Question for the classically trained Astro navigators; when Dec approaches Lat

Skylark

Skylark

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Just musing while under house arrest ?

Is there lore regarding sun sights as Dec and Lat approach the same number?

A sun-run-sun becomes unobtainable, what forenoon / afternoon PL intersect angle is deemed acceptable?

Similarly for MP, what difference between Dec and Lat is generally considered minimum to get an accurate sight?

Is there a time when the sun becomes unusable so other heavenly bodies are commonly used to obtain a fix in waters to 23 deg N and S?
 
The Position Line is a circular arc perpendicular to the azimuth of the Sun or body - so makes no difference at all if Dec = Lat ....

Its only due to scale we work at - that we draw the PL as a straight line.
 
Skylark said:
Just musing while under house arrest ?

Is there lore regarding sun sights as Dec and Lat approach the same number?

A sun-run-sun becomes unobtainable, what forenoon / afternoon PL intersect angle is deemed acceptable?

Similarly for MP, what difference between Dec and Lat is generally considered minimum to get an accurate sight?

Is there a time when the sun becomes unusable so other heavenly bodies are commonly used to obtain a fix in waters to 23 deg N and S?
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Yup. Cant be done! I can give you a 'staff answer'.

When the suns declination and the observers latitude are the same, the sun will rise due east and set due west. Therefore all am and pm sights will give an LOP that runs N/S. With no difference in the LOP's angles, a running fix is not achievable.

A noon sight is all but impossible too with the sun directly overhead. Ive actually tried this. You have to be careful not to fall over backwards! You just dont know which part of the horizon to use.

Put the kettle on and do some star sight planning!
 
Noon sight for lat (Sun ALT close to 90° if lat and Dec are nearly the same), morning and afternoon sights for long.
Calculate using long by chronometer. The only position lines you have to plot are the actual lat and long.
Shouldn't be any problem no matter what lat and Dec are as long as you can see the sun a reasonable height above the horizon.
 
If the sun is nearly overhead just face due south or north as appropriate, sit down and lean against the mast.
 
Ha Ha ....

"When the suns declination and the observers latitude are the same, the sun will rise due east and set due west. Therefore all am and pm sights will give an LOP that runs N/S. With no difference in the LOP's angles, a running fix is not achievable. "

Assuming you do not move - this will provide an inaccurate longitude .....

'A noon sight is all but impossible too with the sun directly overhead. Ive actually tried this. You have to be careful not to fall over backwards! You just dont know which part of the horizon to use."

Having had this situation ... I can honestly say that I never even been close to falling over backwards !! A Sextant has 'back-angle' capability. Second because you already know your expected MP result ... you know you will be traversing a larger portion of horizon ...

Last - the likelihood of the scenario is rare anyway .... so I return to my original answer in post #2
 
capnsensible said:
Yup. Cant be done! I can give you a 'staff answer'.

When the suns declination and the observers latitude are the same, the sun will rise due east and set due west. Therefore all am and pm sights will give an LOP that runs N/S. With no difference in the LOP's angles, a running fix is not achievable.

A noon sight is all but impossible too with the sun directly overhead. Ive actually tried this. You have to be careful not to fall over backwards! You just dont know which part of the horizon to use.

Put the kettle on and do some star sight planning!
Click to expand...

The Running fix in this situation using non meridional sights is not suitable - but a MP against any morning / afternoon sight is acceptable ..
 
Refueler said:
Ha Ha ....

"When the suns declination and the observers latitude are the same, the sun will rise due east and set due west. Therefore all am and pm sights will give an LOP that runs N/S. With no difference in the LOP's angles, a running fix is not achievable. "

Assuming you do not move - this will provide an inaccurate longitude .....

'A noon sight is all but impossible too with the sun directly overhead. Ive actually tried this. You have to be careful not to fall over backwards! You just dont know which part of the horizon to use."

Having had this situation ... I can honestly say that I never even been close to falling over backwards !! A Sextant has 'back-angle' capability. Second because you already know your expected MP result ... you know you will be traversing a larger portion of horizon ...

Last - the likelihood of the scenario is rare anyway .... so I return to my original answer in post #2
Click to expand...
OK, you know better than the RYA course compilers who've done it. Well done. Do let them know.

