The instrument and the idea
A sextant measures the altitude — the vertical angle — of a celestial body above the visible horizon. The principle is comparison: knowing the exact time and the body's position from the Nautical Almanac, the navigator calculates the altitude the body should have from an assumed position and compares it with the measured altitude. The difference, the intercept, together with the body's azimuth, gives a position line at right angles to the bearing; two or more crossed lines give a fix. The sextant altitude is corrected for index error, dip, refraction and, for the sun, semi-diameter before it is used.
- Measure the sextant altitude, rocking the sextant to read the lowest point.
- Note the exact time in UTC to the second.
- Correct the altitude for index error, dip, refraction and (sun) semi-diameter to get the true altitude.
- From the almanac take the body's GHA and declination, calculate the altitude/azimuth for the assumed position, and lay off the intercept and position line.
NOTE — Time is everything Because the Earth turns, an error of about four seconds of time misplaces a sight by roughly one nautical mile. An accurate timepiece checked against a time signal, with its error logged, is essential to celestial navigation.
TIP — One sight is a line A single observation places the ship only on a line. It takes two or more crossed lines, or a running fix from one body, to give a position — exactly as with terrestrial bearings.