Home » Cruise » Julian Polishes his Astrolabe… Life Before GPS

Julian Polishes his Astrolabe… Life Before GPS

LIFE … before GPS

Forty years ago the Global Positioning System, a satellite computerized mapping system was not even on the horizon, so to speak, for the private yachtsman. If you wanted to cruise offshore you had to know celestial navigation. And the knowledge of this art/science kept many would-be cruising sailors either coastwise sailing or firmly anchored in their home port – the mysterious art being just too uncertain. An instrument as obscure as a sextant, a chronometer, almanacs and sight reduction tables were enough to fill the neophyte with dread. Learning the uses of the above might be understood in a warm, bright classroom. However, at sea, grabbing a celestial body, bringing it to the horizon and timing the event with accuracy was another thing … as was computing the results in a damp, rolling cabin.

For those with a burning ambition to explore the world by sailing over the horizon, various methods of ‘simplified’ celestial navigation could be used. The North Star lies permanently above the North Pole, and its angle above the horizon (altitude) is your latitude. In other words if the North Star is sitting on the horizon you are feeling a bit warm as you are on the equator; if the same North Star is directly above, you are feeling a tad chilly; you’re in polar bear country. Anywhere in between and you can find your latitude easily by measuring the angle between Polaris and the horizon. In the 17th century an astrolabe was used; today a cheap, plastic sextant would be adequate.

Say you knew your latitude on departure you could sail west keeping the pole star at the same height above the horizon (say with a pencil at arm’s length), you would maintain the latitude. Longitude was more difficult but DR (deduced reckoning); distance run along your course line would give you a rough idea. As you neared your destination, it would be prudent to slow down or even stop at night. Then a Radio Direction Finder (RDF) would be an important tool. A RDF was most useful in picking up a bearing from a land station. All airports in the 70s broadcast radio signals in Morse code and the identifying letters were recorded on maritime charts and on a published list of radio beacons. You’d tune into the frequency, identify the signal and then rotate the antennae or sometimes the radio itself until the signal died or dimmed, then you’d have the bearing. From this you could plot a line of position. Two lines of position and you’d have a fix, and three you’d have a ‘cocked hat’ where you’d put your position in the middle, an even better fix. Good RDF signals were usually audible from a hundred miles distant, perhaps more. Today many radio beacons have been deemed obsolete and no longer transmit but commercial AM stations can still be useful.

Know where you are NOT.

On long ocean passages it’s not so necessary to know where you are as to know where you are not. It’s important to know that you are not near that coastline, rocky outcropping or reef but not important to know exactly where you are. On long passages it’s easy to veer off your rhumb line given the vagaries of weather and wind vanes but it’s practically irrelevant as long as you are not near a hazard, and that includes shipping lanes.

The North Star has long been the sailor’s best friend at sea but celestial navigation using the sun, especially for a noon sight, was essential before GPS. A noon sight with either a morning Line of Position and/or an afternoon LOP gave you a good fixed position. If you managed to get a good morning LOP you would advance it along your course line to your latitude. If you got a good afternoon LOP you’d advance the latitude line to this LOP. Either way you’d have a good fix.

The basic navigation methods above stood me in good stead to undertake yacht deliveries in the mid seventies and I can remember well over a dozen passages from the States to the Caribbean and vice versa. Deliveries earned me enough to fit out a derelict Hollandia 28, acquired in Road Town, Tortola in 1973. I threw out the rusty old engine, got rid of stanchions and pulpit and pushpit. I gutted the interior and installed a platform that was a bed, dining area and living room. I had a car battery for lights and a camping gas stove. An old Zenith short wave radio gave me accurate time and a sextant and portable RDF completed my navigation ensemble. After exploring the islands for several years I departed for Bermuda. She was ill-prepared for offshore passage making but I went anyway. They say the Lord looks after drunks and fools and at various times I fitted both categories. But I’m still here – and still doing the same thing.

A simple, well illustrated book on celestial navigation is written by Mary Blewitt. Good second hand copies are available on Amazon for about $5.00

‘Longitude’ by Dava Sobel is a fascinating book: the true story of a lone genius who solved the greatest scientific problem of his time.

Julian Putley is the author of ‘The Drinking Man’s Guide to the BVI’, ‘Sunfun Calypso’, and ‘Sunfun Gospel’.

Check Also

yacht insurance cuba

Caribbean News: Pantaenius Offer Yacht Insurance For U.S. Yachts Traveling To Cuba

Pantaenius Offer Yacht Insurance Coverage for U.S. Yachts Traveling to Cuba Due to American laws …

PHP Code Snippets Powered By : XYZScripts.com