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Astro-navigation from Square One: Introduction

Why learn astro-navigation? It appeals to all sailors to use the Earth’s natural elements such as wind, current and tide to propel them across seas and oceans and a natural extension of this must be to use the multitude of heavenly bodies as a guide. It matters not that the means of electronic navigation have proved, so far, to be extremely accurate and reliable. There is little more satisfying for a sailor than to gaze in to the heavens and use the Sun, Moon, planets or stars, together with a little maths, to produce lines on a chart indicating a likely position.  For the training of professional sailors, astro-navigation remains a mandatory subject.

Many who try fail to finish a book on learning astro as the subject is relatively complex. It requires knowledge of an entirely new language of several dozen terms and the use of seemingly impenetrable maths.  Others simply stick to proformas and sight reduction tables, content not to understand the underlying science. 

Many astro books leave much to the imagination, explaining little from first principles with scant attention to the language and detail.  This book endeavours to leave nothing unexplained, no word undefined and aims to teach the subject by UNDERSTANDING and not by ROTE. Those section paragraphs immediately relevant to the current RYA Yacht Master Ocean course syllabus are identified in the contents pages and section paragraph in italics.

Emphasis is needed that the astro-navigator does not need to know a myriad of stars and that most of the work in astro-navigation has been done for you. The precise predicted positions of all bodies at all times are recorded in the astro-navigational almanac.  Essentially all that needs to be done is to compare the height in the sky of a body from your unknown position to the known height of the body from a known position.  It is even easier using the Sun for latitude at mid-day.

The subject requires an ability to think in three-dimensional space, problematic for some, less so for others and to ease this potential pitfall I have used diagrams freely with every explained concept.  If background information helps in the understanding, then I make no apology for including what might seem to be superfluous detail.  Astro-navigation is a wonderful mix of art and science. The art is in the use of the sextant, in its care and adjustment and the taking and reading of a sight. The maths and physics may appear complex but can be rendered less fearsome. 

Do not become over-anxious at the thought of spherical trigonometry, intimate knowledge is unnecessary and most techniques minimize its use on the chart table, reducing the maths to careful addition and subtraction. Of course the modern wizardry of computer programs and internet access can provide useful help and take some of the work out of “astro”, but all sailors will recognise the importance of self-reliance in navigation as they do in boat-handling; a £250 sextant, a scientific calculator and a set of annual tables make a good place to start, for an outlay of £300.

Many when tackling this book may find the early pages too simplified but as with swimming; it is better to start gracefully in the shallow end rather than to immerse oneself in the middle, only to drown. The glossary is meant to be comprehensive and at first sight is daunting. Although it is at the beginning of this book, it is probably best not read at the beginning but referred to as necessary from within the text. The book’s divisions into sections will be self-explanatory, addressing the basic vocabulary, the details of the theory, the sextant and the tables used before considering in turn, time and the heavenly bodies. The examples shed light and explain the available options in handling the data. The appendix is a digest of the relevant plane and spherical trigonometry and explains the maths used in detail; an understanding being useful but not imperative.

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