maxi77
Well-Known Member
All the computed altitude and azimuth tables can now be downloaded as a pdf from the NOAA web site.
Using a Sextant....anyone out there ??
- Thread starter Matata
- Start date
alant
Well-Known Member
That's why you have to subtract a correction for height of eye!
------------------------------------------------------------------------
Thats an adjustment, which, is made because it assumes you are at the centre of the Earth, when taking your sights.
------------------------------------------------------------------------
Thats an adjustment, which, is made because it assumes you are at the centre of the Earth, when taking your sights.
maxi77
Well-Known Member
[ QUOTE ]
That's why you have to subtract a correction for height of eye!
------------------------------------------------------------------------
Thats an adjustment, which, is made because it assumes you are at the centre of the Earth, when taking your sights.
[/ QUOTE ]
Well when I was taught astro it was to correct for the height of eye ebove sea level so that you actually measured the elevation above the horizontal. It is a correction that does not need to be added when using an artificial horizon sextant because the elevation is already being measured against the horizontal.
That's why you have to subtract a correction for height of eye!
------------------------------------------------------------------------
Thats an adjustment, which, is made because it assumes you are at the centre of the Earth, when taking your sights.
[/ QUOTE ]
Well when I was taught astro it was to correct for the height of eye ebove sea level so that you actually measured the elevation above the horizontal. It is a correction that does not need to be added when using an artificial horizon sextant because the elevation is already being measured against the horizontal.
alant
Well-Known Member
Well when I was taught astro it was to correct for the height of eye ebove sea level so that you actually measured the elevation above the horizontal.
========================================
Thats because the tables for dip, have already taken the distance from the centre of the Earth, to zero height of eye, into consideration already.
" is a correction that does not need to be added when using an artificial horizon sextant because the elevation is already being measured against the horizontal."
========================================
So, how does it do that then?
========================================
Thats because the tables for dip, have already taken the distance from the centre of the Earth, to zero height of eye, into consideration already.
" is a correction that does not need to be added when using an artificial horizon sextant because the elevation is already being measured against the horizontal."
========================================
So, how does it do that then?
maxi77
Well-Known Member
The periscope sextants I used had magic mechanical devices that used pendulums and gyros and mechanical dividers to record 1/60 of the elevation every second as you kept the object between to horizontal bars and after 60 seconds you read of the elevation with no height of eye correction. It is over 30 years since I did it last so bits of the memory are vague. Gave sights good to about 5 mile though.
Refueler
Well-Known Member
[ QUOTE ]
[ QUOTE ]
That's why you have to subtract a correction for height of eye!
------------------------------------------------------------------------
Thats an adjustment, which, is made because it assumes you are at the centre of the Earth, when taking your sights.
[/ QUOTE ]
Well when I was taught astro it was to correct for the height of eye ebove sea level so that you actually measured the elevation above the horizontal. It is a correction that does not need to be added when using an artificial horizon sextant because the elevation is already being measured against the horizontal.
[/ QUOTE ]
BONG ! The corrected altitude is actually referenced to centre of earth - NOT horizontal.
In effect HoE / Dip correction can be said to be average radius of earth + your eye height above sea-level applied to the observed altitude to obtain referenced to CoE in plain terms.
You could of course look at this :
http://physics.gmu.edu/~hgeller/astr402/...served Altitude
/forums/images/graemlins/grin.gif
[ QUOTE ]
That's why you have to subtract a correction for height of eye!
------------------------------------------------------------------------
Thats an adjustment, which, is made because it assumes you are at the centre of the Earth, when taking your sights.
[/ QUOTE ]
Well when I was taught astro it was to correct for the height of eye ebove sea level so that you actually measured the elevation above the horizontal. It is a correction that does not need to be added when using an artificial horizon sextant because the elevation is already being measured against the horizontal.
[/ QUOTE ]
BONG ! The corrected altitude is actually referenced to centre of earth - NOT horizontal.
In effect HoE / Dip correction can be said to be average radius of earth + your eye height above sea-level applied to the observed altitude to obtain referenced to CoE in plain terms.
You could of course look at this :
http://physics.gmu.edu/~hgeller/astr402/...served Altitude
/forums/images/graemlins/grin.gif
peterb
Well-Known Member
[ QUOTE ]
BONG ! The corrected altitude is actually referenced to centre of earth - NOT horizontal.
In effect HoE / Dip correction can be said to be average radius of earth + your eye height above sea-level applied to the observed altitude to obtain referenced to CoE in plain terms.
