Practical limits observing Polaris

My question is polling at sea experience and may be in the wrong category. Please move it or guide me to movebit to the correct category.

My celestial nav experience did not involve taking any Polaris sights near the equator. I beg of you who have used the north star near 0° latitude I pose a question: what are the practical limits of Polaris near the equator? Given severe clear conditions, is it visible or useable at 0°? 5°N? 10°N?

What about the Southern Cross? Same questions except for south latitudes?

I thank you for delving your experience(s).

Karl

If something is 90 degrees from your zenith it is on the horizon and unless it is the sun you are not going to see any sign of it.

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I appreciate your respnse so quickly and suspect you are right. So how close to the horizon did you actually use a star? Any star?

General rule of thumb is anything below 15 degrees is not used.

I don’t know what the celestial "priesthood’ would say about it.

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I am far from a priest, maybe a junior altar boy at best, but I recall refraction errors get increasingly large close to the horizon.
This was part of a thought exercise about taking a sun-sight without a sextant, yes you can see the lower limb of the sun kiss the horizon with the naked eye, but the errors can be large right there.

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If you only want to know, where the true geographic North is, then even a very low Polaris will tell it; here, the refraction does not matter.
However, if you will calculate your geographical position, the refraction at low altitudes will produce not controllable errors.

Astronomers use a mean refraction of -50 ArcMinutes, e.g. for the prediction of sunrises.
However, the air’s humidity or the dust floating in the air may influence the real local refraction.

From the ship to the local horizon, you could know the air composition just above the sea.
Then, the sight line continues a long grazing way up beyond the stratosphere, crossing different layers. The consistence of these layers, with their vertical borders influencing the refraction, is impossible to know on the ship.

In rare cases, the refraction may be even inversed.

I’ve had to do zero altitude sun and moon sights with no sextant when the GPS died. The result was within 15 miles of our DR, and confirmed by a passing ship the next day. Mid-ocean (or even approaching coastal waters) that is accurate enough.

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As already said. Seeing Polaris at or near the equator is less than likely. It would have to be exceptionally clear. It tends not to be exceptionally clear. Near the equator.
I would watch for it disappearing heading south or appearing again heading north.
I’d also watch for the Southern Cross.
Lat by Polaris was something I rarely bothered with. I did occasionally just because why not. Particularly when I was learning and getting ready for D.O.T
Planets or Moon same same.
I prefered to focus on getting 3 or 4 good obs of the best selected stars. Avoiding those with Alt less than 20 or more then 80. Preferably between 30 and 60.
The advantage of a Latitude not being so great if you got a good 3 way cross.
And we would get a good latitude at noon anywho.
The closer you are to the Equator the shorter twilight is and less time you have.
I did take the Sun at or near the zenith when I could for fun it worked quite well.
Often depending on where i was and weather at the time i just got what i could get. If i could get anything at all.
Planets were worth taking when there was high thin cloud cause they are bright. Otherwise I didn’t bother.
Ocasional if things worked out I might try for a planet in the morning with a obs of the sun. but like the moon if i could get a planet it was usually a crap cross cause it would be almost opposite the sun.
But sometimes it passed the time.
On rare occasions I took stars on nights with a bright moon. They used to say it was unreliable because the horizon was not right. Buggered if I could think of or ever heard a good explanation of why. Although air temp at night might affect refraction, which it does during the day anyway.
It was 40 odd years ago.
I still did routine observations up until I quite deep sea. 34 years ago.
I doubt if anyone actually bothers now.
When the opportunity arises, I do occasionally dust off the old sun gun and take a shot or two to show a cadet how its done. its all still part of the syllabus and exams.
Same with compass errors. Mostly use transits but pull the binnacle cover off and dust of the azimuth mirror occasionally.
These days just to show a cadet someone going for a cert.

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Not recommended transpacific as lots of reefs.

Taking stars non a cloudy night because that was all you had a shot of Polaris was always an option. I have used a meridianal pass of Jupiter to confirm a DR around Indonesia when radar was primitive.
Taking stars at night was practised by Subs in times gone by. The altitude was observed by the navigator who was blindfolded while another officer read the altitude. The time and altitude being recorded in the control room. The navigation officer spent 20 minutes adjusting to the light level.

Looking at the altitude correction tables the situation can be better understood.

The correction is non-linear of course so there are two tables, one for 90-10 degrees and a second for 0-10.

I’ve never tried shooting stars below 15 degrees that I recall but just looking at the table the corrections below 10 start increasing quickly and would be less reliable.

I think there is such a thing as a night vision scope on a sextant to find a dark horizon.
Also you can dig up an old aircraft sextant with a bubble level:

A mariner can use the horizon as this line of reference, but when an airplane is above the clouds or flying at night, its navigator can’t see the horizon. The bubble sextant solves this problem by providing an artificial horizon. It takes its name from an air bubble in a liquid-filled chamber that functions like a carpenter’s level, indicating when the sextant is aligned horizontally. When I look through the eyepiece of my sextant, I locate a star and, with a drum on the side of the instrument (like a camera’s focus ring), adjust the angle of a rotatable prism until the star showing in the eyepiece is aligned beside the bubble. The prism and drum are geared to circular scales, marked off in degrees. From these scales I read the star’s altitude.

  • is there some reason this won’t work on a ship? I am not sure, never have I used one.

A bubble sextant is difficult to obtain any useable accuracy without a large number of readings and taking the mean using assistants for time and recording altitude . The acceleration of the bubble is the problem with the ships motion. I vaguely recall that the US navy did trials before WWII on a battleship and the roll period is somewhat ponderous.

I have and the roll of any ship of the sea is way different than the motion of any airplane. Averaging works in the air for a bubble sextant but not on the ocean. Plus, those things are HEAVY! Even the little circular very light ones get tedious for a two minute sight. Hogsnort is right.