Astronavigation - what do I need to take besides a sextant?

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SteveSarabande

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I have a few long distance deliveries coming up, the perfect time to practice astronav. The aim is to get to RYA YM Ocean standard.

I haven't time to fit in the theory course, so was thinking of reading a few books and trying it out while on passage.

I have a selection of books, i will get a sextant and ebay it after the exam. What I want to know is what else I need to take on the boat and where to get it from. Can I download the tables and almanac or do I need to buy them.
 
SteveSarabande said:
I have a few long distance deliveries coming up, the perfect time to practice astronav. The aim is to get to RYA YM Ocean standard.

I haven't time to fit in the theory course, so was thinking of reading a few books and trying it out while on passage.

I have a selection of books, i will get a sextant and ebay it after the exam. What I want to know is what else I need to take on the boat and where to get it from. Can I download the tables and almanac or do I need to buy them.
Click to expand...

Sight reduction tables can be downloaded here: http://msi.nga.mil/NGAPortal/MSI.portal?_nfpb=true&_pageLabel=msi_portal_page_62&pubCode=0013
 
You will need an accurate source of the time. A watch is fine. I glued mine to the plastic Ebbco I used so I could take the site and note the time without having to put the sextant down.

I used a small pre programmed calculator which allowed me to take a dozen sites and enter them in to the calculator which would average them out for me. The second part of the program then simplified the reduction process.

Plotting sheets save you having to ferkle about with charts.
 
It doesn't sound like you have much idea about what astronavigation involves. This isn't something you can fudge without a bit of effort. If you can't attend a course, read a basic instruction book well before you go. I like Mary Blewitt "Celestial Navigation for Yachtsmen".
 
AndrewB said:
It doesn't sound like you have much idea about what astronavigation involves. This isn't something you can fudge without a bit of effort on your own part. If you can't attend a course, get a basic instruction book. I like Mary Blewitt "Celestial Navigation for Yachtsmen".
Click to expand...
Correct, I have no idea.

I have that book and a few others. I have a week on a 45' powerboat going from france to scotland and 30 days on a 60' yacht transatlantic to work it out. Hopefully the transatlantic skipper will give me a few pointers if stuck.

I just dont want to get offshore and read that I forgot a vital table
 
SteveSarabande said:
I have a few long distance deliveries coming up, the perfect time to practice astronav. The aim is to get to RYA YM Ocean standard.

I haven't time to fit in the theory course, so was thinking of reading a few books and trying it out while on passage.

I have a selection of books, i will get a sextant and ebay it after the exam. What I want to know is what else I need to take on the boat and where to get it from. Can I download the tables and almanac or do I need to buy them.
Click to expand...

A gps so that you know where you are with reliability and accuracy. A sextant is a nice toy but just like horses have been replaced by cars so the sextant has been replaced by gps.
 
OR4751 said:
Sight reduction tables can be downloaded here: http://msi.nga.mil/NGAPortal/MSI.portal?_nfpb=true&_pageLabel=msi_portal_page_62&pubCode=0013
Click to expand...

Or if you want the same facility as you get from those six volumes in a handbook, Reeds Astronavigation Tables for the current year. Then get Tom Cunliffe's 'Celestial Navigation' for an intelligently written, concise explanation that's fun to read and will get you doing all you need. Both available from Amazon. Get a sextant as you say, and the cheapest quartz digital watch you can find. And a pad of paper and a pencil.

That's it. That's all you need. Go to sea, preferably on a fairly calm day and night. You'll come back with a skill, a sense of accomplishment and a wonder for the cosmos that rotates (that's how you have to think about it) around our world at the centre of it all, with you bobbing about at a location on its surface which you can now pin down to about a square mile.
 
BelleSerene said:
Or if you want the same facility as you get from those six volumes in a handbook, Reeds Astronavigation Tables for the current year. Then get Tom Cunliffe's 'Celestial Navigation' for an intelligently written, concise explanation that's fun to read and will get you doing all you need. Both available from Amazon. Get a sextant as you say, and the cheapest quartz digital watch you can find. And a pad of paper and a pencil.

That's it. That's all you need. Go to sea, preferably on a fairly calm day and night. You'll come back with a skill, a sense of accomplishment and a wonder for the cosmos that rotates (that's how you have to think about it) around our world at the centre of it all, with you bobbing about at a location on its surface which you can now pin down to about a square mile.
Click to expand...

I have Tom's book, doy you mean this almanac?
http://www.amazon.co.uk/Reeds-Astro...475&sr=8-1&keywords=reeds+astro++almanac+2015
 
Depending on what authority you decide to follow, in addition to a sextant (and a standard textbook which will tell you how to measure and correct its simpler errors; also standard navigation equipment like a compass and parallel rule, or similar), you will need:

1. A watch, of accuracy known to the nearest 2 seconds;
2. The current HMSO Nautical Almanac, giving hourly GHA and DEC (position) of sun, moon, planets, stars;
3. Altitude Correction Table, included in the Nautical Almanac;
4. HMSO Sight Reduction Tables for Air Navigation, which does the spherical geometry involved, and includes the necessary interpolation tables.
5. Plotting Charts, e.g. Baker Charts, with parallel rule or similar.

But few people these days would bother to take 4., as there are simple computer programs for solving spherical geometry, even a good pocket calculator will do this.
Also, standard computer programs are available for predicting the position of celestial objects, to a perfectly adequate degree of accuracy for a yacht, so no need for 2.
No 3. is reproduced in every textbook.
Plus it is possible to work on standard charts rather than special plotting charts, so strictly no need for 5.