You could of course look at this :
http://physics.gmu.edu/~hgeller/astr402/...served Altitude
[/ QUOTE ]........which shows that the dip correction is related to the dip of the horizon below the observer's eye, and not to the offset from the inter-centre line. That offset is called the horizontal parallax, and although mentioned in the PowerPoint slides it's calculation isn't dealt with in any detail. Anyway, it is only significant when using near-by objects such as the Moon, and decreasingly Mars and Venus. By the time that you reach solar distance (92,000,000 miles) the maximum horizontal parallax you can achieve from Earth is only about 9 seconds of arc, equivalent to an error of about 300 yards, and it's unlikely that your sights will be that accurate.
BONG ! The corrected altitude is actually referenced to centre of earth - NOT horizontal.
In effect HoE / Dip correction can be said to be average radius of earth + your eye height above sea-level applied to the observed altitude to obtain referenced to CoE in plain terms.
You could of course look at this :
http://physics.gmu.edu/~hgeller/astr402/...served Altitude
[/ QUOTE ]........which shows that the dip correction is related to the dip of the horizon below the observer's eye, and not to the offset from the inter-centre line. That offset is called the horizontal parallax, and although mentioned in the PowerPoint slides it's calculation isn't dealt with in any detail. Anyway, it is only significant when using near-by objects such as the Moon, and decreasingly Mars and Venus. By the time that you reach solar distance (92,000,000 miles) the maximum horizontal parallax you can achieve from Earth is only about 9 seconds of arc, equivalent to an error of about 300 yards, and it's unlikely that your sights will be that accurate.
duncanmack
Well-Known Member
Nigel,
Thanks for that link, It's pure dead brilliant!
/forums/images/graemlins/smirk.gif
Thanks for that link, It's pure dead brilliant!
/forums/images/graemlins/smirk.gif
stephenh
Well-Known Member
" equivalent to an error of about 300 yards, and it's unlikely that your sights will be that accurate. "
and there's me happy with 2 miles ( one on a good day ) - I have obviously got to sharpen up my act...... !!!!!
and there's me happy with 2 miles ( one on a good day ) - I have obviously got to sharpen up my act...... !!!!!
duncanmack
Well-Known Member
and your pencil.......... /forums/images/graemlins/grin.gif
Sgeir
Well-Known Member
To digress slightly from ocean navigation, of which I have no experience, the sextant is a very good instrument for coastal navigation if your GPS goes down. Not for taking sights off the sun (what's that?), but for using horizontally by measuring the two angles between three identifiable fixed positions.
Compared with a sighting compass it is a far superior method as it is much easier to fix an accurate angle (there is no compass rose swinging around). Instead of a "cocked hat", the final fix is much more precise. I've ever had to do it in anger, but when practising with it, found it to be pretty quick and straightforward, and was surprised at the accuracy when compared with a compass reading; the sextant position was very close to the GPS.
We use a three arm protractor to plot the position; this is a very fast and simple operation and vastly preferable to plotting compass readings on to the chart. Three arm protractors might be hard to come by, but worth their weight in gold.
Compared with a sighting compass it is a far superior method as it is much easier to fix an accurate angle (there is no compass rose swinging around). Instead of a "cocked hat", the final fix is much more precise. I've ever had to do it in anger, but when practising with it, found it to be pretty quick and straightforward, and was surprised at the accuracy when compared with a compass reading; the sextant position was very close to the GPS.
We use a three arm protractor to plot the position; this is a very fast and simple operation and vastly preferable to plotting compass readings on to the chart. Three arm protractors might be hard to come by, but worth their weight in gold.
Refueler
Well-Known Member
[ QUOTE ]
" equivalent to an error of about 300 yards, and it's unlikely that your sights will be that accurate. "
and there's me happy with 2 miles ( one on a good day ) - I have obviously got to sharpen up my act...... !!!!!
[/ QUOTE ]
Yeh well ... Sun sight to a couple of miles if you're lucky. Star sights and position to a mile or so if lucky.
Anyone who says they can do better than that are kidding themselves. Particularly if you are running up morning sights to a noon position.
" equivalent to an error of about 300 yards, and it's unlikely that your sights will be that accurate. "
and there's me happy with 2 miles ( one on a good day ) - I have obviously got to sharpen up my act...... !!!!!
[/ QUOTE ]
Yeh well ... Sun sight to a couple of miles if you're lucky. Star sights and position to a mile or so if lucky.
Anyone who says they can do better than that are kidding themselves. Particularly if you are running up morning sights to a noon position.
stephenh
Well-Known Member
" Yeh well ... Sun sight to a couple of miles if you're lucky."
I have always suspected that getting within a mile is down to errors in my favour !!
Now I know....
