Taking Multiple Sun Shots

[barnakiel]

Back in the celestial days we used Tamayas (calculators, not only sextants). Alas, over time I got addicted to simplified printed tables that may give me up to 5 Nm of error but are superbly fast and easy to use.

In fact, though slowly, I can still calculate the line from sine / versine / haversine and logarithm tables and that's the beauty of it: look ma: no electronics, no toys, just the Sun, the sextant, the tables and the pensil!!!

b.

[bajardine]

Wow! I’m thrilled by all the responses I’ve gotten. This is great. I’ve read a lot of posts on celestial nav so far so I love all the replies. When I do end up going offshore, I’m sure I’ll have several GPS’ onboard. A plotter, a handheld, etc. I imagine I’ll even tuck a hand held away in a metal box for safety. I still feel the need to learn astro nav though. There’s a lot of attraction to the sea and the idea of navigating by the sun. moon, planets, and stars makes it even that much more appealing. Don’t get me wrong though, when I’m on the eve of making a landfall, I’ll be following my GPS with whatever confirmation I can get from the stars.

Jimbo - you hit the nail right on the head. That's what I was trying to say. How many shots do you take and then average them?

I just recently bought a used Davis mark 25 sextant to start learning with. I know the limitations to the plastic sextants as mentioned everywhere. I know they won't be as accurate as an aluminum or brass one but I didn't want to spend $4-500 dollars yet on an Astra IIIb. Those are what are catching my eyes these days. I like what I've seen of them so far. If anyone has any cons against them, I'd like to hear them.

The first day I took a couple of sights but only worked up one sight from morning, noon, and afternoon. My triangle was way too big. I was really disappointed. I bought the book "Celestial Navigation in the GPS Age" by John Karl. In it he shows taking multiple shots and plotting them out to see if you have a personal error or if your errors are random and therefore self cancelling. By taking the average of several shots, your accuracy will increase.

Roverhi mentioned I may be obsessing over something that is not that accurate on a small moving boat. Not obsessing, just trying to learn through practicing. My goal is to be able to get a good fix every time with 3-4 shots and averaging them. My hope is that if I can get a good fix of 1-2 nm every time from my driveway, I should be able to get accuracy of 4-5 nm at sea given decent conditions.
Jim Cate mentioned an old DOS program that would show the outliers in your shots. That’s was I was trying to do with the sheet I made in Excel. Putting in the times and elevations, it plots them, averages them, and shows which shots are way off. Using this on my second day of sun sights, I was able to plot a fix where the center of the triangle was 2nm from my actual position. I was quite pleased with this.
Right now I’m contemplating which is the next book I’m going to buy to further my studies. So far I’ve thumbed through “The Complete Onboard Celestial Navigator” and have read most of Mary Blewitt’s “Celestial Navigation for Yachtsmen”. I’ve also read most of the book by John Karl. Mary’s was the easiest to understand and was the one that made it all click for me so far. From there, everything I’ve read from the other books is adding to it. I really like John Karl’s treatment of calculators and computers. So far I’m taking my shots, averaging them, do the sight reductions using the tables, and then I check what I have against the formulas using my scientific calculator. So I guess here is my next question: Which book would serve me the most, How To Use Plastic Sextants By David Burch or The Sextant Handbook by Bruce A. Bauer ? The first is an ebook sold and recommend by the Davis website. The Sextant Handbook is one I’ve looked at on Amazon. Has anyone read either of these two books?

[S/V Alchemy]

I have a Freiberger Yachtsman, a Davis 25 and an Astra IIIB. I think I like the Astra and the Freiberger equally, but I purposely got them in different "styles", one is whole horizon and the other is split mirror, so my opinion varies with my needs and the conditions...each style has its strengths. The Davis is mainly for my kid, because it's light enough for kid arms to keep steady. It's cruder, but not obviously so. Don't let it get warm.

The only issue with the Astra is potential "operator error" in that if you leave the battery for the night use LED in the handle too long, it will go bad and leak onto the contacts. So change it once a year whether you need to or not!

I've read two Blewitt books and a couple of others. I use an almanac, Table 5 and HO 249 as needed, and use a simplified plotting chart that basically requires only addition and subtraction. I still own a venerable Tayana calculator for doing the whole thing in trig, but I can't say I recall the method without cracking a book! The calculator comes in a lovely dovetailed wooden box quite similar to the Astra box.

