This is the angular difference between the altitude measured between the apparent horizon and the body as seen through the observer’s eye and the altitude which would be measured between the body and the horizon as seen from the surface of the sea.

The observer will always be a certain distance above the sea surface and the altitude obtained will always be greater than that observed at 0° above the sea. The correction is referred to as DIP and must always be subtracted from the observed altitude. In order to find the value of dip, the nautical tables must be entered with the height of eye of the observer above the sea surface. Once applied the observed altitude becomes the “apparent altitude”. In the diagram XOD is the observed altitude, whilst angle H’O’X’ (and angle HOX) is the apparent altitude and angle HOT is the angle of Dip.

Apparent altitude.
This is the altitude obtained after correcting the sextant altitude for index error and dip.