NavList:

At the risk of being labelled a pedant, when is index error actually index error plus arc error?  I thought index error was the angle at the eyepiece between the lines of sight through the index and horizon mirrors when the reading on the sextant arc reads zero (or the angle on the arc when the two horizons merge).  At any other angle, e.g measuring the height of a celestial body or the angular distance between two celestial bodies) your measurement includes index error which remains constant plus arc error, which is caused by how accurately the scale is cut on the arc or arc/micrometre combination.  Asking for a friend.  DaveP