Celestial Navigation 101, Interlude: What sextant should I buy?

The question has come up as to which sextant I would recommend for people working through this course. This question happens to come up in any celnav course at some point, so it happens that I have a "stock" essay answer to the question which I usually give out on the first day of class. What follows is a slightly modified version of that, tailored to a wider audience than just students sitting in a Seattle classroom. If you are not interested in purchasing a sextant at this time but are following along with the Celestial Navigation 101 posts, do please read this, as it contains good general information about sextants which will be helpful later in the program. Because this is largely cut-and-pasted from a pdf, there may be some formatting issues. I'm endeavoring to correct the worst of these as I update the essay for this purpose, but please bear with.

First off, you do not need your own sextant for this course. You will eventually need one with which to practice sextant sights, and of course you'll need one on the water if you intend to incorporate celestial navigation into your ocean navigation routine.

Disclaimer: I do not sell sextants, nor do I have any vested interest in which sextant you purchase. For this reason, I will in some cases recommend one sextant or type of sextant over another without reservation. However, if you already own a sextant, and it does not happen to be one of the ones I recommend for purchase, don't worry; any marine sextant at all will do everything you need to safely navigate across an ocean. I will also discuss purchasing used sextants, which in some cases can be a very good option.

What does a sextant do?

A sextant measures angles. In celestial navigation, a sextant measures angles from the horizon up to an object in the sky, whether it is a star, planet, the sun or the moon. There are instruments which can measure angles more accurately than a sextant, but a sextant is the best tool for measuring angles from the deck of a small boat rocking in big seas.

Metal Sextants

Plastic, by its nature, allows more freedom of movement between the working parts of a sextant, and so creates more error in any given sight. However, with proper sight-averaging techniques (which we should be using with any sextant) a good plastic sextant will yield similar results to a metal sextant. The advantage of a metal sextant is that if we do not use proper sight averaging techniques we will still get a pretty acceptable fix. In other words, a metal sextant allows us to be a bit lazier and still navigate safely.

Metal sextants are made of either brass or aluminum. Aluminum sextants are lighter and less expensive than their brass counterparts. Lighter is generally better. The old argument that heavier sextants are somehow superior because they have “more inertia” is frankly absurd. If you are using proper sight averaging techniques you will be shooting a lot of sights for every fix, and if your arm is getting tired from lifting 10 lbs of brass up to your eye your sights are going to get sloppier.

The other common argument favoring brass over aluminum is that the worm-gears of a brass sextant are inherently more precise than those of an aluminum instrument. This is technically true. However, the difference in accuracy is somewhere in the neighborhood of .05' of arc. This is nominally a difference of about 100 meters, which seems like a lot to us in a post-GPS world. However, in order to make our math easier, celestial navigation is based entirely on the notion that the earth is a perfect sphere, which it is not. So the inherent errors in this assumption alone will generate errors of a quarter mile or more for a dockside fix, and from the deck of a small boat in large seas it is unrealistic to imagine that we will get a fix much more accurate than one nautical mile from our actual position. Which sounds like a fairly large error, but is actually far more accurate than we can possibly steer a boat to compensate for on a voyage from, say, Seattle to Honolulu. So the advantage of a brass sextant in accuracy is as negligible as that of, for example, using a scale calibrated to micrograms to sell apples by the pound.

A better argument in favor of brass sextants is that they are more durable than their aluminum counterparts. This is demonstrably true. A well maintained brass sextant under rigorous daily use will last generations, whereas an aluminum sextant under the same conditions might last only a few decades, due to friction slowly degrading the worm-gear. Realistically, in this day and age when celestial navigation is mostly a redundant back-up for GPS and we are typically only shooting one celestial fix each day, a good aluminum sextant will last your entire seafaring career, but it will not also last through your grandchildren's seafaring careers. An aluminum sextant is a working tool, a brass sextant is an heirloom.

The last (and best) argument in favor of brass sextants is that they simply look better than aluminum sextants. They do. Personally, I love the way a really well-made brass sextant looks and feels. The mechanisms move more smoothly, and they just look and feel “nicer” than an aluminum sextant. Standing on the dock, or in the classroom, there's nothing better to do celestial navigation with than a brass sextant. But I probably wouldn't actually take one to sea with me on a small boat, because if I were to lose a $2000 sextant over the side I would be very, very sad.

Brass Sextants

The best and most easily available new brass sextants are from Cassen and Plath in Germany, and Tamaya in Japan. Typically you must purchase the sextant and the telescope (a 3.5x40 scope is standard, and probably the best all-around scope for most applications) for the sextant separately, and the combined cost will be around $2000. This is actually about half the price the same sextants were being sold for 10 years ago, due largely to the impact aluminum sextants have had on the market.
Tamaya sextants are sold by Captain's Nautical Supply, www.captainsnautical.com, Cassen and Plath sextants are available online from Celestaire, www.celestaire.com
Old US Navy surplus Mark 5s are excellent (if somewhat utilitarian) sextants if you can find them.

Aluminum Sextants

The best aluminum sextant available new is the Astra IIIb, from China. This is the instrument used by the Chinese Navy and Merchant Marine, as well as most of the navies and merchant marines around the world. It is by far the best selling sextant in the world, with good reason. My own preferred sextant is an Astra IIIb. They retail for about $600. Astra sextants are available locally from Starpath School of Navigation, www.starpath.com.

Another good aluminum sextant is the CHO-T sextant (the more correct transliteration from Cyrillic is SNO-T, but nobody seems to want to buy one of these) from Russia, which is a knock-off of the Freiberger sextant from Germany. These instruments can usually be found in good condition used for somewhat less than the price of a new Astra IIIb. Be careful to buy a marine sextant if you get a CHO-T, because they also make astronomical sextants which are otherwise identical but leave the telescope image inverted.

