I would say NO on the plastic sextant for learning. The reason is, the inherent inaccuracy of the plastic instrument can mask small errors in sight reduction and in taking the actual sight. As a backup, for someone already well versed in celestial navigation who is on a strict budget
, okay I give it a cautious half thumb up. For learning, you need to KNOW that you have a reasonably accurate instrument, so the error is all you. In like regard, I must caution against a used sextant. A limb could be slightly warped and you could zero out the index error but still be a couple of minutes off at a high angle, or even several minutes off. Again, for a backup, a verrrrry inexpensive used sextant of presumably good quality (plath, frieberger, tamaya, etc) would be a reasonable option for a budget
constrained sailor. For learning, go with a new one.
Now a new metal sextant is gonna cost you some big bucks. UNLESS you go for the Chinese Astra IIIb. www.celestaire.com
is your friend. Probably around $500 now. I bought mine back when they were $275. This is an ideal learner sextant, nearly as accurate as a top of the line Plath, but waaaaay cheaper. The box is pretty cheesy but you can get a Pelican case for it if you don't mind that it doesn't look very nautical.
GPS? That will be your primary position-fixing method. It is easy. Fast. Convenient. Accurate. Celestial is none of the above. It has class, and you get an elitist rush out of it, maybe, but the main advantage of celestial tools and skills is that it is still useful in a post-apocalyptic absence of GPS satellites, or the system crashes due to some North Korean cyber attack, or you get hit by lightning
and all your electronics
are fried, or you lose the ability to charge yoru batteries. Celestial has potential even when forced into low-tech mode. BUT... as a low tech backup, it MUST be TOTALLY independant of electronics or electricity. So, while it is cool to whip out a scientific calculator and having memorized the formulae, refer to the almanac for ephemerical data and reduce your sight, or even more cool to just plug
your Hs into a computer program or an iphone
or droid app, you ought to instead go with the classic method that uses HO 229, which is the sight reduction tables, and the Nautical Almanac, paper, and pencil. And your plotting tools. Even if you have no chart, you can, if you know how, construct a mercator plotting sheet and plot your LOPs on that, advancing as needed for a fix. No electricity? No problem. You wil never know within feet where you are, but in the middle of the ocean if you can determine your position to within a mile, you are good to go. In pilotage waters, you can determine your position by ranges, crossed bearings and special cases, and other visual aids. Again, low tech. Anyway, your celestial skill set and tool set MUST include a non-electrified method of sight reduction.
Even before you buy a sextant, you can practice the MOST BASIC form of low tech navigation... DR, Dead Reckoning. The Dead actually probably is derived from DEDUCED reckoning, but who's counting. Anyway, at some point in the voyage, presumably the beginning, you know precisely where you are at. If you know you went at x speed in x direction for x time, then you have a pretty good idea where you are at then, too. And Y more hours later, at Y course and Y speed, then Z hours more, and you should be able to make a fairly accurate determination of your position. The more accurate you can judge your speed, (no electronic wizardry, or it isn't low tech) and the direction that the boat actually moves, which can easily be 20 degrees from the direction the boat is pointing, and the more careful you are about recording your observations and putting them on the chart, the more accurate your DR position will be. The ultimate low tech speed indicator is a chip log. This is simply a piece of wood, typically triangular or pie shaped, with the sharp corner weighted so it just floats, and a long piece of string. Now, at 1 knot
, if you deploy your chip log, and let the string run for a timed hour, 6060 additional feet of string will pass over yoru transom. Not very practical. Especially when it is time to pull it all back in. Divide that figure by 60 and at one knot
, 101 feet of string will pass over your stern. Still not very practical, is it? Okay then... 1/10 of a minute, 6 seconds, should see 10 feet of string pay out. Ahhh... that's more like it. Not quite as accurate but very handy. So you make a knot in the string where the chip is being dragged about a boat length back, then make another knot at 10.1 feet and another at 20.2 feet, and so forth. At 50.5 feet, the 6th knot, you are looking at a speed of... 5 knots. Careful timing and taking the average of several tries will give you a speed quite accurate enough for a DR. Your course through the water
you can determine by hand bearing compass
, sighting back down your wake. Unless you are running downwind, you are going to be making some leeway, so your compass
heading is not good enough. Plotting the distance and the speed for your hour's run gives you a DR position. You can further refine this into an Estimated Position by plotting the set and drift of the current from the DR, for the same hour, since sighting down your wake gave you only the course through the water
, not course over ground. It is possibly more convenient to simply do all this with arithmetic and only plot the Noon DR... up to you. But keeping a proper DR is the essence of navigation. Compare your DR position every noon with your GPS, and you will eventually find your DR getting quite accurate, sometimes within a couple of miles. Your DR position goes hand in hand with celestial navigation. You have to start with an assumed position, when using the azimuth/intercept method, and this assumed position will be based on your DR. In other words, you have to know where you are, in order to figure out where you are at LOL!
More stuff on DR and celestial, and in fact all facets of navigation, in American Practical Navigator, also known as Bowditch, or Pub no.9. Get it. It is your Bible.
And BTW, another additional backup that could be good to have, is a small hand-held GPS, even an iphone
or other smartphone with internal GPS. If you only turn it on once a day to get a fix and then turn it right back off, the battery
ought to last quite a few days. This daily electronic fix would be a wonderful add-on to your DR. In fact, the thing to do in such a situation is to begin your DR plot anew, from the daily fresh electronic fix.
Last but not least, in order to reduce a celestial observation to an LOP, you have to have an accurate time reference. A battery
powered portable SW reciever will work for that. Tune in to WWV or WWVH at 5, 10, or 15MHz for the time ticks, and either set your watch or clock by the time ticks, or note the difference between the official time and your watch or clock. Knowing this difference and recording it will allow you to calculate the rate by which the clock gains or loses, so in the absence of a time signal, you can still calculate the correction. You should know the time of a shot to within the nearest second. You don't need a fancy chronometer but you do need a timepiece that gains or loses at a predictable rate, and you need some way to check your time by a coast station, or even by the top of the hour time tick from the BBC. A Timex or Casio wristwatch is good enough, especially one that is kept by your navigation station instead of on your wrist.