suns declination and the vessels movement. Is it accurate enough? Yes. Particularly on slower vessels.
The traditional Running fix is used in preference because it is more accurate.
At one time a set of tables were printed for use in lifeboats to determine a position based on sunrise. I have not seen a set for many years. Not sure who printed them.

Instead of trying to determine noon from sunrise and sunset.

You can determine a rough Longitude and latitude from the time of sun rise. Or sunset. Without a sextant. If you have a compass an almanac and a watch.

It is not something I would routinely do. having a sextant I would take a set of star sight prior to sunrise or after sunset.

I would routinely use sunrise or sunset to check my compass error. It would simply be going backwards to determine my latitude then longitude. The accuracy would be limited. But still a useful check an upgrade from an EP or DR.
John Barrie mentions using this technique in his book sextant. But does not go into detail on how to do it.

If anyone is interested I can post how to check you compass by amplitude. Its quite simple.[/QUOTE]

Also covered on the RYA course.
Click to expand...
 
Finding your compass error by Amplitude.

Azimuth and Amplitude are two different angles. For most observations the azimuth is used.

The Azimuth is the angle between the observers meridian and the celestial body for example the sun. The Azimuth is calculated for most observations to determine the direction of the position line or to obtain a compass error.

The Amplitude is the angle between due east or due west and the sun at sunrise or sunset.

The Amplitude is a simpler calculation based on the unusual, but very convenient 90 degree triangle which occurs at sunrise or sunset.
When the sun (or other object) is on the observers horizon. There is a convenient 90 degree angle from the horizon to the observers zenith.

The Zenith is the point on the celestial sphere directly above the observer.

The visual horizon and the true horizon or Observers Rational Horizon are not the same.
The visual horizon is bellow the true horizon due to "dip"
Dip is due to the observers height of eye or altitude above the horizon.
In addition to dip there is also refraction and parallax errors so when the sun appears to be on the horizon it is actually still below the horizon.

As a general rule or convention in order to approximate those combined errors an Amplitude of the Sun observed or taken when the suns lower limb is approximately 1/2 its diameter above the visual horizon.

The Time of sunrise or sunset can be determined from the nautical Almanac. The LMT of sunrise and sunset is listed for latitude. Applying your Longitude to the time predicted time of sunrise or sunset. will give you the GMT for sunrise or sunset at your location.

The Amplitude is the angle calculated between due east or due west and the Sun when the Sun is on the observers horizon. (Works Just the same for Moon or other objects Stars and Planets can only rarely be seen right on the horizon)

With a calculator.
The Sine of the Amplitude= The Sine of The Declination/Cosine Of The Latitude.
Sine Amp=Sine Dec/Cos Lat.

Or if using Norries
Sine Amplitude=Sine Declination x Secant Latitude.
Using Norries Logarithm tables
Log Sine Amplitude=Log Sine Declination x Log Sine Secant Latitude.

To determine the calculated compass Bearing of the Sun.

If The sun is rising.

The sun will be in the east. The LHA will be Greater Than > 180 Less Than < 360
If The Declination is North the Calculated Compass bearing = 90 - Amplitude.
If The Declination Is South the Calculated Compass Bearing =90 + Amplitude.

If The sun is setting

The Sun will be in the west. The LHA will be Greater Than > 0 Less Than < 180
If The Sun's Declination is North The Calculated Compass Bearing = 270 + Amplitude.
If The Sun's Declination Is South The Calculated Compass Bearing =270 - Amplitude.

To determine the Compass Error.
Compare the Observed Compass Bearing of the Rising or setting Sun to the Calculated Bearing.
The difference is the compass error.
Apply the Variation to the compass Error.
This will give your Deviation.
 
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Foolish Muse said:
Yes, I'm only talking about the LHA used to derive Hc because I only care about that. The point of determining LHA is because it tells us what page to look at in the sight reduction tables. Is there another reason for some other LHA that I'm not considering?

I also eliminated any discussion of celestial sphere because I really can't see a purpose for it. The latitude and longitude of the sun in the celestial sphere is the same as the latitude and longitude of the very tall flag pole on Earth directly beneath it. The almanac tells us the exact position of the base of that flag pole for every second of the year. That is all we care about. We determine our position as a circle around the base of that flag pole.

Can I ask that you take a look at the sextant users guide and go through the steps that I outlined. I'd be really appreciative if you found something wrong with my approach. I've tried to simplify something that was previously very complex.
Click to expand...

