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Sextants revolutionalized navigation and were the absolute must haves for anyone going beyond the sight of land. So do we still need sextants on pleasure craft for ocean voyages? That is a question of much debate.
I think there are three groups of people who are thinking of getting a sextant and (maybe) learning how to use it.
If you are in the first category and want a sextant for decoration, your answer may be the easiest. There are many different brass sextants on Amazon and elsewhere (like this one or like this one). If you buy one of those, please do not expect them to work (just to give you a reference point: A working, high quality brass sextant will be around $2,000).
If you are in the second category, things are a bit more complicated. Before we go much further, let’s also do a contemporariness test. It is fair to ask if a sextant is even relevant and needed for sailing these days, with multiple different services like GPS, GLONASS, Galileo and Beidou and abundance of cheap receivers. It is fairly easy to carry multiple (sometimes in the order of 10 including mobile phones and tablets) receivers onboard and keep them at different places, including in faraday cages (oven) for lightning protection. That said, we sailors are opinionated people and will have very different but equally strong opinions. To get some idea, check this poll on Cruisers Forum, a leading web forum that brings long distance sailors and cruisers together. If you have time, read 15 pages of comments as well (did I say sailors are opinionated?). You will see that more than 80% of these contemporary sailors saying it is not required in varying degrees. If you find it convincing and are in that 80%, you may be better off by carrying more GPS receivers and faraday cage bags.
If not completely convinced, it may be good to give some reference point for pricing. As I mentioned before working brass sextants sell around $2,000, aluminum ones around $600. When it comes to plastic ones, Davis Mark 15 is around $150, Mark 25 is around $200 and Mark 3 is around $50, which is as basic as it gets (which some people argue is better but we will come to that later). You need to decide between these three options. As you can see, it is a very wide range. It is important to mention that these are all brand new. Sextants last LONG if used properly, especially considering that they are not used that regularly. I used a sextant from 1944 and it worked like a charm. So there is a huge second hand market. Prices vary significantly but you can find a good aluminum sextant like Astra IIIb for anywhere between $250 to $300 on eBay.
The question for a used sextant is not how old it is, how well it has been taken care of and whether it actually works. But there lies a problem. Most of the time, sellers of used sextants are not very knowledgable on sextants or celestial navigation. This doesn’t mean that there is foul play on their side, there usually is not. I have seen many sellers saying “I don’t know if this works because I am not knowledgable on sextants, I got this from my
It is also good to ask when you willl use the sextant. One thing important to know about sextants is they are almost useless for proper navigation unless you have other accompanying stuff. At a minimum you need to have an almanac, which changes yearly (although there are ones that go until 2050). For finding your longitude, you also need an accurate clock (you can find your latitude without a clock). An error of just one minute of time means 15 miles, so the clock needs to be accurate. And on top of everything you need quite a bit of practice, it is not the most intuitive thing when you do it for the first time. Obviously there are apps and computer programs that automate the calculations but again, you assume the electronics might have fried in the very first place otherwise one of the backup GPSes would work.
Also it is important to think about the context here. If you were hit by a lightning in the middle of the ocean that was severe enough to fry all electronics onboard, even if you were lucky not to have a structural damage on your boat because of that lightning, I would assume you wouldn’t be continuing your regular cruise. Instead, you would head to closest port for repairs or simply go for a landfall anywhere you can. So you will need some emergency navigation skills and tools. A sextant will be handy then, even if you may not have an up-to-date almanac. If what you are concerned about is this scenario, I would highly recommend the book Emergency Navigation: Improvised and No-Instrument Methods for the Prudent Mariner by David Burch. For such use, I think a plastic sextant (like Davis Mark 15 or even a Davis Mark 3) is good and useful to keep onboard. Would having a more expensive metal sextant be better in this scenario? Presumably yes. But I don’t think the difference will be materialistic enough given all other suboptimal conditions. Remember, this assumes you are in an emergency and trying to do a landfall somewhere, many more things will be suboptimal and a plastic sextant as opposed to a metal sextant may be least of your worries.
And finally if you are in the 3rd category, into serious celestial navigation and plan to use your sextant frequently for actual navigation. Then I’d definitely look for higher end metal sextants. Astra IIIb is a very decent choice when you look for reasonably priced metal sextants. Used ones sell around $300 on eBay and around $600 new.
One last note on plastic sextants. If you buy a plastic sextant I also recommend buying the book How to Use Plastic Sextants: With Applications to Metal Sextants and a Review of Sextant Piloting by David Burch. Plastic sextants have their quirks. Many techniques in the book can also also be recommended for metal sextants but they are more important for plastic sextants. And when it comes to Davis plastic sextants, Burch has a note on Davis Mark 15 vs Mark 25: "[...] the full mirror on Mark 25 makes the easier sights easier and the hard sights harder."