Practicing celestial navigation at home

Professional Mariner Forum
#1

Dear all,
Greetings to you all, I am a physicist and sailor, and have been long fascinated by celestial navigation. I have studied the related chapters on Bowditch’s American Practical Navigator, understand well the theory behind it, and I am able to do the sight-reduction exercises with no difficulties.

However, I have never taken a sight. I would like to take advantage of this period where we are stuck at home :disappointed_relieved: to practice some sight reductions from home.

I live in a place where I cannot see the horizon. However, I can make an artificial one with a pan with a dark bottom filled with water (I have tried, and I can see in it a clear, beautiful reflection of the Moon), and I plan to purchase a sextant to practice sights with this artificial horizon.

Do you have any suggestion of a sextant model (new or second-hand) that I could purchase to this purpose? I am not looking for a very high-quality one, but for a model that is good enough to reproduce my correct position with these sights with a reasonable accuracty, i.e., a precision good enough to convince me that I am not getting that position by luck :stuck_out_tongue_winking_eye:. In other words, I would like to see and be convinced that the sight reduction is working!

Also, given a suggested model, with what accuracy do you expect that it can reproduce my position with such an artificial horizon?

Thank you for you help!
Joshua

#2

Don’t get a plastic yacht one, they are awful.
Get something like this: Nauticalia Captain Campbell’s Vernier Sextant | Force 4 Chandlery

If you are doing it in your garden then your life probably isn’t going to depend on it so you don’t need to spend mega bucks.

However, you must ensure that you apply the correct corrections to the instrument.
Most people forget about collimation.

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#3

That’s a small-size replica, 20x16 cm. From the seller:

Hello, Is this a practical working item that can be used for navigation purposes???.Mr A S Brace.

This Sextant is intended to be decorative. It is functional, but is not intended for serious use. There are instructions included.

#4

Sorry, couldn’t find ‘tongue in cheek’ emoji.
You’re a physicist and a sailor; make your own.

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#5

During the Second World War the Long Range Desert Group ( later formed into the British Special Air Service) used engine oil in a pan as an artificial horizon and a nautical sextant with good results. I’m sure you will be able to convert the sextant angle obtained to the true altitude.

#6

One of the sailing or cruising forums likely would be a better bet for info on this topic. Or just a google search.

This is from the third post on that thread.

I learned using a cheap $35 Davis sextant and an artificial horizon. Try using just a bowl of water as an artificial horizon instead of buying one. It’s a neat skill to learn.

As to accuracy don’t know about on land but beginning cadets learning on the ship generally get within 5 miles of the GPS position after a little practice.

#7

I remember a quite heated argument with the Chief Officer (I was 3/O, anyone remember them?).
He was insistent that my position be extrapolated to the MPP, the Most Probable Position.
I denied that this had any more relevance or accuracy to my initially derived position and that his position was just a guess.
Unfortunately, this was years before I had heard of, and could successfully prosecute the knowledge of, Schrodinger’s Cat.
This was also before the advent of Transit GPS so sextant nav was a quite a grown up enterprise.

#8

Wonderful project, Josh! Was a Navy Navigator that was refined by a couple of merchants!
Sextant: lotsa money? Tamaya. Huge lotsa money? Plath. Reasonable to start and get the idea? Davis Instuments. You may even find some of the Navy issue sextants fairly reasonable somewhere.

Books: Bowditch H.O.1 definitely. William Buckley wrote one just for lead keel sailors would be great. No way can you have too many on the subject but Navy Quartermaster 3 & 2 and QM 1 & Chief will give you lots of practical.

Artifical horizon: the floater is great but it doubles the altitude. A bubble attachment would be better. The best would be a tripod like for a surveyor with a sturdy adapter to clamp on the handle. I have never tried this so is just a “bright idea” that may be a bad idea.

Sight reduction: old style pencil and sight reduction form with H.O.229. There was an Aviation table (H.O.60 I think) that was easiest of all. Stay away from H.O.214 as it is limited and huge! I wonder if there are sight reduction “calculators” still available to take the drudgery out of sight reduction? Bet there are computer programs!

