When we take a sextant altitude of a heavenly body, we measure the angle between the body and the visible sea horizon as witnessed from the observer’s position on the bridge of a vessel. (angle DOX in the diagram). This is referred to as the observed altitude. In converting it to a true altitude we need to change the observation position from the bridge of the vessel to the centre of the earth, the true altitude being angle RCX in the diagram.

Observed altitude to true altitude.