Index Error

Index error is the sextant's zero reading error. It appears when the index mirror and horizon mirror are not perfectly parallel while the instrument is set to zero, so the observed altitude needs a small correction before it is reduced.

Before reducing a sight, the navigator checks whether the error is on the arc or off the arc. On the arc is normally subtracted from the observed altitude; off the arc is added. The exact sign convention should match shipboard practice, the instrument certificate and the logbook note for that observation.

Recording the correction beside dip, refraction and other altitude corrections keeps celestial navigation auditable. It also prevents a small mechanical offset from being mistaken for set, drift or a poor observation when several sights are compared.

CaptainCalc keeps celestial and voyage calculation support native on your device, so reference checks remain available even without an internet connection.