You still need some form of current Almanac but I am sure you know that. There are many considerations and compromises when doing stars on a sailboat that are more involved than from either bridge or main deck of a nice, big, stable ship!

Have fun! And remember: Co-altitude by any other name is Zenith Distance!

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#9

Used to do 7 sights a day;
3 in the morning for the run up to noon.
Noon, by the (corrected by the time signal but not adjusted) chronometer.
3 evening sights.
The Mate (C/O) did the stars which we ignored because he was obviously on a different celestial body to the one that the rest of us were on.
I preferred the Marq St. Hilaire method but others liked the ‘Long by Chron’ style.
Always got there.
I did see the post on here a couple of years ago that the USCG (is that the examining authority?) is bringing back celestial nav which is good.
Probably harder now with all the space junk that is floating around up there.
Anyway, I do remember the most important thing for accurate sextant navigation.
A 2B pencil to plot it.

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#10

Bleary eyed winding of the chronometer after the time check at 9. Take one sight, see how I felt about the sight and if OK work out DR for noon and move on to ablutions etc. The daily grind of the second mate of a tanker before there was anything to switch on.

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#11

Wow, @CROSSBOLT, @Hornblower and Hogsnort, thank you for your nice replies, your stories make me dream. Please post other celestial navigation related stories if you have some!

I just went for a Tamaya MS833 from 1998, I can always return it if I am not happy. But I am very excited about trying it! Any suggestions about some first tests, besides removal of index and side error, that I should make to convince myself that that the instrument is working fine?

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#12

Figure out what the included angle between two nav stars should be then super impose them with the sextant (just like bringing a star to the horizon). Compare.
Note: I have never tried this! Would have to study just to do the math!

Once you have the mirrors all adjusted sight the same star or very distant point of light. Should read zero altitude when super imposed.

The experienced merchies will have better methods than this I wager!

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#13

The angular distance “d” between two stars is:

Cos d = sin(DEC star1) x sin(DEC star2) + cos(DEC star1) x cos(DEC star2) x cos(RA star1 – RA star2)

Right Ascension RA in degrees, not in hours.
Try with stars up in the sky, near your horizon, the refraction may reach half a degree.

#14

Thank you for the suggestion, but I see an issue with correcting properly for the refraction of the light beams coming from the two stars.

In fact, when the altitude of a body with respect to the horizon is measured, there exist standard tables which yield the altitude correction due to refraction for a light beam travelling on the plane perpendicular to the plane of the horizon. So far, so good.

However, when one measures the angular distance between two bodies, the situation is different. Here, the plane which goes through the two bodies and the observer (P) is no longer perpendicular to the plane of the horizon: the tables above no longer give the right correction for refraction. This correction will depend on the relative orientation between P and the plane of the horizon.

Of course, this very same problem arises with the lunar-distance method , so there must be a practical way around it.

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#15

As for your last comment: thre is a delicate issue with refraction, see my response to @CROSSBOLT

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#16

Now, see here, boy! Who is the expurt here? I was a Navigator so that’s the “ex” part. A “has been” more than not. Have been known to “drip under pressure” so that’s the “spurt!” Har, har! Just could not resist!

Like Urs suggested, use stars above the horizon, preferably near zenith to minimize the refraction. My Tamaya corrections were all zero for the whole limb and the fixes pretty well showed that. I took great pains with the mirrors to minimize error. Like making sure one second magnitude star passed exactly through itself and when superimposed the vernier read zero.

#17

Thank you for your response, @CROSSBOLT ! Just to have an idea, what is roughly the minimal altitude of the two bodies to safely neglect refraction?:nerd_face:

#18

I would say whatever your neck and back could stand shooting two bodies directly opposite with equal zenith distances ( or nearly so).
One star for zero. Two each for included angles of 20, 45 and 90 if you wanted to prove the whole limb. Tamaya instrument error of zero for the whole limb valid and typical if it has not been dropped. You realize you will be the expert on this when you are through, don’t you? This is akin to collumation in the instrument shop. Did I spell “columation” right?