Celestial Navigation – Learn How to Master One of the Oldest Mariner`s Arts - Softcover

Cunliffe, Tom

Softcover

ISBN 10: 0470666331 ISBN 13: 9780470666333

Publisher: Fernhurst Books Ltd., 2010

Synopsis

Guiding a craft using one of the oldest of the mariner's arts-celestial navigation As romantic--sounding as sailing to Tahiti did centuries ago, using the sun, moon, planets, and stars to guide one's boat on the seas is enjoying a comeback. For those who do not wish to be caught short when modern technology fails on board, knowing how to chart a path with the aid of a sextant is an essential navigating skill. In Celestial Navigation , veteran yachtsman, Tom Cunliffe, reveals how any boat owner can master this seafaring technique, without complex mathematics, using his simplified approach. Readers begin with a sound foundation of basic concepts and definitions, before moving on to the hardware-the sextant and how to use it. Within a few pages, you'll be working out your latitude from a noon sight, and learning how to plot a position from observing the sun, planets, moon or stars-or wherever you may be on the world's oceans.Helps readers prepare for the RYA Yachtsmaster Ocean Exam, which includes celestial navigation as a requirement Includes clear diagrams and worked examples to help guide the novice navigator New edition has downloadable material, a new chapter on electronic navigation as well as the bonus feature of online video demonstrations Whether setting out to sea on a short or long journey, Celestial Navigation is an essential guide to using the sun, moon, planets, and stars as your guideposts home.

About this title

About the Author

Tom Cunliffe is Britain's leading sailing writer. He is a worldwide authority on sailing instruction and an expert on traditional sailing craft. He learned his sextant skills during numerous ocean passages, many of which in simple boats without engines or electronics, and voyaged to both sides of the Atlantic from Brazil to Iceland and from the Caribbean to Russia. He has cruised the coast of America and Canada and logged thousands of miles exploring both sides of the English Channel.Tom's nautical career has seen him serve as mate on a merchant ship, captain on gentleman's yachts and skipper of racing craft. His private passion is classic sailing boats and he has owned a series of traditional gaff-rigged vessels that have taken him and his family on countless adventures from tropical rainforests to frozen fjords.Tom has been a Yachtmaster Examiner since 1978 and has a gift for sharing his knowledge with good humour and an endless supply of tales of the sea.He also writes for Yachting Monthly, Yachting World and SAIL magazines, and wrote and presented the BBC TV series, The Boats That Built Britain.

From the Back Cover

Celestial navigation is one of the oldest of the mariner's arts - and one of the most awe-inspiring. To guide a small boat across the trackless oceans using a simple measuring device - the sextant - and a knowledge of the Sun, Moon and stars is a skill that borders on the magical. It is also essential for every ocean sailor who wants to be able to fix his position should the GPS fail.

In these pages Tom Cunliffe shows how to master the art in easy stages. His talent is to educate without intimidating. Starting from a sound foundation of the basic concepts and definitions, he moves on to the hardware: the sextant and how to use it. Within a few pages you'll be taking your first sight. From there it is a short step to plotting your position, wherever you may be on the world's oceans.

NEW EDITION FEATURES: Revised text and new photographsVideo tutorials accessed onlineAccess to downloadable calculation sheets

Whether you need to pass an exam, want a back-up to GPS positioning or simply choose to delight in the wonder of the cosmos, this is the perfect guide.

Tom Cunliffe is Britain's leading sailing journalist and writer. He learned his sextant skills during numerous ocean passages, many of them made in the days before GPS. Tom is a RYA(R) Yachtmaster[R] Instructor Examiner and has acted as consultant for US Sailing. He has lectured extensively around Britain and the USA and writes regularly for Yachting Monthly, Yachting World and SAIL magazine. He also wrote and presented the BBC TV series, "The Boats That Built Britain."

Wiley Nautical recommends this title for the Celestial Navigation section of the RYA [R] Ocean Yachtmaster [R] course.

ISBN: 9780470666333

UK 11.99 / US $19.95 / CDN $23.95

www.wileynautical.com

Front cover image: TAMAYA TECHNICS INC.

