Good beginners guide to celestial navigation

[Frank Holden]

I wrote and illustrated this some 20 years ago...

When published they paid for 'first Australian publishing rights' so its mine to do with as I wish.


A few typos slipped through the net ( I had to do my own proof reading.. never a good idea) but they would be picked up and a note included in the following month. So look ahead.

Thats it... enjoy

https://www.dropbox.com/s/oxduof5rsz8zwtb/Offshore Navigation.pdf?dl=0

 

[MM5AHO]

I'm in the middle of a course (RYA YM Ocean), and being taught by a ex merchant marine guy who sailed from the 60s onwards, and he is teaching me the traditional way.
In addition, I bought every low cost book on Amazon, and read them all. Of those I bought (most were 2nd hand), I found Tom C's best, but for practicing the problems the Adlard Coles exercises invaluable.

Now back to practice some haversines and traverses!

 

[alant]

Don't have the book to hand, not on boat right now but I'm fairly sure he says people worry about it without trying. It's a problem at some times with the melon slice, but I've tried a good few times with spot on results. Practice........

Oh, and on a good full moon plus horizon, you can get a star or two if the declination is within your tables and you can reduce it using normal form. He mentions this, tried it, worked.

To boldly go.......:encouragement:

Just stating what he told me some years ago.

 

[LadyInBed]

I have a couple of short beginners guides (pdf's) in my Downloads section
http://montymariner.co.uk/downloads/

PS Astro Nav is long winded and heavier on the maths.
Celestial Nav is a good starter
and the Sextant Users Guide is quite good.

Replaced heavy to placate alant

 

[alant]

I have a couple of short beginners guides (pdf's) in my Downloads section
http://montymariner.co.uk/downloads/

PS Astro Nav is long winded and heavy on the maths.
Celestial Nav is a good starter
and the Sextant Users Guide is quite good.

Not "heavy on the maths" at all.
If you can add up & subtract, then thats it. OK, some of it is in degrees/minutes, so it isn't tens & units, but that is as hard as it gets. The most difficult stuff, is extracting stuff from tables, which for those born into the electronic rather than rady reckoner age, can be daunting. Using a pro forma helps tabulate logically, but the basic stuff aint rocket science.

 

[laika]

You're in the middle of a sports stadium with just one floodlight and not sure where you are, so you pretend to know where you are then look in a book which will tell you the angle of the floodlight above the ground if you were there. Then you measure the angle of the floodlight for real and from that it's quite easy to work out how far from your pretend position you are, though it's on a line, you could walk around the floodlight and the angle would remain the same.

What if you guessed you were at wembley but you were actually in fratton park?

 

[capnsensible]

Just stating what he told me some years ago.

Tall, isn't he.

 

[Sandrena]

none of the sources seemed to present a beginners overview, so here goes ...

I haven't got Mary Blewitt or Tom Cunliffe's books handy, but the RYA book says:-

The basics
Imagine, for a moment, that you are somewhere in open water, with a single bright star directly overhead.

Astronomers have spent centuries watching, recording, and predicting the movement of stars, and the results of their efforts are published in star charts and almanacs. So if you can identify which star you are looking at, you can find out where it is.

And if you know where the star is, you know where you are.

But suppose the star isn't directly overhead?

If it's almost directly above you, then you must be somewhere fairly close to the spot at which it is directly overhead. And if it's low on the horizon, then you must be a long way away.

If we could measure, accurately, the angle between the star and our zenith (a point in the sky that is immediately overhead), we could be more precise than "fairly close" or "a long way away" because -- as a rule of thumb -- every degree corresponds to sixty miles.
Unfortunately, it's very difficult to be absolutely certain where the zenith is. But it's relatively easy to measure the angle between a star and the horizon, to a high level of accuracy.

Suppose, for instance, that there's a star directly over London, and another directly over New York, and that to us they appear to be 70 above our north west horizon and 50 above the horizon in the north-east.

Being seventy degrees above the horizon means that the London star is twenty degrees away from directly overhead. Using the "one degree is sixty miles" rule, this means that we must be somewhere on the perimeter of a circle whose centre is directly below the star, and whose radius is 1200miles.

Applying the same logic to the New York star means that we are also on another circle whose radius of 2400 miles and whose centre is in New York.

There are only two places where we can possibly be on both circles at once -- and as one of them is off Iceland and the other is off the Azores, it shouldn't be too difficult to tell which is which!

I'm sure the others must include something similar.

 

[LadyInBed]

Not "heavy on the maths" at all.

I can't help thinking that you didn't look at the pdf I referred to.
i.e. P 2-5 goes into atmospheric refraction influenced by atmospheric pressure and air temperature, which is a bit more that a 'Beginners Guide' which the OP is after.

 

[RobbieW]

I cant recommend a simple book mainly because I'd not found one, most seem to want to teach at least some of the spherical trig that underlies astro nav. Having got, in the first ten minutes or so, that astro relies on a variant of the running fix what I really wanted was a set of steps to work up that fix. Sure, there are a lot of variables and getting them in the right order is vital but with a decent proforma, or a spreadsheet, its not especially difficult - but it does require concentration. Foolish Muse's stuff looks pretty good on a swift scan

 

[RobbieW]

What if you guessed you were at wembley but you were actually in fratton park?

