All subjects have their own vocabulary, and in these first chapters we have introduced you to the majority of the terms and expressions, which are used in Astro-navigation and at the same time explained the concept of the Celestial Sphere and the measurements upon this sphere, which concern the navigator.

New terms and definitions are always a little bewildering to a beginner, but in any subject, a proper understanding of their meaning is essential in order to be able to grasp what follows. In learning any language you cannot escape memorising the vocabulary.

The Astro-navigational terms and quantities we have explained in the preceding chapters must be fully understood and committed to memory before proceeding further with this study of ‘Ocean Navigation’. This cannot be achieved with a single reading, nor even with two or three readings.

These initial chapters must be read and re-read, every statement considered, every figure and diagram studied, and analysed until the principle behind it is appreciated and understood. Only on such a firm foundation will the ability to fix your vessels position from your own observations of celestial bodies be achieved.

If the concepts described in these chapters seem somewhat baffling at first, this is merely because they are new and strange, not because they are difficult, and beginners should not throw their hands up in despair and say I’ll never grasp all this stuff! A little perseverance will soon show them that it is all basically quite simple, that no obtuse mathematics is involved – only the ability to grasp some new term and concepts, extract some figures from tables, and make a few simple additions or subtractions. After all, many fine navigators on merchant ships started their seagoing careers as deck hands in the foci’s with a very little education behind them, if they can master this subject, then you most certainly can.

There now follows a glossary of the terms and definitions that have been introduced so far. This is not intended to be learned parrot-fashion or to be in any way a substitute for the text of these chapters, which must be understood. It is the underlying principle behind each term, which is important, but the glossary will prove a useful refresher after these principles have been understood and an aid to memory to refer at later stages in this ‘Ocean Navigation Study.

It is equally important to become thoroughly familiar with the contents and use of the daily pages in the N.A. and the examples in the preceding text should be closely followed step-by-step, later covering the working and answers and attempting to achieve the same answers on your own. Only in this way, will proficiency be gained?


Formulae introduced in § 1 – 5.
L.H.A. = G.H.A. (Greenwich Hour Angle) ± Long.
L.H.A. = G.H.A. + E. Long. and L.H.A. = G.H.A. – W. Long.
G.H.A. Star = G.H.A. Aries + S.H.A. Star
Polar Distance (PX) = 90º- Dec..
Zenith Distance (ZX) = 90º – True. Alt. (AX)

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