Sextant altitude, observed altitude and index error.

The altitude is measured by a sextant and although they are precision instruments, there is always a slight difference between the sextant altitude and the observed altitude caused by the design and manufacture of the instrument. This difference is referred to as the index error and must be either added to or subtracted from the sextant reading, depending whether the actual zero position of the sextant is “on” or “off” the arc.

If the actual zero position is to the left of the one marked on the graduated arc, it is referred to as being “on the arc”. This means that the instrument is over reading and the error must be subtracted from the altitude obtained. If the zero position is to the right of the one marked on the arc, then it is referred to as being “off the arc”. In this case the instrument is under reading and the error must be added to the altitude obtained.

The error has to be established each time the sextant is used as it is not a constant one. After applying the correction, the sextant altitude then becomes the observed altitude.