As I said, Ive tried a noon with the sun directly and I mean directly overhead. Cannot be done. I dont know what you mean by back angle in this case......the sun cant be 'higher' than 90 degrees. It was something Id read about and found myself in a position to try. So I put weight on my practical experience of doing it.
 
capnsensible said:
OK, you know better than the RYA course compilers who've done it. Well done. Do let them know.

As I said, Ive tried a noon with the sun directly and I mean directly overhead. Cannot be done. I dont know what you mean by back angle in this case......the sun cant be 'higher' than 90 degrees. It was something Id read about and found myself in a position to try. So I put weight on my practical experience of doing it.
Click to expand...

Give it a rest ... I passed all my Nav courses - Basic and Advanced to take effin great ships round the world ... 17years as a Professional Navigator before slinging my anchor ashore ...

If you don't know about back angle on a sextant - then you are lacking in one of the great attributes of the sextant.

The scenario of the near 90 deg MP is not a problem - because even if you face the opposite way to the MP horizon - the sextants range is able to go PAST the 90 into the 'back-angle' ... (Alan S post is based on that ...)
IF you find that - you stay as you are and NOT fluff about trying to get the correct horizon ... coz you will miss MP !

As to RYA ... I was until couple of years ago - consultant advisor to RATS as well as CA .....
 
capnsensible said:
As I said, Ive tried a noon with the sun directly and I mean directly overhead. Cannot be done. I dont know what you mean by back angle in this case......the sun cant be 'higher' than 90 degrees. It was something Id read about and found myself in a position to try. So I put weight on my practical experience of doing it.
Click to expand...

You’re winning in the credibility stakes for the moment (purely based upon the fact that your response echos my thinking). ?

For MP, how can you know when sun is at its Meriden if it’s altitude is largely unchanged all day ?
 
Refueler said:
Give it a rest ... I passed all my Nav courses - Basic and Advanced to take effin great ships round the world ... 17years as a Professional Navigator before slinging my anchor ashore ...

If you don't know about back angle on a sextant - then you are lacking in one of the great attributes of the sextant.

The scenario of the near 90 deg MP is not a problem - because even if you face the opposite way to the MP horizon - the sextants range is able to go PAST the 90 into the 'back-angle' ... (Alan S post is based on that ...)
IF you find that - you stay as you are and NOT fluff about trying to get the correct horizon ... coz you will miss MP !

As to RYA ... I was until couple of years ago - consultant advisor to RATS as well as CA .....
Click to expand...
Two possiblities then. Either you have not experienced the rare occasion the sun is directly r directly overhead at you local noon, in which case you must only agree. Or... bull poo. :cool:

I cant believe a pro navigator wouldnt know this.
 
Skylark, the sun's ALT starts at zero at dawn, climbs to a max of 90 at noon, then decreases to zero at sunset. You take your noon site a at the maximum
 
Skylark said:
You’re winning in the credibility stakes for the moment (purely based upon the fact that your response echos my thinking). ?

For MP, how can you know when sun is at its Meriden if it’s altitude is largely unchanged all day ?
Click to expand...
Ah, the suns altitude changes! It still rises and sets. But you simply cannot measure it when its directly overhead. Its neither north or south of you. This is pointed out in most training books.

It sounds like for those on 20 knot vessels, the am and pm cuts wont be a problem. But if you are bobbing along at 5 knots they most certainly are. Perhaps its something these 'big ship' navigators forget but people like Tom Cunliffe with a lifetime on small vessels understand when passing their knowledge on.
 
capnsensible said:
Two possiblities then. Either you have not experienced the rare occasion the sun is directly r directly overhead at you local noon, in which case you must only agree. Or... bull poo. :cool:

I cant believe a pro navigator wouldnt know this.
Click to expand...

Stop talking through your back *****

I HAVE been in the ~90 deg situation more than once and EXACTLY as I posted ....
 
If you found it impossible to take a noon sight with the sun exactly overhead how did you know it was exactly overhead?
 
capnsensible said:
Ah, the suns altitude changes! It still rises and sets. But you simply cannot measure it when its directly overhead. Its neither north or south of you. This is pointed out in most training books.

It sounds like for those on 20 knot vessels, the am and pm cuts wont be a problem. But if you are bobbing along at 5 knots they most certainly are. Perhaps its something these 'big ship' navigators forget but people like Tom Cunliffe with a lifetime on small vessels understand when passing their knowledge on.
Click to expand...

Ha Ha ....
 
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