Like Kevin Ruscoe (see Flying Fish 2007/1, page 89), I started to practice using a sextant at sea during my first transatlantic crossing in 1992. I was extremely lucky, as the instrument I had been given by my late father-in-law, a retired master-Mariner, was a half-size brass one, which I now realise is much easier to keep steady in a small boat than a full-sized aluminium-framed sextant, as it is relatively heavy and more compact. It is fitted with a 2 x telescope which, because it magnifies the boat’s motion less and has a larger field of view, makes it easier to use, particularly for star sights. Like Kevin, I found the stars easier to shoot than the sun, as when bringing a star down to the horizon it seems to ‘click’ into place at the magic moment whereas the exact ‘cut’ for the sun or moon is sometimes less easy to be certain of from a low height-of-eye.

To use astro successfully one needs four things – a sextant, a watch, an almanac and tables with which to reduce your sights. I had the ideal sextant, and my GPS gave me accurate time (this, of course, is cheating as the whole idea of astro is to be independent of electronic aids, though radio signals from WWV or the BBC are usually allowed). I had bought a cheap, waterproof, digital Casio watch in Gibraltar and, to my surprise, found it to be more accurate than my old Rolex which I had lost some years before. I later bought another, an analogue Pulsar for £20, which rates utterly regularly at –3 seconds per month, so I wasn’t really cheating. To prevent both of these watches being damaged in the event of a lightning strike I kept one in a small waterproof tin (a Faraday cage) on top of Kinsa’s 3.5 ton lead keel.

My first books consisted of Mary Blewitt’s Celestial Navigation for Yachtsmen and Tom Cunliffe’s Celestial Navigation, both excellent though Tom’s is the simpler; the Nautical Almanac; and AP 3270, Sight Reduction Tables for Air Navigation (Latitudes 0°-40°). Later, in Panama, I bought a volume of Selected Stars. These are only the basics but they weigh heavily and bulk large, they need to be kept dry, and the Almanac must be renewed annually (expensive at £35 plus postage and packing, and not always easy to get hold of in far-away places).

Then at a boat show I found The Complete On-Board Celestial Navigator by Dr G G Bennett, published by McGraw Hill, (US $29.95 for the current volume for 2007–2011) which contains, in one slim A4 ring-bound volume, sight reduction tables, star-finder tables, and a five-year nautical almanac. George Bennett is an Australian who teaches navigation and has sailed in nine Sydney Hobart races as navigator. His really is the only book you will need for astro navigation for five years. The bad news is that you must forget all you have learned about sight reductions and learn to use his method instead. It is, however, clearly explained and fairly simple to do. The big advantage is that, while he uses the familiar Marcq St Hilaire intercept method, his tables are derived from modern computerised data and your estimated position does not have to be converted into a hypothetical assumed position. Thus intercepts are usually short, often less than five miles. He also includes all the forms you need – these are intended to be photocopied, but with practice one can do the sums on one sheet of an A5 notebook.

Having successfully converted to GG Bennett, my next indulgence was to put on my birthday wish-list a Celesticomp V. This is the ultimate lazy man’s astro tool since it contains in one small pocket calculator all, and more, that you will ever need. Sadly it is no longer available, but there are others. You enter sextant sights and times directly, which it averages for you and, having put in other data including an EP, course and speed, it will reduce your sights in a few seconds and give you a position allowing for the movement of the vessel. It contains its own ephemeris to AD 2100, including data for 157 stars, and also does a host of other useful things such as compass checks, tidal range sums and will even keep your dead reckoning up to date. It really is a fascinating tool.

If using the Celesticomp for star sights, it will print a list of suitable stars – each with its altitude and azimuth – for use at dawn and dusk each day. It even has a programme for shooting an ‘unknown star’, which it identifies by giving the azimuth of all the possibilities – useful in cloudy weather. I have even, by pre-computing, been able to shoot Venus during the day. A Venus / Sun fix taken at 1100 local time really is fun! So:

Yes – actually, you can!

 * The signs of the Zodiac