The Sextant: Purpose and Principle
The sextant measures the angle between two observed objects — most commonly a celestial body and the visible horizon. This angle, the altitude, feeds into sight reduction tables or a calculator to yield a position line. The instrument's accuracy depends entirely on keeping its errors understood and corrected.
Picking Up and Handling
Always lift a sextant by its frame, never by the arc or mirrors. This prevents distortion of the frame, which would introduce error into every subsequent observation. Check the index glass and horizon glass are clean before use; smears cause indistinct images and false altitude readings.
The Arc and Micrometer Drum
The index arm moves the index glass, changing the measured angle. Coarse movement is along the arc; fine adjustment is via the micrometer drum and vernier, giving resolution to fractions of a minute of arc. Always approach the final reading by turning the drum in the tightening direction to eliminate backlash.
Errors of the Sextant — in Logical Examination Order
Errors are dealt with in sequence because some must be corrected before others can even be checked.
1. Error of Perpendicularity The index glass must be perpendicular to the frame. Set the index arm to roughly 60°, hold the sextant horizontally, and look into the index glass — the real arc and the reflected arc should appear as one continuous line. If they form a break, adjust the index glass retaining screws.
2. Side Error The horizon glass must be perpendicular to the frame. Set the index near zero, aim at a star, and adjust the micrometer slowly. If the direct and reflected images pass through each other rather than merging cleanly side by side, side error is present. Correct with the horizon glass side-error screw. Side error must be eliminated before index error can be accurately assessed.
3. Index Error (IE) The remaining non-adjustable error: the reading when the horizon glass and index glass are parallel, which should be zero. Aim at the horizon, bring the two images into exact coincidence, and read the arc.
- Reading on the arc (positive): IE is off — subtract from every observation.
- Reading off the arc (negative): IE is on — add to every observation.
Memory aid: "On the arc, take it off; off the arc, put it on."
Parallax and Dip are corrections applied to the altitude after IE correction — they are not sextant errors but external factors. The examiner may link them here.
Taking a Sight
Shade the index glass appropriately for sun or bright bodies before pointing at the sun — never look at the sun unshaded. Rock the sextant about the line of sight to find the true vertical; the body should describe an arc, and altitude is read at the lowest point of that arc (for a lower limb sun sight). Record time to the nearest second at the moment of observation.
Routine Care
Store in its box away from extremes of temperature and humidity. Sudden temperature changes cause the mirrors to mist and the frame to distort temporarily. Check IE at the start of every passage and log the value.