Plastic Sextants

Plastic sextants have a bad rep. This is not entirely undeserved, but some of the bad reputation which plastic sextants have is plainly spurious. One of the most frequent complaints I've heard about plastic sextants is that if you leave them out in the sun, the plastic expands and thus degrades your sight. Yes, plastic left in the sun will expand. However, it will tend to expand evenly, so this should not actually affect your sight. Also, no navigator worthy of the name is going to leave any sextant out in the hot sun, so the point is moot. What is not moot is the fact that plastic worm-gears (or even plastic-on-metal worm-gears) are too malleable for the precision desired of a sextant. Even so, a sextant with a plastic worm-gear is still significantly more accurate than any other means of deriving an angle from a moving boat at sea, such as an astrolabe or kamal, so it is still a worthwhile tool to have on board.

Plastic sextants are available from Davis Instrument Corporation, which is the only company in the United States which manufactures sextants. Davis sells three basic models of plastic sextant. Unlike metal sextants, and utterly counter-intuitively, with Davis sextants what you pay for is inverse to what you get. The “standard” Davis sextant is the Mark 15, which runs about $170 new. It has all of the problems previously noted with plastic worm-gears, but is otherwise a functional sextant. The “deluxe” Davis sextant is the Mark 25, which retails for about $220 new. It has all of the problems associated with the Mark 15, and additionally has a “whole horizon” beam-splitting mirror, which is generally inferior to the traditional split mirror of the Mark 15 (more on mirror differences later). The main selling point of these two sextants is that they look and operate more or less like most metal sextants. Frankly, for the same price you will probably be much better off getting a used metal sextant on eBay, which I would not otherwise ever recommend.

Davis, however, makes one more sextant, which more or less redeems them from the sins of manufacturing the other two. It is called the Mark 3, and it retails for about $50. The Mark 3 is about the very least thing that could ever consider itself to be a functioning marine sextant. It is made out of stamped plastic and held together with model airplane glue, and has only one significant moving part, which is its saving virtue. There is no worm-gear, just a simple pivot point, and instead of a traditional micrometer drum to read minutes of arc it has a very simple vernier scale. The Mark 3 isn't pretty. It doesn't seem to work in the same way as a more traditional sextant (although the basic principles are the same), has no telescope, and instead of a beam-splitter or a traditional split horizon mirror has a simple glass mirror and thin air. In spite of this extreme simplicity (or rather, because of it), if proper sight-averaging techniques are used, the Mark 3 will yield results very comparable to those of an aluminum or brass sextant of $2000 or more. Neither of Davis' more expensive sextants can make this claim. So, for $50 for the sextant, $30 for a Nautical Almanac and $20 for a decent quartz watch, you can be adequately equipped to navigate across an ocean for about $100, roughly the same price as a bottom-shelf handheld GPS or a good hand-bearing compass.

Used Sextants

Given normal maintenance and usage, there really isn't a great deal that will go wrong with a good sextant. A mirror might need to be replaced or resilvered, and also realigned, but that's usually about it. Unless you are very knowledgeable about sextants, do not buy a used one on eBay. The vast majority of sextants sold on eBay or similar venues are ornaments not intended for navigation, even if the ad says otherwise. This is not because the sellers are dishonest, it is because they are not navigators and do not know the difference between a sextant and a wall-hanger.

As far as buying a sextant from a second-hand boating supply place, chances are good that if you stay with a known brand that you will get a decent instrument. My best recommendation, however, is to wait to buy a used sextant until after you have completed this course, and are more knowledgeable about sextants generally.

What type of horizon mirror is best?

There are generally two types of horizon mirrors for sextants. The traditional or “split image” type is a piece of glass which is only silvered on one half of its surface. This allows you to see the reflected image (from the index mirror) of the star in the silvered side, while viewing the horizon through the unsilvered (clear) side.

“Whole horizon” or beam-splitter mirrors are partially silvered across their entirety, and so partially reflect the star and partially display the horizon in the same field of view. In doing so, 50% of the light of the object is lost, and 50% of the light of the horizon is also lost. This is not a problem when both the object and the horizon are very visible, such as shooting the sun on a clear day. However, it makes shooting a dim star with a faint horizon nearly impossible, and shooting a daytime moon or Venus amid any amount of cloud cover utterly impossible. The only virtue of the “whole horizon” mirror is that it makes sun-sights a little bit easier for a rank beginner. After a few minutes of sextant practice, you will have no difficulty using the traditional mirror to obtain a sunline, and so will have no further use for a whole-horizon mirror.

What about bubble-sextants and artificial horizons?

Some military sextants are equipped with a “bubble” attachment for use by submarines on dark nights. An actual, functional bubble attachment is very, very expensive, and of very little utility for any vessel which is always on the surface. The “practice bubble attachment” sold by some companies is basically a carpenter's level, and for actual navigation (or even sextant practice) is essentially worse than useless.

Some companies also sell an “artificial horizon”, which is little more than using a tray of water to reflect the image of an object back onto itself. You can achieve the same thing with a pie-plate or a rain puddle and save yourself the $30 or so that most companies charge. But this isn't the way we shoot sights, so it really isn't even useful for training. You are much better off using even a small body of water and computing for the difference between the near shore and the horizon. However, if you have even three miles of water between you and the opposite shore (such as is true for most of Puget Sound, for example), you have a real horizon, even though you can see land beyond the curvature of the earth.

Okay, so what is my best bet?

Based on all of this, my recommendation is that an Astra IIIb aluminum sextant, with a traditional split mirror, is your best bet for the money.
If that's out of your price range, a Davis Mark 3 is a great value at a very small cost.