Thorny subject: Sextant and GPS era

[ZULU40]

Originally Posted by captmikem

It appears many people believe one can just unbox a sextant and get a position.

That is possible for a noon site, which will only give you latitude, not a position.

longitude

[robert sailor]

First of all there are only a hand full of sailors crossing oceans as a form of relaxation and only a few of them might have actually learned how to take sights and how to manually do sight reductions. Its very unlikely that youll see any mount of sailors that will go to the trouble of actually learning celestial navigation, get the tables,get the paper for drawing LOP's and buy the paper charts. Its a real commitment that very few will ever actually do. My Mom used to say " talk is cheap, it takes money to buy whisky "

[captmikem]

Originally Posted by ZULU40

------ Originally Posted by captmikem View Post
It appears many people believe one can just unbox a sextant and get a position.

That is possible for a noon site, which will only give you latitude, not a position.-----

longitude

Longitude?
No, a noon site gives one LATITUDE.

For many years before accurate time was able to be kept, sailors “ran down their latitude”. They could find their latitude with either a pole site (Polaris, the pole star) or a noon site, (which basically means they use their octant (predecessor to the sextant) to follow the sun up around noon, then at LAN (Local Apparent Noon) the sun appears to stop climbing for a few seconds, then starts down. That angle is then subtracted from 90 and gives what is called Zenith Distance (ZD) this number is then added to or subtracted from Declination (the sun’s latitude, remember the sun appears to move north and south as the year progresses) which gives your latitude. Sailors would know the latitude of their destination off the chart, sail to this latitude, and then head east or west staying on that latitude until it was raised on the horizon.

Think of this…
You are in a field with a telephone pole. You use your sextant to measure the angle from the bottom of the pole to the top of the pole. At the distance you are from that pole you could move in a circle around that pole and the angle would remain the same. This is called a “Circle of equal altitude” . This is basically what you are measuring anytime you take a sight with your sextant. It gives you a Circle of Equal Altitude on the earth based on the body you are shooting. You are somewhere on that circle, which given the scale of the earth appears as a line on your chart, which is called a Line of Position (LOP).

So with the sun at It’s Zenith to you (Highest point), knowing it’s latitude (declination) you do the simple calculation of subtracting your observed angle of it from 90 (the angular difference between the equator and the pole) and adding or subtracting the sun’s latitude (adding if it is in the opposite hemisphere from you, subtracting it if it is the same) gives you your latitude. (If the sun did not move, and was always at the equator, you would not have calculate in the sun’s declination).

Just a note here… with accurate time, we can find circles of equal altitude for bodies at any time, like having more telephone poles in the field. Where these circles of equal altitude (lines of position) cross, is where you are at!

I hope this helps understand what is going on in Celestial Navigation.

M

[captmikem]

SIGHT... not site.. been around computers too long...

M

[double u]

...I didn'r write that arriving without chart was smart...

[double u]

Originally Posted by ZULU40

longitude

OMG!!!

[ZULU40]

Originally Posted by captmikem

Longitude?
No, a noon site gives one LATITUDE.

For many years before accurate time was able to be kept, sailors “ran down their latitude”. They could find their latitude with either a pole site (Polaris, the pole star) or a noon site, (which basically means they use their octant (predecessor to the sextant) to follow the sun up around noon, then at LAN (Local Apparent Noon) the sun appears to stop climbing for a few seconds, then starts down. That angle is then subtracted from 90 and gives what is called Zenith Distance (ZD) this number is then added to or subtracted from Declination (the sun’s latitude, remember the sun appears to move north and south as the year progresses) which gives your latitude. Sailors would know the latitude of their destination off the chart, sail to this latitude, and then head east or west staying on that latitude until it was raised on the horizon.

Think of this…
You are in a field with a telephone pole. You use your sextant to measure the angle from the bottom of the pole to the top of the pole. At the distance you are from that pole you could move in a circle around that pole and the angle would remain the same. This is called a “Circle of equal altitude” . This is basically what you are measuring anytime you take a sight with your sextant. It gives you a Circle of Equal Altitude on the earth based on the body you are shooting. You are somewhere on that circle, which given the scale of the earth appears as a line on your chart, which is called a Line of Position (LOP).

So with the sun at It’s Zenith to you (Highest point), knowing it’s latitude (declination) you do the simple calculation of subtracting your observed angle of it from 90 (the angular difference between the equator and the pole) and adding or subtracting the sun’s latitude (adding if it is in the opposite hemisphere from you, subtracting it if it is the same) gives you your latitude. (If the sun did not move, and was always at the equator, you would not have calculate in the sun’s declination).