BTW what standards of accuracy are asked for in the MN / RN, say under exam conditions ?
I have always suspected that getting within a mile is down to errors in my favour !!
Now I know....
BTW what standards of accuracy are asked for in the MN / RN, say under exam conditions ?
Refueler
Well-Known Member
[ QUOTE ]
" Yeh well ... Sun sight to a couple of miles if you're lucky."
I have always suspected that getting within a mile is down to errors in my favour !!
Now I know....
BTW what standards of accuracy are asked for in the MN / RN, say under exam conditions ?
[/ QUOTE ]
You won't believe this. But I never remembered sitting a Sextant altitude question in all my MN exams. It was down to you to learn at sea. College Navigation was more chartwork related.
MN training was a mix of on-board required work and correspondence backed up by College periods to fill you with all the Spherical Trig Maths stuff, Ships Construction etc. ...
But mine was quite a few years ago.
" Yeh well ... Sun sight to a couple of miles if you're lucky."
I have always suspected that getting within a mile is down to errors in my favour !!
Now I know....
BTW what standards of accuracy are asked for in the MN / RN, say under exam conditions ?
[/ QUOTE ]
You won't believe this. But I never remembered sitting a Sextant altitude question in all my MN exams. It was down to you to learn at sea. College Navigation was more chartwork related.
MN training was a mix of on-board required work and correspondence backed up by College periods to fill you with all the Spherical Trig Maths stuff, Ships Construction etc. ...
But mine was quite a few years ago.
bilbobaggins
N/A
[ QUOTE ]
"......We use a three arm protractor to plot the position; this is a very fast and simple operation and vastly preferable to plotting compass readings on to the chart. Three arm protractors might be hard to come by, but worth their weight in gold....."
[/ QUOTE ]
I share your thoughts on the value of this. Some might find it useful to have a 10" Douglas protractor to use as a 'Station Pointer'; faster, cheaper and much lighter. Oh, and much more readily available than the now-scarce and expensive Ship's Station Pointer...
Instructions are inside the cardboard protective case.
"......We use a three arm protractor to plot the position; this is a very fast and simple operation and vastly preferable to plotting compass readings on to the chart. Three arm protractors might be hard to come by, but worth their weight in gold....."
[/ QUOTE ]
I share your thoughts on the value of this. Some might find it useful to have a 10" Douglas protractor to use as a 'Station Pointer'; faster, cheaper and much lighter. Oh, and much more readily available than the now-scarce and expensive Ship's Station Pointer...
Instructions are inside the cardboard protective case.
Sgeir
Well-Known Member
calamitys38
Well-Known Member
This is a real humdinger of a question to answer as there are so many variables involved. However, I can confirm that the answer is expected to be within 1 mile. Having said that, due to rounding errors etc made in the calculations, sometimes the answer can exceed this minimum criteria, yet still be marked as correct if the methodology was correct in principal and it can be clearly seen why there is a difference from the students answer to the staff answer.
In another subject area - secondary port Tides for example - we would expect the answer to be calculated to within +/- 5 mins and +/- 0.3m. However, we would accept an answer outside these parameters if we could see why (e.g. can't draw a straight line on the tidal curve graphs, or a minor mathematical mistake had been made (classic 2+2=5 sort of thing)), although the student in this instance would be penalised with an error of 10% of the marks being docked. If however, the student perfectly calculated the times and heights of a secondary port, but had looked up the wrong port on the wrong day in the wrong month (or any combination) they would be awarded a grand total of ZERO. Confusing eh!!
Not sure if this helps and almost impossible to answer on this forum as there can be some subjectivity in how/why marks are docked, based around have the students clearly shown they understand what they are doing and just had a 'brainfart' or, is it a more serious error on their part!! /forums/images/graemlins/confused.gif
In another subject area - secondary port Tides for example - we would expect the answer to be calculated to within +/- 5 mins and +/- 0.3m. However, we would accept an answer outside these parameters if we could see why (e.g. can't draw a straight line on the tidal curve graphs, or a minor mathematical mistake had been made (classic 2+2=5 sort of thing)), although the student in this instance would be penalised with an error of 10% of the marks being docked. If however, the student perfectly calculated the times and heights of a secondary port, but had looked up the wrong port on the wrong day in the wrong month (or any combination) they would be awarded a grand total of ZERO. Confusing eh!!
Not sure if this helps and almost impossible to answer on this forum as there can be some subjectivity in how/why marks are docked, based around have the students clearly shown they understand what they are doing and just had a 'brainfart' or, is it a more serious error on their part!! /forums/images/graemlins/confused.gif
maxi77
Well-Known Member
[ QUOTE ]
[ QUOTE ]
[ QUOTE ]
That's why you have to subtract a correction for height of eye!