Excerpt. © Reprinted by permission. All rights reserved.

Celestial Navigation

By Tom Cunliffe

John Wiley & Sons

Copyright © 2010 John Wiley & Sons, Ltd
All right reserved.
ISBN: 978-0-470-66633-3

Chapter One

The Earth and the Heavens

We all learn as infants that the Earth revolves once a day and that the stars remain, to a greater or lesser extent, stationary. We also become aware that the Moon is in our own back yard, that the stars are plunging through space at various mind-boggling distances from us and that the Earth is travelling on an annual voyage around the Sun. Whether or not all this is true is of no relevance to the practical astro navigator.

For our purposes the Earth, otherwise known as the terrestrial sphere, may be taken to be a perfectly round ball swimming in a vacuum at the centre of the known universe. At the outside of the vacuum, an indeterminate but fortunately irrelevant distance away, is a further big ball which marks the perimeter of the universe. This ball is known as the celestial sphere. For our purposes all the heavenly bodies move in their courses on its inside surface, and its centre coincides exactly with the centre of the Earth.

THE TERRESTRIAL SPHERE

Any location on the Earth's surface can be expressed in terms of latitude and longitude.

Meridians of longitude

To define our position on the globe in an eastwest direction we make use of the meridians of longitude. These are great circles which converge at the poles of the Earth, a great circle being the line described on the Earth's surface by a plane passing through the centre of the Earth. In the case of a meridian, it is best thought of as what you would see if you pulled a segment out of a perfectly round orange. The segment starts and ends at the opposite poles of the orange. Its curved surface is the shortest distance between them on the surface of the orange. This definition becomes more important when great circle sailing is discussed later. For now, it is enough that a meridian runs direct from pole to pole on the surface of the terrestrial sphere.

Position is measured in terms of angular distance (see below) east or west of the zero or datum meridian. This passes through the Greenwich Observatory in England, and is known as the Greenwich Meridian. Those in denial of Britain's contribution to astronomy and longitude can choose to call this the International Reference Meridian, or the Prime Meridian. Longitude is measured in degrees east or west of Greenwich until east and west meet somewhere in the remote Pacific Ocean.

Parallels of latitude

Having determined our angular distance east or west of Greenwich we need another set of co-ordinates to fix us in a north–south direction. These are the parallels of latitude, which define angular distance north or south of the equator, which is actually the great circle on a plane at right angles to the Earth's axis, halfway between two poles.

The equator is the only parallel of latitude which fulfils the definition of a great circle. All the others are small circles (see diagram).

Geographic position

Any point on the Earth's surface fixed by its terrestrial co-ordinates (latitude and longitude), is known as a geographic position (GP).

Angular distance

For the non-specialist, distances between locations on Earth are generally expressed in miles or kilometres. This is convenient because we need to time our journeys. For the astro navigator, things are somewhat different. It would be impossible to try to handle the north–south distance between the stars Sirius and Aldebaran in terms of miles, but to say that it is 33° measured from the centre of the Earth is comprehensible and very easy to work with.

When dealing with spheres, the most convenient unit of distance is one degree of a circle. The Earth turns through around 25,000 miles in a 24-hour day at the equator. Because the meridians come together at the pole, it won't be anything like this far in Northern Norway. This inconvenience is done away with if we think of Earth as turning through 360 degrees in a day. This is angular distance. It's the same in Norway, the Caribbean and even for a masochist camped out a few yards from the North Pole.

Subdivision of degrees

A degree subdivides into 60 minutes (60), and each minute into 60 seconds (60). One minute of latitude is equal, at all latitudes, to one nautical mile (1M). One second of latitude is equal to 101 feet, or a few boat lengths for the average yacht. Since this is clearly too small to be of any serious use, minutes of arc are now more conveniently subdivided into decimal points, thus: 36°14.1N. A tenth of a mile is around 200 yards, the length of a unit of anchor rode in Nelson's navy, hence the term 'cable' when used for distance.