Finally of course, you can get a 'colour by numbers' computer program, where you just feed in the results of a couple of sextant readings together with their times, answer a couple of other questions, and it spits out your estimated position.

So, it all depends on how fundamental an approach you want to take with regard to astronavigation. Do you want to assume, for example, that it is for the situation where every piece of electronic equipment aboard is out of action; or just a situation where the GPS isn't working?
 
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AndrewB said:
Depending on what authority you decide to follow, in addition to a sextant you will need:

1. A watch, of accuracy known to the nearest 2 seconds;
2. The current HMSO Nautical Almanac, giving hourly GHA and DEC (position) of sun, moon, planets, stars;
3. Altitude Correction Table, included in the Nautical Almanac;
4. HMSO Sight Reduction Tables for Air Navigation, which does the spherical geometry involved, and includes the necessary interpolation tables.
5. Plotting Charts, e.g. Baker Charts.

But few people these days would take 4., as there are simple computer programs for solving spherical geometry, even a good pocket calculator will do this.
Also, standard computer programs are available for predicting the position of celestial objects, to a perfectly adequate degree of accuracy for a yacht, so no need for 2.
No 3. is reproduced in every textbook.
Plus it is possible to work on standard charts rather than special plotting charts, so no need for 5.

Finally of course, you can get a 'colour by numbers' computer program, where you just feed in the results of a couple of sextant readings together with their times, and it spits out your estimated position.

It all depends on how fundamental an approach you want to take with regard to astronavigation. Do you want to assume, for example, that astronavigation is for the situation where every piece of electronic equipment aboard is out of action, or just the GPS isn't working?
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I own a sextant. Only used in the garden.

If every piece of electronic equipment was fck'd, what are the chances that my paper tables would survive?

Counter argument: Worsley's navigation on the sail to South Georgia.
 
Vega1447 said:
I own a sextant. Only used in the garden.

If every piece of electronic equipment was fck'd, what are the chances that my paper tables would survive?

Counter argument: Worsley's navigation on the sail to South Georgia.
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Well, they might. Or maybe your scientific pocket calculator would still be OK in its waterproof pouch in your grab bag. Some sextant boxes include highly simplified tables for the sun that with just a watch will give positions accurate to about 20 miles. If the watch is gone, then the sextant will at least enable you to "sail down a latitude" like Shackleton and Worsley (but this needs good DR skills, and a compass). Of course, if the storm was bad enough to blow all your electronics, it's probably trashed your sextant too.
 
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When I bought my sextant, it was suggested that I google and download Andrew Evans' Astro Nav manual. It's free to download and takes you through a beautifully explained process to arrive at your first plot.

if you read it cover to cover before you go, you will also be able to download, free, any tables and plotting charts that you might need for the areas you plan to visit.

I've managed to get a plot only 10NM out, which apparently isn't bad, but I don't think it would have impressed Mr Worsley!

Good luck.
 
You do get the impression that TC isn't big on maths and technology from reading his stuff. However if you aren't really interested in the maths and just want to plug some numbers in and get a result back his is probably the simplest book I've encountered. If going with this method, which is what you get taught on the RYA ocean theory course, the standard works to have with you are:
- Almanac
- NP303 vol 1
- NP303 vol 2, 3 or both
- printed proformas
- plotting paper
- Your book for when you forget what it is you were supposed to be doing.

If you don't download stuff, the version I usually get is the "nautical almanac commercial edition":
http://www.amazon.co.uk/2015-Nautical-Almanac-Commercial-Edition/dp/1937196151

Smaller and cheaper than the bigger UKHO version, exactly the same info. Don't know how it compares to others.
NP303 is detailed here:
http://www.ukho.gov.uk/ProductsandServices/PaperPublications/Pages/AstronomicalPubs.aspx
Now whether you want to take both vol 2 *and* vol 3 with you depends on the latitudes you'll be in. 0-40 degrees is vol 2, 39-89 degrees is vol 3. These things are BIG and HEAVY so you don't want to take both if you're only doing this for practice and most of your trip will be in one range or the other.
The printed proformas are handy if nothing else because there're an aide-memoire. They remind you of what you need to plug in where and how to do the calculations. The cunliffe book has some in. There's probably others available on the Interweb. The Tom Cunliffe book, much as I dislike it for its tack of technicality is very practical not only in putting across the standard RYA method succinctly but also in being very light and easy to carry.

Now for plotting paper the "proper" stuff which I think I got from Kelvin Hughes at a boat show one year is very nice but does suffer from being really big. You don't actually need it.

If you're on delivery you might not have anyone on watch with you. Although I've not done this a suggestion in previous threads has been to have a voice recorder. You do a time "mark" verbally with reference to your watch at the beginning of a session, then "mark" the stars and read off the sight while recording and you can subsequently play back and get the time of the sight relative to the start. In this day and age I suppose downloading a voice recorder app for your phone would be the way to approach that these days. Not such an issue if you're only doing the sun.
 
There's a lot of stuff to remember and a fair bit of 'fetching and carrying' with numbers and rules (i.e is east best or least and what does that exactly mean! And if latitude is LESS than declination but of the same name..do you add or subtract zenith distance??

Buy, download or even better make up your own step by step pro forma so you dont end up leaving something out or adding when you're supposed to subtract. Easy enough to do at home, never mind down below when you're attempting to work up a sight for the first time!
 
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