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Choosing a sextant

Jan 1, 2003

In these days of inexpensive electronic alternatives, it's easy to treat celestial navigation as an unnecessarily complicated back-up system. But, for many serious navigators and ocean voyagers, mastery of celestial navigation is a worthwhile goal. At some point, these navigators will want to own a sextant. How should the navigator go about making a wise investment in this most traditional of navigation tools?

First, come up with a realistic budget for purchasing a sextant. With the wide range of choices in sextant frame materials, optics, and options, it should not be surprising that there is an equally wide range of prices. Until 1986, when Ken Gebhart of Celestaire in Wichita, Kans., first began importing the Chinese-made aluminum Astra IIIB, plastic sextants were the only option for those unwilling to step up to the counter with a big check for a German or Japanese brass sextant. Now, with the availability of the Astra and other similar sextants, navigators can pick up a quality metal sextant for considerably less than $500. From these lighter-weight aluminum sextants, the price for metal sextants extends up to more than $4,000. The range of Davis plastic sextants, by comparison, starts at less than $50 and goes up to around $200. Be realistic about what you can afford, and then purchase the best value for your money and your needs.

Second, the navigator needs to make a realistic appraisal of how the sextant will be used. Is the instrument going to be part of an introduction to celestial navigation before being put away in the box? Or will the sextant be brought out often for practice and navigation over the next several years? Certainly, no one wants to invest thousands of dollars in something to be stored away, and if regular use is planned then a durable and effective instrument is recommended. Plastic vs. metal

The choice between plastic and metal sextants, although often a budget decision, brings up other issues as well. Plastic sextants are often seen as good training sextants; they're inexpensive, lightweight, and readily available. The plastic sextant is considered the trusty "back-up," brought on board for the occasional ocean race or passage. A traditional metal sextant, on the other hand, is seen by many as an heirloom instrument, owned by the saltiest of sailors, to be used reverently and passed along for generations, though not necessary for infrequent use.

Given these differing viewpoints, how can a buyer choose between plastic and metal sextants? Putting aside the cost factor for a moment, there are several considerations. First, plastic as a material is much less thermally stable than the metals used in sextant construction. On a boat, where dramatic temperature changes can be encountered just by moving the sextant from the sextant box belowdecks to the hot sun on deck, up to several degrees of error may be introduced in some plastic sextants due to expansion of the frame. This can make precise measurements very difficult to obtain. Metal sextants, while not immune to this thermal change, are subject to much smaller variations, making adjustment or compensation more straightforward.

This wandering error is more of a frustrating problem for the novice celestial navigator unused to taking sights, as the errors are largely unrelated to sight-taking ability. Primarily for this reason, Gebhart of Celestaire usually recommends that the novice celestial navigator opt for a metal sextant (this is actually contrary to the way many approach buying a sextant, thinking that a plastic sextant is a good "beginner" sextant). An experienced celestial navigator, on the other hand, can adjust technique to accommodate the shifting errors.

Generally, metal sextants at all price points also have better-quality optics (mirrors, telescopes, and shades), again making sight taking easier and more consistent. And, in this age of bells and whistles, more options are generally available for metal sextants: additional telescopes, filters, and more. The weight difference between sextants should also be taken into consideration, with sextants weighing in at from one to four lbs. The argument for light weight is obvious to anyone who has held a four-pound sextant at eye level for several minutes, but proponents of heavier sextants say the instrument is much more stable in windy or otherwise unstable conditions.

All that said, it should be pointed out that, with practice, any competent navigator can achieve good results with any sextant, whether it is made of plastic or metal. The plastic sextant is still a good value, and many are sold each year. Plenty of ocean passages have been successfully completed with low-budget sextants. Indeed, even some facsimiles of sextants have been written up as producing get-you-home results. But, for lasting quality and consistent results with all celestial bodies, aluminum or brass sextants offer the best value for the money as a primary instrument.

Aside from the metal/plastic debate, what are some of the nuts and bolts of the sextant that need to be considered when making your choice?

Mirrors. The full-view mirror has risen in popularity due to the ease with which a body can be brought down. But, for some, the slight decrease in light transmission (due to the thin coating on the mirror) makes enough of a difference to make the split-view the only choice. That fact plus personal preference (usually defined as "what I learned with, or feel most comfortable with") decides the issue. If possible, buy both types and experiment to find what works best.

Telescopes. A 4 x 40 telescope, or equivalent, is usually standard for metal sextants. The 4 x 40 provides a good field of view for both sun and star sights. A higher-power telescope (usually 7 x 35) narrows the field of view, making it slightly more difficult to use on a pitching deck. However, higher-power telescopes with 7-power eyepieces usually have a larger exit pupil, the result of which is to get more light into your eye. This allows you to find stars early or late in twilight when they are dim. Some navigators use the high-power scope for sun sights also, since the sun appears much larger through the eyepiece, making the sun's limb easier to place on the horizon. If your sextant box includes both telescopes, be sure to check one against the other for accuracy, as the differing magnifications can occasionally make for variations.

Lighting. Usually consisting of two small LEDs mounted over the arc and the micrometer, powered by batteries located in the handle, this feature is handy for navigators going it alone, without an assistant to help hold a flashlight and record data. Remove the batteries when the instrument is in storage.

The case. If your sextant doesn't come with a rugged box, purchase a waterproof case. The extra cost will be far outweighed by the gain in your peace of mind when at sea. A good case has room for extra batteries, a timepiece, additional mirrors or telescopes, and a supply of lens paper and a cleaning cloth.

Options. These can include practice and bubble horizons (for practicing and using a sextant without a horizon), sight tubes, polarizing filters, astigmatizers (to turn a star's pinprick of light into a line of light), calibrating devices, brackets to attach timing devices, bearing compasses, and night-vision devices. They add to the cost of a sextant, and their utility varies with each navigator.Used sextants. Quality instruments in good condition retain lots of value over many years. Sextants of a variety of vintages, manufacture, and condition are available from many sources. But, unless the navigator knows just what to look for, a reputable dealer is the best place to shop (though some surprising bargains have been found at flea markets, in want ads, and in attics). Care and feeding. A sextant is a high-quality instrument, whether plastic or metal, and should be cared for appropriately. Regular cleaning of the optics with lens paper, wiping off of salt water with a soft cloth (even, in extreme cases, a liberal freshwater rinse), and light oiling of the moving parts will, in most cases, be sufficient to keep a sextant in good working order for years. Once every couple of years (more often with hard use) take the sextant apart and give it a thorough cleaning. For all its seeming complexity, it really is a simple instrument and is easy to care for. Of course, for serious problems or the occasional mirror resilvering, a dealer can provide an estimate for repairs.

If possible, give your new instrument a try before plunking down the cash to make sure it feels good in your hands and operates smoothly. Get used to how to read the arc and micrometer drum, flip through the shades, and try out the different telescopes and mirrors. Take a couple of sights, ask someone else to do the same, and then compare the results. If everything checks out, head to sea and practice, practice, practice!