Surely pretty unlikely for anyone actually in Fratton Park to consider being in Wembley though

 

[alant]

Tall, isn't he.

Not on email.

 

[alant]

I can't help thinking that you didn't look at the pdf I referred to.
i.e. P 2-5 goes into atmospheric refraction influenced by atmospheric pressure and air temperature, which is a bit more that a 'Beginners Guide' which the OP is after.

Cunliffe doesn't get that complicated, it's based upon the RYA course.

 

[GHA]

I haven't got Mary Blewitt or Tom Cunliffe's books handy, but the RYA book says:-

I'm sure the others must include something similar.

Exactly, not a mention of how it actually works in a couple of sentences.

 

[Skylark]

I'm sure that we all work through Astro nav in different ways.

Contrary to RobbieW, above, for me the spherical trigonometry concepts demystifyed the process.

Based upon an assumed position (more later) we have three pieces of info. Lat, something similar to Long (easier to understand if shooting the sun) and declination of the heavenly body.

Enter a Table of spherical geometry using the three arguments above and read off altitude (bearing from horizon) and azimuth (bearing from north.

The altitude should be the same as that taken from the sextant. But it will not be because you have assumed a position for mathematical convenience. The difference between tabulated altitude and sextant altitude equates to the same difference between your assumed and actual position, along the line of azimuth.

This gives a Position Line so you must be somewhere on that line.

Now repeat later in the day (sun) and plot a running fix from the two position lines.

Using the sun as the heavenly body is the basic building block of Astro. Using the moon, planets and stars are later progressions. Deriving a 3 point fix from selected stars during the twilight observation period is more challenging in terms of preparation and sight reduction but it follows a similar process to that of the sun.

Personally, I don't use templates or plotting sheets. Constructing a plotting sheet on plain paper using the trig relationship between actual lat and long is quite straightforward.

 

[Martin_J]

I'm not sure if the Teacup Navigation (by Rodger E Farley) website has been mentioned but I was looking into this recently and like others, I wanted an 'easy to follow' guide but one that also covered everything!..

There appears to be a pretty good 122 page guide on the website, although I must admit I put it down on our mid summer cruise and haven't found time to pick it up since. I must start reading again.

Teacup Navigation - Formerly known as The Armchair Navigator

 

[Barnacle Bill]

I think it can be made too complicated.

I take the 'recipe' approach: a sequence of simple instructions. Show exactly what to copy from the table onto the form, and exactly how to plot your position line at the end of the process.

It's here http://www.aztecsailing.co.uk/newaztec/ocean%20theory/ch1/astro1s2.html and the same approach is taken in Reeds Ocean Handbook.

 

[laika]

PS Astro Nav is long winded and heavy on the maths.

Not "heavy on the maths" at all.

Cunliffe doesn't get that complicated, it's based upon the RYA course.

I think the problem was that you parsed LadyInBed's original comment incorrectly. The statement was not that Astro Nav (the practice) is (necessarily) heavy on the maths but that "Astro Nav", the specific pdf being referred to, was, relative to the other three titles, heavy on the maths. Is that the misunderstanding here?

FWIW...

I have Cunliffe's earlier "ocean sailing". It's very much:

"here's some hand waving theory which I am trying but somewhat failing to convince readers that I actually understand, now just do this step by step, don't worry about the mathematical theory and just use the lookup tables". I found it quite annoying because I actually wanted the maths but for someone who doesn't, who just wants something straightforward to plug the numbers into and magically get a result, I don't think it gets much simpler. [EDIT: absolutely not intended as a criticism of those that just want a position and to get on with sailing!] I'm guessing his more recent book is no different. These publications also weigh nothing compared with the weight of 3 volumes of NP303 and an almanac....

 

[Martin_J]

You will find the best, simplest, most common sense, and FREE, sextant users guide right here on Backbearing.com.
http://backbearing.com/files/Sextant-Users-Guide.pdf
In fact the author gave a lecture on this topic to the local bluewater crusing group just a week ago. I know, 'cause it was me
You'll be finding yourself within an hour.

I might just take a look... Thank you

 

[dom]

I have Cunliffe's earlier "ocean sailing". It's very much:"here's some hand waving theory which I am trying but somewhat failing to convince readers that I actually understand, now just do this step by step, don't worry about the mathematical theory and just use the lookup tables".

I found it quite annoying because I actually wanted the maths...

Re first para: Yup, people like to pretend they understand this stuff -- it's cool see! In reality all they know is how to apply/teach a fairly pedestrian process, which is not so cool. So peeps get all confused by the ensuing poor explanations and wobbly metaphors.

What many never find out is that the mathematical concepts are not in the slightest tricky. Solving the equations is of course another matter. If one simply wants a quick conceptual explanation up until the celestial spherical trig equations then try this:
http://onboardintelligence.com/CelestialNav/CelNav4.aspx

And if one wants to roll one's sleeves up here's a great paper written by a charming kraut scientist:
http://www.waypointamsterdam.com/Handy_stuf/Short_Guide_To_Astro_navigation.pdf