I had a quick look at the back bearing site. And The sextant users guide. It is appears to be as you say just the minimum required information to complete the form and come up with an answer. For some people this is ideal for others more information about why is required. I tend to ask why?
It might be worth while explaining a little bit more about why? you use an assumed position (AP). There is no need to be confused by different LHA. There is only one LHA.
When you choose an (AP) you are choosing your DR. a slight complication in terminology. On one hand your DR or Dead Reckoning position. is just your course and speed from your last known position. Your DR is just a convenient position which will hopefully be near your current Unknown position. The Accuracy of the DR is not critical. The position line will be the same regardless of the DR the distance to the position line or intercept will vary. over short distances the azimuth does not change enough to be significant.
When you Choose an (AP) all you are doing is adjusting your DR to make the tables easier to use. By adjusting your DR to provide whole numbers for your Latitude and LHA.
You can avoid having to interpolate the printed data in the tables.

Simply put instead of using the DR you plotted from your last position. You changed your DR to something more convenient.

I'm sure you can find a less long winded way to say it.
 
capnsensible said:
Yeah but you can get close enough for us and I've used it too, no prob. Its also in the Davies sextant manual.

http://www.manualsdir.com/manuals/406143/davis-mark-15-25-sextant.html?page=14

This is not a difficult guide either.
Click to expand...

An old Captain of mine once told me that "Navigation is the science of approximation", he was right of course. The method described is known as "equal altitudes", and is as old as the hills. The trick is to take a sequence of altitudes and produce a curve, from which a good approximation of true noon can be had. From there its just an almanac away from a longitude. I've been teaching young officers this technique for many years, purely to put some fun back into navigation. You can get a fix that is accurate to a couple of miles of longitude, and maybe half a mile of latitude if you are good. On our ships we are sitting in the middle of a 25 mile diameter circle of ocean, so a mile or two here or there is of absolutely no consequence.

Interestingly, while there is no longer a requirement for a chronometer on a ship - the use of the sextant is being promoted "in case GNSS goes down". I wonder where they'll been getting their time from ?...... most ships simply use their GNSS receivers as chronometers, as I imagine most leisure sailors do these days.

CC
 
Capt Cautious said:
......"in case GNSS goes down". I wonder where they'll been getting their time from ?...... most ships simply use their GNSS receivers as chronometers, as I imagine most leisure sailors do these days.

CC
Click to expand...
A half-decent quartz watch seems adequate to me.
Mine drifts quite predictably a few seconds a month against the GPS.
 
Capt Cautious said:
An old Captain of mine once told me that "Navigation is the science of approximation", he was right of course. The method described is known as "equal altitudes", and is as old as the hills. The trick is to take a sequence of altitudes and produce a curve, from which a good approximation of true noon can be had. From there its just an almanac away from a longitude. I've been teaching young officers this technique for many years, purely to put some fun back into navigation. You can get a fix that is accurate to a couple of miles of longitude, and maybe half a mile of latitude if you are good. On our ships we are sitting in the middle of a 25 mile diameter circle of ocean, so a mile or two here or there is of absolutely no consequence.

Interestingly, while there is no longer a requirement for a chronometer on a ship - the use of the sextant is being promoted "in case GNSS goes down". I wonder where they'll been getting their time from ?...... most ships simply use their GNSS receivers as chronometers, as I imagine most leisure sailors do these days.

CC
Click to expand...

You shouldn't use a navigational GPS as a time source; it may be off by a second or two. Although the embedded time signal is very precise, the navigational processing (connected, I think, with the cycle of the GPS transmissions) means that the displayed time is not so precise. There is also the issue of "leap seconds" - thesemay not be corrected, leading to errors of 12+ seconds. GPS time sources (used by web servers and such like) are different beasts from the positional GPS we all use.

However, a modern quartz watch - even a cheap one - will probably be amply accurate for navigational use.
 
AntarcticPilot said:
You shouldn't use a navigational GPS as a time source; it may be off by a second or two. Although the embedded time signal is very precise, the navigational processing (connected, I think, with the cycle of the GPS transmissions) means that the displayed time is not so precise. There is also the issue of "leap seconds" - thesemay not be corrected, leading to errors of 12+ seconds. GPS time sources (used by web servers and such like) are different beasts from the positional GPS we all use.
Click to expand...
That rather depends on the receiver manufacturer. Although typical receivers output position every second, they are well capable of outputting accurate time, converted from GPS system time to UTC including leap second corrections.
 
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