Entry & progression
Certificate: RYA/MCA Yachtmaster Ocean — the highest RYA qualification, covering astro-navigation and ocean passage-making. Assessed by an oral and written exam plus a submitted qualifying ocean passage. Entry: hold Yachtmaster Offshore; complete a qualifying non-stop ocean passage of at least 600 miles as skipper or mate of watch, including celestial sights, with a full passage record and workbook. Commercial use: extends the commercially endorsed Yachtmaster Offshore to unlimited area with an approved qualifying passage; underpins ocean command in the wider yacht CoC pathway.
The instrument and the idea
A sextant measures the vertical angle, the altitude, between a celestial body and the visible sea horizon. The principle of a sight is comparison: knowing the exact time and the body's tabulated position from the Nautical Almanac, the navigator calculates what the altitude ought to be from an assumed position and compares it with what was actually measured. The difference — the intercept — together with the body's bearing (azimuth) gives a position line drawn at right angles to that bearing. Two or more position lines crossed give a fix, just as with terrestrial bearings.
- Set the sextant to zero and measure the index error so it can be corrected out.
- Bring the body down to the horizon and rock the sextant gently to read the altitude at its lowest point.
- Note the exact UTC of the observation to the second.
- Correct the sextant altitude for index error, dip, refraction and (for the sun) semi-diameter to get the true altitude.
Why time matters so much
Because the Earth turns, an error in time becomes an error in position: roughly four seconds of time misplaces a sight by about one nautical mile of longitude. The ocean navigator therefore carries an accurate timepiece, checks it against a time signal, and logs its error and rate of gain or loss so every sight can be corrected. Accuracy in the sextant reading, the time, and the arithmetic together determine how good the position line will be.
NOTE — One sight, one line A single observation never gives a full position — only a line somewhere on which the boat lies. It is the crossing of two or more lines (or a running fix from one body) that produces a fix.
WARNING — Guard your time Four seconds of error moves a sun sight about a mile. Keep an accurate clock on UTC, check it against a time signal, and record its error — a good sextant sight worked on bad time is a bad position.