One minute of longitude equals one mile at the equator, but diminishes to zero at the poles. Working out what it represents in between in terms of miles would mean yet another calculation, so there, straight away, is a very good reason for the concept of angular distance.

THE CELESTIAL SPHERE

Just as it is possible to fix a position on the Earth's surface using its terrestrial co-ordinates of latitude and longitude, so the exact situation of a heavenly body on the surface of the celestial sphere can be defined by its celestial co-ordinates.

All the main features of the terrestrial sphere are mirrored in its celestial counterpart.

The terrestrial poles, if projected outwards from the centre of the Earth onto the celestial sphere, form the celestial poles. The terrestrial equator is projected outwards to throw a great circle onto the celestial sphere equidistant at all points from the celestial poles. This is called the celestial equator.

Celestial longitude – or Greenwich Hour Angle (GHA)

Since the first edition of this book, the notion of Greenwich Mean Time (GMT) has been replaced by Universal Time (UT). Modern almanacs and data in general refer nowadays to UT, but Greenwich remains the centre of operations for the celestial navigator. As for time alone, there is no practical difference between the two.

The celestial zero meridian is the projection of the terrestrial zero (Greenwich) meridian. However, whereas terrestrial longitude is measured from the Greenwich Meridian in degrees east or west around the world to 180 on the opposite side, celestial longitude, which is known as Greenwich Hour Angle (GHA), is measured to the westward only in degrees from 0 to 360°. When considering matters concerning the concept of Greenwich Hour Angle, never forget that it is merely a way of expressing celestial longitude.

You will see in the diagram on page 4 that 40°W longitude is the equivalent of a GHA of 40° on the celestial sphere, and that 120°E longitude marries up with GHA 240°. A second glance shows that if 120E were expressed in a 0° to 360° notation, beginning at Greenwich and working westward, it would represent a longitude of 240°. It is just a question of convention. For better or worse, longitude is expressed as 0° to 180° east or west, and GHA as 0° to 360°.

To convert east longitude to 360° notation and tie it in with the corresponding GHA, simply subtract the figure from 360°. Thus 120° east is equivalent to a GHA of 360 minus 120, or 240°.

To find the GHA of a body for a given time (and it changes by the second as the Earth turns) you need to consult The Nautical Almanac or one of the other available books containing the required data, known as the nautical ephemeris. By far the easiest of these to use, although not the cheapest, is the almanac itself, published jointly by HM Nautical Almanac Office, United Kingdom Hydrographic Office (NP 314) and, in the United States, by the United States Naval Observatory. These two books are one and the same. Illustrated on pages 28–29 are the pair of 'daily pages' from the almanac for 1st, 2nd and 3rd May of a given year. (The year for the examples in this book is actually 1986. In practice you would turn to the current year in your almanac.) The far left column of the right-hand page refers to hours of GMT and the next column gives the GHA of the Sun for the hour exactly. To find the increment by which it varies for minutes and seconds of time, turn to the 'increments' tables in the back pages of the almanac, an example of which is illustrated on page 30. Read off the answer, making sure that you take it from the correct column.

Note that since the heavenly bodies are moving westward, their GHA goes on increasing until it reads 360°, when it starts again. This means the minutes and seconds increments are always added to the hourly value of the GHA.

Example

What is the GHA of the Sun at 10h 15m 47s GMT on 1st May?

Notice that 43'.5 + 56'.8 equals 1°40'.3. Sixty minutes make one degree, not one hundred. In this case, 43.5 + 56.8 = 100.3 minutes. At 60 minutes to the degree, that makes 1°40'.3.

Celestial latitude, or declination

The cross co-ordinate used on the celestial sphere to fix the position of a heavenly body north or south on its GHA co-ordinate is its declination. As you'll by now be able to guess, it corresponds exactly to terrestrial latitude.

Declination is actually angular distance north or south of the celestial equator and, like terrestrial latitude, it is conveniently named north or south. A body with a declination of 42°N will, at some time in the 24-hour period, pass directly over the head of an observer in 42°N latitude.

