Celestial navigation glossary
Index correction (IC)
Correction for the sextant reading not quite 0° when the mirrors are parallel: “on the arc, take it off”.
Where the course teaches it
Index correction and dip give Ha, in the module Sextant corrections: Hs to Ho. Turn a sextant reading into the observed altitude, in the right order.
Taught alongside it
- Sextant altitude (Hs)
- The angle read off the sextant, before any corrections.
- Dip
- Correction for height of eye: the sea horizon is below true horizontal, so subtract dip.
- Apparent altitude (Ha)
- Hs with index correction and dip applied.
- Main correction
- Refraction, semi-diameter and parallax combined in one almanac table.
- Observed altitude (Ho)
- The fully corrected altitude: the true angle of the body’s center above the horizon.
- Side error
- Sextant error from the horizon mirror not being square to the frame; adjusted out with its screw.