Buyer’s guide: Sextants
James Turner explains how to buy a sextant you can trust, and how to use the thing when you do
T he sextant is a wondrous device. it’s a power-free way to find your rough position on this planet. The only other things you’ll need are a chart, a watch and a book of tables. I learnt astronavigation en route from the UK to Tahiti in 1977, using an Ebbco plastic sextant, whilst the skipper used a Zeiss drum sextant. We navigated independently and usually found our noon fixes to be within 4 miles of one another.
You may be thinking of crossing oceans where an incident of one sort or another could rob you of electronics, or you could just be interested in astronavigation. Whatever your reason for wanting one, you need the right one for you.
Which one is for me?
The majority of purchases you make for your boat will probably have been influenced, at least in part, by budget. That’s fine, but in my view, when you’re deciding on a sextant you need to ask yourself what you’re going to use it for. Unless you’re entering into the round-theworld race in 2018 where all electronics are banned, the chances are that the one you buy will be used either as a backup in case of GPS or electrical failure, and/or out of general interest in astronavigation. If your interest is solely as a backup to GPS, you need only take sun sights, and perhaps also moon sights on occasions when the moon is visible in daytime and can be crossed with the sun for an instant fix, whereas if you want to explore the wonders of astronavigation to the full, you’ll want to take sights of the planets and stars as well.
The point of making this decision first is that there are a number of sextants on the market that are fine for shooting the sun and moon, but nigh-on useless at stars and planets. This is due to the size that we observe the heavenly bodies in the sky. The sun and moon appear big, and you don’t need a telescope to observe them, the planets and stars appear much smaller, so you do.
The next things to think about should be the sextant’s construction, features, and your budget. There’s a simple choice in construction between a metal sextant and a plastic one. I’ve never been biased against plastic sextants. Davis make some superb models, and if you’re up by the mast, arms wrapped around the rigging as the boat tosses around on a big swell whilst you wait for ages for a gap in the clouds to grab a quick sun sight, you’ll be very happy you bought a lightweight plastic sextant. On the other hand, the metal ones, though much more expensive, do have a lovely feel to them. It’s worth noting that any additional accuracy or calibration in a metal sextant is blown away by the disadvantage of taking sights from the low, pitching deck of a yacht.
What features should I look for?
As far as features are concerned, there are two main considerations. First, do you opt for the traditional ‘split screen’ or the more modern ‘all-view’ type? Traditional sextants show the horizon on the left side and the heavenly body on the right, and by jiggling the sextant you observe the body in the centre. The technologically more advanced ‘all-view’ mirror type superimposes the heavenly body on the whole screen and the horizon can be viewed all the way across. My view is that if you’re new to astronavigation, the ‘all-view’ screen is easier to use, though it generally costs a bit more. Having grown up with the traditional ‘split screen’ I’m not about to change, and it does make horizontal sextant angles – used in coastal
navigation – much clearer. Whether you’re shooting the sun, moon, stars or planets, the type of view is irrelevant; you can use either type for all astro shots, it’s just a question of which type you are comfortable with. The number of shades is also worth considering, as is arc radius. The more shades the sextant has, the greater your ability to adjust the view through the sextant to something comfortable for your eyes. With fewer shades you risk, in some light conditions, the view being either too bright or too dark to take an accurate reading. The longer the arc radius, the larger the sextant and the greater the accuracy and ability to read the angle clearly.
The other feature to consider is whether there is a light on the arc. If you’re planning to do star sights, which you take at dawn and dusk, a light on the sextant is a huge advantage, the alternative being a head torch or torch between your teeth.
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