Just a note here… with accurate time, we can find circles of equal altitude for bodies at any time, like having more telephone poles in the field. Where these circles of equal altitude (lines of position) cross, is where you are at!

I hope this helps understand what is going on in Celestial Navigation.

M

no? au contraire

otherwise, what is the point of knowing Greenwich MT
the suns orbit 360 degrees in 24 hours, 15 degrees per hour

finding the time of your local noon, which is relatively easy
take multiple readings of the sextant shooting the sun each side of noon noting the exact time,
which finds its highest altitude,
determines your local noon,
determines your longitude straight off the clock.
Easy peasy

the problem they faced was in finding an accurate enough chronometer that would hold its time after 6 months in various temperate zones. James Harrison was the clockmaker that conquered it

https://en.wikipedia.org/wiki/Longitude_by_chronometer

[double u]

minimum error for a first sight, never mind noon or otherwise, is 15 miles...

[double u]

in the "olden (pre-gps)days" you could recognize the boats navigating by noon-latitude by their route: no lowlying islands, only mountainous ones...
&btw: the actual taking of the sight is the same, local noon or not (if anything: the higher the sun the more difficult the taking of the sight is. the noon-latitude only saves on "maths" (if you really want to call it that...).
& by taking timed identical altitudes you CAN determine longitude...(Noon sight longitude - Ocean Navigator - November/December 2011 )but this isn't what was meant...

[Adelie]

Learning celestial navigation is not easy but neither is it especially hard. Nor is it particularly expensive. For about $125 you can put together the kit you would need to get home or into a repair harbor.

Taking 1 sight per week and reducing it should be sufficient to stay in practice for an emergency.

[captmikem]

ZULU40

Yes, an accurate noon sight with accurate time can be used to get an approximation of longitude. it was used in the 1800's when accurate timepieces became available. It is just not used nowadays as much more accurate sights are available. That is why a noon sight is also called a latitude sight.

M

[Jammer]

If all you want is longitude and you have time and a reasonable approximation of latitude, all you need is a sunset or sunrise time. No sextant required.

As I understand it, the point of a noon site is that you can get a line of position during the day, with a reduction that is relatively trivial.

[Jim Cate]

FWIW

Back when we were voyaging using only celestial, I did noon sights that gave fairly good longitude results. Did the usual series of sights approaching and leaving noon and then entered the data into a little program I wrote for my trusty HP-41. The routine did a least squares fit to a parabola and gave a surprisingly good estimation of the peak of the parabola. This, coupled with accurate time (cheap quartz watch plus WWV) allowed a surprisingly good longitude result as well as a good latitude... ie, a fix!

Long time ago... I'd be hard pressed to get that back up to speed today!

But the fallacy of the simple "easy peasy" routine described upthread is that when taking sights from the heaving deck of a small boat with a height of eye of maybe 7 feet, accurately capturing that magic moment where the "sun stands still" is simply beyond human capability... at least for this human!

Jim

[double u]

Jim, the way the krauts do it is:
take a sight shortly before noon & time the same altitude after noon - noon is exactly in the middle between these two moments (that is of course if you don't move...otherwise: corrections)
what the whole thing is for I cannot imagine: the difficulty is taking an exact sight from, as you say"heaving deck" - this requires practise. the calculations are easy, the most difficult thing being how many minutes (of arc) to add to get a full degree...

[double u]

reading again a passage in John Kretschmers "Cape Horn to starboard" I came across:

“All this talk of stars may seem obsolete with the dawning of electronic navigation. Even line-of-position celestial navigation is a dying art. I suppose the sextant is an evolutionary dinosaur compared with an instanteneous, talking SatNav (then…), but it seems a pity to me. Celestial navigation is more than just finding your way across an ocean, it’s a communion with nature. I have spent hours studying the sky, waiting for a brief glimpse of the sun. Landfalls are imminently more gratifying when you’ve used the sun, moon & stars to guide you. Do we really want to make offshore voyaging as effortless as possible? In the elimination of risks, do we also eliminate the rewards? I know that resistance to change is never the right tack, but at what point do we destroy the very purpose that has compelled us to take to the sea in the first place?”

This from a navigator with much more experience than most (if not all) of us.
Remains to add:
These rewards cannot be recaptured any more, they are gone (on purpose voyaging without a GPS seems very foolish even to me…). The closest you can come is by learning & practicing this “old art” & seeing how close you come to the GPS’ positions. I guarantee you, that you will NOT regret it, never!