Use of a sphere to represent the Earth in the teaching of navigation has the advantage that the mathematics is relatively simple and has sufficient accuracy for most practical purposes. It also allows relatively simple instructional material to be developed for plane and Mercator sailing. Unfortunately, and as remarked by Williams [1], bad practice based on an error of principle has pervaded some navigation texts for some time. This error has passed into various instructional settings where it has gone un-noticed or un-remarked. The error lies in the un-rigorous use of meridional parts for the spheroidal earth together with latitude differences for the spherical earth in the teaching of plane and Mercator sailing. This is exemplified in [2, example1, p 585]. It is often argued that the resulting error in calculated distance or mid-latitude caused by this un-rigorous treatment is small and within what practical course keeping allows. However, permitting the discrepancy to remain unexplained is a disservice to students of navigation. Students should be given a clear explanation of the differences between the fictitious though useful spherical model of the earth and the spheroidal earth as presently described by WGS84. Teaching of plane and Mercator sailings should be based upon either the spherical model or upon the spheroidal model but not on parts taken from both.