[boasun]

When I worked as a 3rd officer on the Naval ships (civilian crewed), we were required to obtain at least 3 sun lines and if the LAN (Local Apparent Noon) fell on your watch catch that one also. Only took one sighting per line... Didn't do any averaging. So on a four hour watch, we were pretty busy, from looking out and radar watch, supervision of the non-rates. Navigation was done also. Caught a Sun line after coming on watch, then one in mid watch and one near the end of watch and advance them all into a running fix. The LAN could fall at the end of the 8-12 or at the beginning of the 12-1600 watches. But the Deck officer on the 8-12 had to do the math to figure when the vessel will have an LAN.

But if you are on a slow moving Sailing vessel. And far off land masses you could get away with a no-sextant sight at Sunrise and one at Sunset. The noon sight you will need to use a sextant. The sunrise and sunset will give you longitude and the Noon sight will give you lattitude. And advancing those sights will give you a running fix, which allows you to do Dead Rectioning; 1. to determine where you are going and if there are any currents that are pushing you into Nav hazards.

Of course now days: You have GPS, AIS, Radar and Electronic Chart Displays. But only a fool depends on GPS alone.

[alan2]

Originally Posted by S/V Alchemy

Celestial's valuable as long as batteries die.

It is also valuable because there's a load of math (although this can be greatly simplified to just addition and subtraction) and good technique to master. It is a routine activity, like morning sit-ups, that yields better results over time, and it is also a mental discipline that can be an important part of keeping your marbles rolling right on long trade-wind passages.*

*Ever notice how everyone stops chatting about a week in?

Batteries, sooner or later, will always die, or as Murphy offered, "anything that can go wrong will".

[Noname]

Celestial was easy. Figuring out how to do an eye splice in double braid warps my simple mind???

[alan2]

Originally Posted by bajardine

Hi, how many sextant shots do you guys take each time? I've started learning celestial navigation and at first didn't get a very good triangle when I plotted my LOPs. Yesterday I took lots of sights and put the times and altitudes into an Excel file to both average my shots and so I could plot them and see if my errors were systematic or not. I took late morning, noon, and early afternoon sun sights in my driveway and ended up with a pretty good fix. I'm attaching my plotting sheet. So I thought I would put that question out there. How many sights do you take each time? I'm also attaching a picture of the file in Excel that shows each shot. I also measure sextant index error by touching lower sun limb of the reflected image to the upper limb of the sun and reverse it. I do this 4-5 times to get my index error. I take the reading of each sight for index purposes and put it into the Excel file as well. I then use this average to get my Ha by subtrating this error from the average altitude reading and then divide in half.

Your graph of shots looks pretty good to my uneducated eye.

Otherwise, shooting morning and afternoon sun lines from beach, I'm not a sailor, or moon and sun shots, I usually will shoot 5 lower limb, and 5 upper limb sun lines. When the fates smile on my efforts as opposed to laughing sardonically, my intercepts are inside 5 nm. With sun/moon shots, I will do 5 sun shots, upper or lower limb, then 5 moon shots, using whichever limb is available. Results are usually as above noted, within 5 nm, sometimes within 3 nm, though sometines things just seem to go to hell.

Push coming to shove, one of these days, I might return to a previously pursued brand of insanity, long range (1000 yard) rifle shooting. That too being something that might drive one to drink, hopefully good scotch, though some lean toward bourbon.

Noli bastardus carborundum, excuse my rotten latin.

[Tia Bu]

Used to take 5 shots for each line, plot them on graph paper, and then take the one lying closest to the best straight line I could draw through them. Took a LOP one hour before noon, a noon sight, and and LOP one hour after noon. Then advanced the earlier two to the last LOP for a running fix.

A faulty or banged-up plastic sextant can also yield too large a triangle. Set the sextant to 0 degrees, look at the horizon, and rotate the sextant back and forth through 180 degrees. The horizon should appear as an unbroken, straight line throughout the rotation. If not, the mirrors are out of whack.

[Noname]

Originally Posted by alan2

Your graph of shots looks pretty good to my uneducated eye.

