Don’t Leave Home Without It!

GPS and electronic chart plotters are great tools—when they work. But when you’re sailing offshore you’d better have a backup plan. When those systems fail you don’t want to be without your sextant, and the knowledge to use it.

Photo courtesy of James Stapley

Photo courtesy of James Stapley

These electronics have been a great boon for sailors, giving quick, accurate results in any weather. Combining them with radar and Automatic Identification System (AIS) equipment gives a sailor additional security and safety by helping to avoid collisions at sea.

In the past, sailors weren’t as concerned about collisions as they were about knowing where they were. Prompted by a competition launched by the British Admiralty in the 18th century, the development of a reliable seagoing chronometer finally gave them a means to do that.

Photo courtesy of Davor Pukljak

Greenwich Observatory clock. Photo courtesy of Davor Pukljak

They could already determine their latitude by the position of the sun. Now, by knowing their time relative to Greenwich, England, they could finally determine their longitude.

Between sextant shots, sailors keep an hourly deck log, recording their yacht’s speed and their course from the compass, along with the wind and sea conditions. They transfer this information to the navigation chart to advance their position hour by hour, a method called dead reckoning (a contraction of “deduced” reckoning).

With no means of electronic navigation, sailors correct their dead reckoning position with subsequent sextant shots, often taken at noon on ocean passages. Dead reckoning can be extremely accurate if done with precision.

Photos courtesy of Kriss Szkurlatowski

Photos courtesy of Kriss Szkurlatowski

Any difference between the two methods usually indicated
an error in the dead reckoning caused by variations in course or speed between hourly log entries, or sometimes unknown influences such as magnetic anomalies or ocean currents.


We three sailors have all learned the techniques of navigation using the sextant to find our position at sea. With practice it is possible to plot your position within a couple of miles in good conditions.

Photo courtesy of Karin Lindstrom

Photo courtesy of Karin Lindstrom

While GPS is a great tool, there is also a danger of relying on it in the South Pacific, where the charts can be outdated in some of the remote areas. A yachtsman can get a very accurate fix of his position with his GPS and then plot it on a British Admiralty chart surveyed by the HMS Beagle in 1840, which might show features a mile or two out of place.

Unless you are keeping watch, you could easily run onto a reef if you are using these charts. So along with the need to regularly plot your position goes the requirement to verify what is actually happening. There is no substitute for keeping a good watch.

Featured image courtesy of Antonio Jiménez Alonso.


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