Chapter 16: Celestial Instruments

The sextant and its errors

The sextant's index arm, arc, micrometer drum and mirrors measure angles up to about 120°. Index error is found by setting the instrument to zero on the horizon or by star: if the reading is on the arc it is subtracted, off the arc it is added. Non-adjustable errors — prismatic, graduation, centring — are collected as instrument error from the certificate. Perpendicularity, side error and collimation are the adjustable errors.

• Index correction: on the arc, subtract; off the arc, add
• Rock the sextant to find the tangent point of the body to the horizon
• For a star or planet, bring the body down to just touch the horizon

The correction sequence

Sextant altitude hs, corrected for index error and instrument error, gives apparent altitude ha after dip is applied. Dip accounts for the observer's height of eye above the sea and is always subtractive. Then apply the main correction — refraction (always subtractive), semi-diameter (add for lower limb of sun or moon, subtract for upper limb), parallax in altitude (additive, significant only for the moon) — to obtain the observed altitude Ho. Low-altitude sights suffer badly from unpredictable refraction and should be avoided below about 15°.