Otherwise, shooting morning and afternoon sun lines from beach, I'm not a sailor, or moon and sun shots, I usually will shoot 5 lower limb, and 5 upper limb sun lines. When the fates smile on my efforts as opposed to laughing sardonically, my intercepts are inside 5 nm. With sun/moon shots, I will do 5 sun shots, upper or lower limb, then 5 moon shots, using whichever limb is available. Results are usually as above noted, within 5 nm, sometimes within 3 nm, though sometines things just seem to go to hell.

Push coming to shove, one of these days, I might return to a previously pursued brand of insanity, long range (1000 yard) rifle shooting. That too being something that might drive one to drink, hopefully good scotch, though some lean toward bourbon.

Noli bastardus carborundum, excuse my rotten latin.

I think you meant: "non illegitimi carborundum." Sorry but I had to sit through 3 years of the stuff in jr. high school.

[boatman61]

I agree Celestial should be encouraged...
I learnt the basics in the RN then did an Ocean YM Theory in the 80's to refresh...

Having said that I've never used it since...
On the one ocassion I lost GPS I had my 0600 position to work from, a good Atlantic Pilot chart and charts of Europe/UK...
Managed 1200 miles on dead reckoning and found Falmouth (lucky ?)...
timing was out as I was over estimating speed/position... I was 2 days further away than my calculations allowed... oh... and assistance from a small passenger plane doing the Scily Isles run... he confirmed my course after buzzing me a few times...lol
The advantage today is if you have proper charts and a compass, log and depth sounder one can navigate the Coast and Oceans successfully by dead reckoning alone..
the landmarks/knowledge/info availability is so much greater than 100yrs ago...
But if your NOT a lazy sod who hates routines... Learn Celestial...

[sneuman]

How can you take only one noonsight? You need several to determine local noon - which is the whole point, is it not? Or do you just take one sight, get an LOP and cross it with your DR position?

I'm an amateur at celestial, so correct me if I'm wrong - but when I took noonsights in practice, I began taking a series just before and just after my estimated local noon to determine the apogee of the sun and therefore your noon fix.

[SaucySailoress]

Ahoy there, I'm back!! (Bet you never noticed I was gone, did you?)

Thank you, bajardine, for starting such an interesting topic, and welcome aboard. This is one thread I'll bookmark for future reference... Having learnt to sail to distant islands (30km offshore) with a diving compass and laminate copies of charts on my beachcat; and graduated to GPS on account of being a modern chick who likes to move with the times, I look forward to finding the right sextant one day, and perhaps even learning to use it. Bet HWMO will get the hang of it first, mind!

[nigel1]

Originally Posted by sneuman

How can you take only one noonsight? You need several to determine local noon - which is the whole point, is it not? Or do you just take one sight, get an LOP and cross it with your DR position?

I'm an amateur at celestial, so correct me if I'm wrong - but when I took noonsights in practice, I began taking a series just before and just after my estimated local noon to determine the apogee of the sun and therefore your noon fix.

If you know the precise GMT, you can get away with one sight at noon, Meridonal Passage. You just need to pre work when MP will take place, and be ready to grab the sight as the sun hits the high spot. Knowing the precise time of MP removes the guess work of when the sun is at max altitude

[Noname]

Originally Posted by nigel1

If you know the precise GMT, you can get away with one sight at noon, Meridonal Passage. You just need to pre work when MP will take place, and be ready to grab the sight as the sun hits the high spot. Knowing the precise time of MP removes the guess work of when the sun is at max altitude

Its been a while since I did the celestial sight reductions, but, don't you have to know your exact longitude to know the time of meridian passage?? That is the reason for a few sights prior to and after meridian passage, isn't it? Its been 25 years but I thought that was the procedure back then.

[nigel1]

Originally Posted by Seaduction

Its been a while since I did the celestial sight reductions, but, don't you have to know your exact longitude to know the time of meridian passage?? That is the reason for a few sights prior to and after meridian passage, isn't it? Its been 25 years but I thought that was the procedure back then.

If you have a good enough DR longitude, then using the chronometer should be OK.
A sun sight early morning can help in establishing a good DR Long.'
Back ever such a long time ago as third mate on clapped out cargo tramp boats, where sights were about the only means of establishing a position, the routine was star sights at twighlight, morning and evening, then a few sun sights during the morning, Mer Pass at noon, and then perhaps another couple of sun sights in the PM. All these helped to keep a good DR going.

In most cases, waiting until the sun reaches max altitude should be good enough, but most likely difficult to ascertain accurately from the deck of a small boat, so the use of sights before and after noon is good practice