Practical Tips- Astro Navigation - the lost art?

by Sam Jefferson on 3 Jun 2009 Chaldean shepherds, ranging trackless fields
Beneath the concave of unclouded skies
Spread like a sea, in boundless solitude,
Looked on the pole star, as on a guide
And guardian of their course, that never closed
His steadfast eye.

William Wordsworth, 'Excursion'

In these days of GPS, it's hard to believe that it was only a couple of decades ago that sailors heading offshore had no other option other than to reach for their sextant and fix their position using the sun and stars.

On a long passage, most of us simply look across at our GPS and get a very prompt digital readout. Yet there is little doubt that many old salts view the demise of celestial navigation with an element of sadness.

There is something remarkable, almost mystical, about relying on nothing but the sky above you to find your way. When you pick up your sextant and fix your position you are tapping into a tradition and skill dating thousands of years to a time when the Phoenicians looked to the heavens to find their way.

Perhaps it is that tradition and mysticism surrounding celestial navigation which still draws many boaters back to it, not to mention the fact that it can provide a vital back up.

It is also fair to say that the whole process can be pretty intimidating to the novice and this is where the RYA can help. The Yachtmaster Ocean shorebased course can have you fixing your position within the space of a weeek's intensive course. Although you do already need to be at Coastal Skipper standard in terms of navigational skill.

Tim Bartlett's RYA Astro Navigation Handbook also provides an invaluable guide by providing you with clear, simple explanations.

Tim explains: 'the very words 'Astro navigation' can send a shudder through even the most experienced boater.

'The funny thing is, though, that although a lot of Astro jargon has an almost mystical ring to it, most of us already know the 'secrets' on which the whole science of Astro navigation is based.'

Hopefully, this brief excerpt will provide you with the inspiration to take up this ancient art.

Five-Minute Astro

Imagine, for a moment, that you are somewhere in open water, with a single bright star directly overhead.

Astronomers have spent centuries watching, recording, and predicting the movement of stars, and the results of their efforts are published in star charts and almanacs.

So if you can identify which star you are looking at, you can find out where it is. And if you know where the star is, you know where you are.

But suppose the star isn’t directly overhead? If it’s almost directly above you, then you must be somewhere fairly close to the spot at which it is directly overhead. And if it’s low on the horizon, then you must be a long way away.

If we could measure, accurately, the angle between the star and our zenith (a point in the sky that is immediately overhead), we could be more precise than 'fairly close' or 'a long way away' because – as a rule of thumb – every degree corresponds to sixty miles.

As you move away from the point at which a star is directly overhead, the star appears to sink towards the horizon.

Unfortunately, it’s very difficult to be absolutely certain where the zenith is. But it’s relatively easy to measure the angle between a star and the horizon, to a high level of accuracy.

Suppose, for instance, that there’s a star directly over London, and another directly over New York, and that to us they appear to be 70° above our north west horizon and 50° above the horizon in
the north-east (see illustration).

Being seventy degrees above the horizon means that the London star is twenty degrees away from directly overhead. Using the 'one degree is sixty miles' rule, this means that we must be somewhere on the perimeter of a circle whose centre is directly below the star, and whose radius is 1200 miles.

Applying the same logic to the New York star means that we are also on another circle whose radius of 2400 miles and whose centre is in New York.

There are only two places where we can possibly be on both circles at once – and as one of them is off Iceland and the other is off the Azores, it shouldn’t be too difficult to tell which is which!

So the principle of astro navigation is quite straightforward: Measuring the altitude of a body above the horizon tells us how far we are from the point at which that body is directly overhead.

To put it into practice, there are just three problems to overcome:-
• measuring the angle between the heavenly body and the horizon to an accuracy of a fraction of a degree.

• knowing where the heavenly body was at the moment the angle was measured.

• drawing circles with radiuses of hundreds of miles.

All Done By Mirrors - The Sextant

The most fundamental measurement in astro navigation is the angle between the horizon and a heavenly body such as the sun, or a star – known as its altitude.

Mariners have used all sorts of contraptions for the purpose, but for the past two and a half centuries, the first choice has been a sextant.

How a sextant works

Light from the sun, star, or planet is reflected from the index mirror (1), through various removable shades (2), to the horizon glass (3), which reflects it into the telescope (4).

On a 'split view' sextant, the horizon glass is half mirror and half plain glass, so the user, looking through the telescope, can see the reflected image of the heavenly body in the silvered half, but can also see the horizon through the plain glass.

Another set of shades – the horizon shades (5) – is provided to reduce glare from the surface of the water.

On an 'all-view' sextant, the horizon glass isn’t split. Instead, it has an all-over coating that reflects like a mirror, but lets you see through it at the same time.

Whichever type of horizon glass you have, the end result is that you can look in two directions at once, adjusting the angle between the two lines of sight by moving the index arm (6), and reading off the angle on the scale (7) and the micrometer drum (8).

Every complete turn of the micrometer drum moves the line of sight by just one degree, so it’s very good at making small, precisely controlled movements. But it would be tedious – to put it mildly – if it took fifty turns of the micrometer to change the angle by fifty degrees.

To save time and temper, the micrometer can be disengaged by squeezing the clamp (9). The index arm then pivots freely until the micrometer is re-engaged by releasing the clamp.

Excerpts taken from Tim Bartlett’s RYA Astro Navigation Handbook. To find out more about Astro Navigation, you can pick up a copy by going to the RYA Web shop at: www.rya.org.uk or pick up a copy from any good marine retailer, such as Trans Pacific Marine in Auckland. Prices are available on request.

Information on the RYA Yachtmaster Ocean course, which covers Astro navigation, can also be found by going to www.cbes.org.nz/courses/5/ocean-yachtmaster/

For more information on all courses offered by www.cbes.org.nz!Coastguard_Boating_Education download http://www.sail-world.com/NZ/company_n.cfm?compid=978!Coastguard_Boating_Educations_brochure or visit their website www.cbes.org.nz