Shooting stars

[steffen]

Hi all, I have aquired this sextant (Kelvin & Hughes) in the original box with the inspectors certificate still in it. Payed 125 euro for it. Is that expensive or have i been had?

More important, last night was very clear so i went out and tried to shoot a star. I found a clear one (in fact it was Mars) and aimed the sextant to the horizon and then tried to adjust the mirror to the star. I found this very difficult, could never find the star, so ended up doing it the other way, aiming at the star and adjusting back to the horizon while keeping the star in the mirror. Also very difficult.

This was at home in my backyard. How the .... am i supposed to do this on a moving 32ft boat if is so difficult on fixed soil?

Whats the trick to keep looking at the horizon and finding the designated star above you in the mirror?

Fortunately i have all winter to practise.

Happy sailing, Steffen

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[zvidoron]

Hi,

Price not bad at all. Get the Sextan Handbook by Bruce Bauer (not expensive) it will teach you how to adjust it to get best results.

With stars your second method is better, you can also turn the sextant upside down, aim at the star and bring the horizon to it. the turn it back, adjust and read the result. If you practice regularly you will acheive good results in a short time.

You can sit on the beach in the afternoon doing sun sights, wait till twilight and do star sights. Take a GPS position for a check, then go home and work them out from tables or on the internet.

All the best - keep at it it is immensly rewarding to be able to shoot a position to within 0.2 of a mile from a stable platform. I do it regularly. On a boat I suppose 1 mile is more realistic.

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[Talbot]

If it is really bouncing around, work out the correct angle for your DR position for the stars you expect to shoot (the star globe is the best instrument for this, but a bit bulky for a small yacht, but these small telescope like star charts may be a reasonable substitute), then find the star, and low and behold you should only need to adjust the vernier. Otherwise, always track from the star to the horizon, otherwise there is a danger of mis-identifying the star.
Do buy one of these books on the sextant, as it is vital that you set it up correctly, and then record the index error - and then use the index error in your calculations. After that it merely a question of practice - not forgetting that accurate time is also essential. If your GPS is out so you need a sextant, then your best source of accurate time is also out, therefore invest in 3-4 cheap quartz watches, keep them in a protective container and keep a log on their accuracy from the GPS. This will enable you to interpret the accurate time even without an external reference.

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[jleaworthy]

Good advice. I would just add that you should make sure you are using the star telescope for your sights. Its larger objective lens gives a wider field of vision and much better light gathering ability.

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[pathfinder]

Hi,

I would like to get into astro nav but have been detered by sextant prices.What about the cheaper plastic types are they worthwhile?

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[Stemar]

From my (v. limited) knowledge, a plastic sextant is a toy. It's a good cheap way to get a practice instrument so you can learn how to use it, but it won't be stable or rigid enough to give reliable fixes.

When you need one for real, get a real one!

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[tome]

I imagine sextant prices will rocket in the face of falling demand. I thought of selling mine a few years back and am glad I hung on to it as I've watched the prices rise ever since. If you see one at a decent price, buy it!

I had one of the plastic ones but found the optics etc to be very poor quality.

<hr width=100% size=1><A target="_blank" HREF=http://home.btconnect.com/Amaya/Amaya_Web1.htm>Cherbourg Data site</A>

 

[richardandtracy]

You got a very, very good price.

I'm in the process of designing a brass one with a micrometer vernier, and reckon the raw material price is about £50, engraving the numerals £8 (I can do all the graduations myself). The cost of the tooling etc will bump that up a bit too, even though I can make most of it from scrap. Then, if I were to charge myself as little as £1 per hour for labour, it'd work out more expensive than you've paid.

I agree, taking a sight isn't easy. Particularly at night. That's one reason why many of the old sailing ships went aground. They just got their fixes wrong and didn't realise it until too late. Then, without an engine to get them out of trouble.. oh dear.

Regards

Richard.


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[gtmoore]

They do sell some quite expensive plastic ones which I assume are for semi-serious use, (Davis Mark 25 is around £250). Maybe using plastic is an advantage when it comes to expansion and such things. Only a guess and I agree a £50 one seems rather cheap to offer much in the way of accuracy - either that or will be difficult to use in anger perhaps.

Having been into astronomy in the past, I quite fancy learning the theory side of astro navigation purely for the hell of it. Hopefully like you say, the cheaper plastic ones are suitable for this sort of thing cos I can't see me parting with too much cash for one!

<hr width=100% size=1>Gavin

 

[richardandtracy]

For a really cheap plastic sextant, do it yourself from a CD & it's case - look at http://www.tecepe.com.br/nav/CDSextantProject.htm . I bet you've got almost everything for this sextant lying about at home.

Regards

Richard

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[trevor_loveday]

The popular plastic sextants (Davis) need to be adjusted each time you use them, and accuracy is only expected to be in the region of +/- 5 minutes: as 1minute equals 1Nm, that's a big margin of error.

Metal sextants are expected to regularly get within a minute of your position. I use a Celestaire Astra IIIb, and it's a joy to use.

All you ever wanted to know about celestial navigation can be found <A target="_blank" HREF=http://www.celestialnavigation.net/index.html>here.</A>

I am currently doing the Ocean Yachtmaster at evening classes. Most manual, non-electronic navigation is a science but celestial navigation turns it into an art. Most of us have been interested in the stars and planets at some time or another and the use of a sextant is a wonderful opportunity to combine two opportunities.

I realise that the opportunity to use a sextant in anger will be limited to when I'm far from shore and the GPS is not working, but that's not really the point. It's a delightful skill to learn and execute, and provides much satisfaction and a delightful insight into the heavens above us, e.g. my constellation knowledge has increased dramatically, and I could see the Sun's spots clearly with the sextant on Saturday. Highly recommended.

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[zvidoron]

You can get second hand Davis 15 for as little as £15 pounds, just keep your eys open. From a stable platform I easily got psitions within 2.5 miles of GPS position. Repeatability was also good - 8 or 12 readings all within 1 mile span.

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[ponapay]

Star Globe

This is undoubtedly the best tool to use. You can virtually set up you whole fix ffo mthe globe in advance.

All you need then to do is look down the bearing ith the sextant set at the calculated altitude and the star ill be on the horizon or very near to it.

The big problem is that to get a star globe with up to date data is almost impossible, the Roal Navy don't even have them now. You can get gores to recover an old globe but that is tricky and you will have to have an Apple Mac to enable you to scale the gores correctly for your globe.

Basic globes can be obtained via e-bay for about £150, they might need some gentle repair. The most up to date data is likely to be about 1975 - even then it won't be so far out to be useless.

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[trev]

I gather you realise you don't take stars at night, but at dawn or dusk while you can still see a horizon (twighlght).
The best method is to buy a set of aviation sight reduction tables. These are excellent bits of gear which take all the hard work out of it, especially if your calculating rather than computing your intercepts. The intercepts with this method are generally larger than with Marc St Hilaire but the position is the same. The beauty is that the approximate altitude and the azimuth is tabulated so you can set your sextant and get the bearing of the body.

<hr width=100% size=1>Trev

 

[duckmanton]

i dont think, they took a sight then ran aground, how about they where unaware of the dangers as no one had ever been there before. l am relly intrested in how many of the people offering opinions have done much A, sailing B, astro nav

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