ASTRONOMICAL NAVIGATION

This chapter is not complete yet. However it is progressing, and the first few section are worth a read to get a flavour of what will (hopefully) be ready in a few weeks.

In the meantime, contact me if you would like to buy a printed publication which covers this subject.

OVERVIEW

Many people are fascinated by astronomical navigation.

For one thing, it is regarded as the pinacle of the navigator's art - people feel that once you can do this, you have mastered the most complex part of navigation. (This is partly true!)

Secondly it has historical interest - it has been used by navigators for hundreds of years in very much the same way, using pretty much the same tools and methods: sextant, chronometer, tables.

For me the interesting part is that up until comparatively recently, it was the only thing available to the long distance ocean sailor. Before about 1988 GPS simply didn't exist in a usable form, and the earlier global navigation systems (Transit, Omega) were confined mainly to larger vessels.

I genuinely don't know whether I would have had the guts to cross an ocean without GPS, and I greatly admire those that did. I feel that we owe it to those people to take the time to learn to do what they did, even though most of us will never have to use astronomical navigation for real.

For me there is a second, more pertinant, reason to learn astro navigation - and this is probably also the reason why it is still in the syllabus for Yachtmaster Ocean. If, for whatever reason, GPS fails when you are out of sight of land, astro navigation can still provide you with a position fix. No electrical or electronic device is required to achieve this. So if, for example, your GPS (and spare) are swamped with water whilst crossing an ocean - unlikely though this may be - astro navigation provides an alternative method of fixing your position, and there really is no other (save happening upon another vessel and asking them for a position).

Picture: how would you get on fixing this, if the display is blank or unreadable?

INTRODUCTION

The content of this chapter follows directly on from the "reason" set out in the paragraph above.

There is undoubtedly a huge amount of complicated stuff that could be written about this subject, but this chapter is confined to just one question:

• what does the average long distance yacht skipper - who normally navigates using GPS - need to know, in order to do a successful astro navigation position fix?

What you need: equipment and knowledge

The equipment you require for Astro Navigation is as follows:

You need a sextant. This needn't cost the earth: plastic ones are available, and there is a useful second-hand trade in those, and in high quality metal ones (try ebay or trinitiymarine.co.uk for a start). You obviously want one that is in good repair, and reject any that have any "play" in the index arm or signs of wear on the thread - you are going to want to measure to the nearest minute (60th of a degree), and this requires a precise instrument with the minimum of wear or damage.

You need to learn and practice how to use a sextant (not very complicated).

You need an accurate watch or chronometer - modern electric watches, or good quality clockwork ones, are easily accurate enough to give you the time to the nearest second, provided you keep a note of how fast/slow they are and note how many seconds they gain/lose in a day.

You need a source of information to give you the position of the sun (or moon, planets, stars) at any particular time in the day, for every day in the year. The most obvious is The Nautical Almanac, which is an annual publication.

You need a method to do the sight reduction calculation, and you need to learn - and practice - how to do this by your chosen method.

That's it. The choice lies in the last one of these, and it's worth discussing this.

Sight Reduction: Calculator or Tables?

The quickest - and easiest - method of doing the calculation is with a calculator. (Hell, why not use a lap-top?) This, however, represents something of a departure from the ideal situation where you are placing no reliance on electronics or electricity. If your GPS has run out of power or is under a foot of water, it's just possible that your calculator might have suffered the same fate.

Besides, speed is not really an issue. It might be if you were doing this as your primary method of position fixing, but of course you're not: you have a GPS for that. Doing a sight reduction "for real" is going to be an unusual event for you, so it doesn't matter if it takes an hour (the tables process will take about 20 minutes if you practise).

So I am going to assume that you want to make no use of electrics. This means using sight reduction tables, and again we have two choices:

• The Sight Reduction Tables for Air Navigation, which comes in three volumes, and is probably the most frequently used / taught on Yachtmaster Ocean courses

• The Sight Reduction Table in The Nautical Almanac.

The former is slightly quicker (but see comment about speed above), and slightly easier to use. The latter has the advantage that, because you need to have The Nautical Almanac anyway (see above), you already have these tables, and they do not occupy three additional bulky volumes.

Take your pick. This chapter describes how to use both procedures (they have a lot in common).

Practical Steps for Ocean Passage Making

Given that you do need to guard against the possibility of the GPS failing, this is what I recommend:

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

2. Carry information on oceanic currents. (Tidal streams are not relevant for ocean sailing).

3. Plot your GPS position on the chart regularly, and also work up a DR and EP (using currents). 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, so you have a record of how much it gains/loses per day. (Time signals are available from anlogue radio, e.g. SSB - NOT digital radio or internet, where you get a delay; and accurate time is available from a working GPS).

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.

Yachtmast Ocean Qualification?

Some people will be studying Astro Navigation in order to get a Yachtmaster Ocean qualification, and may wish to know whether the choice of method is important in gaining that qualification.

At the time of writing this (check with the RYA for any revisions) a requirement for the Yachtmaster Ocean Certificate of Competence is: "Candidates must have completed work at sea which shows that they have successfully navigated a yacht at sea by astro navigation."

There are some details as to what types of sight you are required to have carried out, but no constraint on the method used for sight reduction. I strongly advise (in this as in other matters) that you select the method that you yourself prefer as skipper - don't be dominated by other people's rules! If your Yachtmaster Ocean Examiner finds it difficult to check your work because he is not familiar with your method, then invite him to check it by doing the sight reduction with his own method: he should get the same answer.