------------------------------------------------------------------------
Thats an adjustment, which, is made because it assumes you are at the centre of the Earth, when taking your sights.
[/ QUOTE ]
Well when I was taught astro it was to correct for the height of eye ebove sea level so that you actually measured the elevation above the horizontal. It is a correction that does not need to be added when using an artificial horizon sextant because the elevation is already being measured against the horizontal.
[/ QUOTE ]
BONG ! The corrected altitude is actually referenced to centre of earth - NOT horizontal.
In effect HoE / Dip correction can be said to be average radius of earth + your eye height above sea-level applied to the observed altitude to obtain referenced to CoE in plain terms.
You could of course look at this :
http://physics.gmu.edu/~hgeller/astr402/...served Altitude
/forums/images/graemlins/grin.gif
[/ QUOTE ]
Bong Bong
The presentation you so kindly referenced quite clearly states that 'DIP' is the correction which is made to correct the observed altitude of the sextant for the true rather than observed horizon, a correction which is clearly not required if some form of artificial horizon is used.
The three corrections applied before calculations start once again given in the presentation you referenced are Index Error, Dip, and Altitude Corection which corrects for the differing refraction of the light from the observed body depending on it's path through the atmosphere.
[ QUOTE ]
[ QUOTE ]
That's why you have to subtract a correction for height of eye!
------------------------------------------------------------------------
Thats an adjustment, which, is made because it assumes you are at the centre of the Earth, when taking your sights.
[/ QUOTE ]
Well when I was taught astro it was to correct for the height of eye ebove sea level so that you actually measured the elevation above the horizontal. It is a correction that does not need to be added when using an artificial horizon sextant because the elevation is already being measured against the horizontal.
[/ QUOTE ]
BONG ! The corrected altitude is actually referenced to centre of earth - NOT horizontal.
In effect HoE / Dip correction can be said to be average radius of earth + your eye height above sea-level applied to the observed altitude to obtain referenced to CoE in plain terms.
You could of course look at this :
http://physics.gmu.edu/~hgeller/astr402/...served Altitude
/forums/images/graemlins/grin.gif
[/ QUOTE ]
Bong Bong
The presentation you so kindly referenced quite clearly states that 'DIP' is the correction which is made to correct the observed altitude of the sextant for the true rather than observed horizon, a correction which is clearly not required if some form of artificial horizon is used.
The three corrections applied before calculations start once again given in the presentation you referenced are Index Error, Dip, and Altitude Corection which corrects for the differing refraction of the light from the observed body depending on it's path through the atmosphere.
anniebray
Well-Known Member
Just an aside re Stokey Woodall (He's the chap who organises the practical sextant course on the Portsmouth-Bilbao ferry) I attended an offshore club evening where Stokey was guest speaker. He was hilarious, informative & extremely entertaining. Invite him to your club !
Mariner69
Well-Known Member
[ QUOTE ]
"BTW what standards of accuracy are asked for in the MN / RN, say under exam conditions ?
[/ QUOTE ]
If I correctly recall, the exam questions all had fixed answers to 0.1 of a Mile. The answers were marked on a basis of deducting 50 percent of the marks for an error in principle and 10 percent for a clerical error. Since the pass mark was around 90 percent for the paper you could not afford to make any errors in principle.
In use at sea, with the old rated chronometers accuracy of 2 miles would be considered good. This is no problem when the nearest land is some 5 miles below your keel /forums/images/graemlins/smile.gif
The sextant was also good for horizontal angles and distance from lighthouses using vertical angles. Also good for operating two vessels together closing from a quarter of a mile to 25 yards away and out again!!
For fast and precise navigation in tight areas we would prepare a plot of angles on the chart to allow quick reading.
"BTW what standards of accuracy are asked for in the MN / RN, say under exam conditions ?
[/ QUOTE ]
If I correctly recall, the exam questions all had fixed answers to 0.1 of a Mile. The answers were marked on a basis of deducting 50 percent of the marks for an error in principle and 10 percent for a clerical error. Since the pass mark was around 90 percent for the paper you could not afford to make any errors in principle.
In use at sea, with the old rated chronometers accuracy of 2 miles would be considered good. This is no problem when the nearest land is some 5 miles below your keel /forums/images/graemlins/smile.gif
The sextant was also good for horizontal angles and distance from lighthouses using vertical angles. Also good for operating two vessels together closing from a quarter of a mile to 25 yards away and out again!!
For fast and precise navigation in tight areas we would prepare a plot of angles on the chart to allow quick reading.