ASTRONOMICAL NAVIGATION - Book & Sight Reduction Pads

Astro navigation is not nearly such a big feature of yachtsmen’s lives as it was just a very few years ago before GPS was available. To “GPS cruisers” like myself, the achievements of skippers sailing less than ten years before I did my first long distance cruise, are just amazing – it is almost unimaginable having no access whatever to electronic navigation, particularly as most of our generation had the perfectly respectable Decca or Loran systems to guide us around coastal waters.

Why Astro?

So, what is the relevance now of this “ancient” system of navigation?

Well, for one thing it is still required for the Yachtmaster Ocean qualification. But perhaps more relevant to the average long distance yachtsman is this: things go wrong on boats, and the GPS is not immune from that. Turn it on and nothing appears on the screen – and what are you going to do? Check the power, and that’s about all you can do. Yes, carry spare receivers, but the golden rule in navigation is pretty much never rely on a single source of navigational information, so let’s not start relying solely on GPS when we’re further from land than we’ve ever sailed before!

So as you would expect, the great majority of ocean sailors use GPS as their primary navigation tool, but most also wish to have an alternative method of position fixing available to them. Ideally the alternative will make no use of electricity, or any gadgets that could fail. Astro Navigation is the natural answer, and is widely used in this role. 

(Actually pretty nearly ALL GPS receivers did fail during the “week number roll-over” which happened for the first time one morning in the summer of 1999, and I heard one cross-channel yacht – who had two receivers on board - call the coastguard for a radio directional position fix that day ...)

Practical Approach on Long Distance Passages

So what do you do in practice? Given that you do need to guard against the possibility of the GPS failing, this is what I recommend for long distance passages:

1. Keep a good written record of course and logged distance, together with weather and barometric pressure. Note your GPS position regularly.

2. Carry information on oceanic currents.

3. Plot your GPS position on the chart regularly, and also work up a DR and EP. They should agree reasonably closely even after a few days. Remember that magnetic variation changes with position (it is marked on the chart).

4. Carry an accurate watch or clock on board, and note to the nearest second how fast or slow it is. Write this in the log regularly. (Time signals are available from the radio and, of course, from the GPS while it's working).

5. From time to time, for amusement and above all to gain confidence in the method, do an astro sight and work out a position line. Compare this with your GPS plot.

6. If you normally use electronic charting on board, carry in addition at least one paper chart covering your whole passage.

The only part of this which needs any study is the astro sight. There are a number of methods and approaches. Having worked out a convenient method for my own use, I have prepared a book and forms to assist other people.

The Compact Tables Method

As discussed above most of us use GPS as their primary navigation tool, and astro is naturally a secondary method. And as a secondary method, we wish to carry it in the most convenient form available.

You WILL need to carry The Nautical Almanac, because that gives the basic “ephemeris” data, i.e. the position of the sun, moon, planets and stars for every hour of every day.

With that on board you DON’T need any other tables – because the concise “sight reduction” tables in the Nautical Almanac are quite adequate anywhere in the world. All you need is a watch and a sextant. But it’s also extremely convenient to have some forms, which guide you through the whole process from sextant sight to position line on the chart. I use these forms. And the book tells you exactly how to work your way through them.

The Book

This book, together with the forms provided with it, gives very detailed instructions for taking the sight and using the tables in The Nautical Almanac. It does include a clear explanation of the method, BUT you don’t have to understand all that in order to use it. 

I have taken the approach that you don’t need to understand food chemistry in order to follow a recipe – there is a detailed, fully illustrated process (45-steps I’m afraid, there aren’t any short cuts) that WORKS.

The book includes practice exercises and – well the “bottom line” is you really don’t need anything else to do an astro sight, and I’ve been relying on this method for years.

I would LOVE to be able to say that this book is available from “all good bookshops”, but at the present time it’s only available from two: Stanfords in Long Acre, and the Sea Chest Nautical Bookshop in Plymouth – or of course direct from Aztec Sailing.

 Price £12.95 + postage for the book and a pad of forms.

For extra forms, download here.