Declination often changes with time. To calculate the declination of a body for a given moment consult the almanac. Look again at the daily pages illustration (pages 28–29) and notice that each column gives not only the changing GHA of the body, but also its declination.

At the bottom of the column is a small letter 'd' with a numerical value beside it. This is the rate of change per hour. Inspection of the hours adjacent to the one you are interested in will show whether the change is to be added or subtracted, depending on whether declination is increasing or decreasing.

Now look at the illustration of the 'increments' page (page 30), and check the column for each minute headed 'v' or 'd' correction.

Suppose you are interested in a 14-minute increment and a 'd' value of +0.9. Go down the column for 14 minutes as far as 'd' 0.9 and read off the value, which is +0.2. This figure is now added to the hourly declination figure you've taken from the daily page. Notice that 'v' and 'd' corrections do not refer to seconds of time. The figures in the column are for minutes only, which is invariably quite accurate enough.

In practice, many people can usually work out the declination for a given number of minutes after the hour by inspection and mental arithmetic, so recourse to the increment pages for changing declination is rare. In the case of the Moon, however, declination varies rapidly and hugely, so the mental arithmetic involved in bypassing the 'd' increment is way beyond me. Here, then, is an example of its use:

Example

What is the declination of the Moon at 2314 on 3rd May?

Dec 23h S 8° 15'.3 - d(14.4) 14m 3'.5 Dec 2314 S 8° 11'.8

Note that in this case 'd' is negative because declination is decreasing, and that the declination is always labelled N or S.

Zenith

An observer's zenith is his terrestrial position projected from the centre of the Earth onto the celestial sphere. In other words, the point directly above his head. The declination of his zenith is the same as his latitude. The GHA of his zenith is the same as his longitude, although in east longitude it will be necessary to adjust the longitude figure to read 0° to 360° notation by subtracting it from 360°.

Opposite the observer's zenith is the celestial position delightfully termed his nadir. Project a line from the zenith through the observer to the centre of the Earth, keep going until you hit the celestial sphere on the other side, and you have it. As the name suggests, it's about as low as you can get.

Local Hour Angle

In the majority of the calculations involved in celestial navigation, the data required will not be the Greenwich Hour Angle of the body concerned, but the Local Hour Angle (LHA).

Just as the GHA of the body at a given time is its angular distance west of the Greenwich Meridian, so the LHA of the same body is its angular distance to the west of the observer's meridian.

Given the GHA of the body from the almanac (see page 3) and some idea of your longitude, working out the body's approximate LHA is straightforward.

As always with angular questions, when in doubt draw a diagram. Below are four examples to illustrate the four most likely calculations of LHA. They are quite simple and it is vital that they are understood. Without a grasp of the concept of Local Hour Angle, the rest of the book will simply not make sense.

Case 1

West longitude: GHA of Sun greater than observer's longitude.

In this case

LHA = GHA minus longitude west.

Example

What is the LHA of the Sun at 16h 15m 27s GMT on 1st May? Your longitude is 15°23'W.

GHA 16h 60° 43'.9 + Increment 15m 27s 3° 51'.8 GHA Sun 64° 35'.7 - Longitude west 15° 23'.0 LHA 49° 12'.7

Case 2

West longitude: GHA less than observer's longitude.

A study of the diagram will show that the logical answer in this case is to find the difference between the longitude west and the GHA, then subtract it from 360 (the remainder of the full circle).

On the face of it, this looks a bit awkward. By far the easiest way to handle these numbers is to add the GHA to 360 and then subtract the west longitude. The answer comes out right every time.

Example

What is the LHA of the Sun at 14h 16m 18s GMT on 3rd May? Your longitude is 40°13'W.

GHA 14h 30° 47'.2 + Increment 16m 18s 4° 04'.5 GHA Sun 34° 51'.7 + 360 360° GHA + 360 394° 51'.7 - Longitude west 40° 13'.0 LHA 354° 38'.7

In both examples, LHA = GHA minus longitude west. If longitude west happens to be greater than LHA and makes the sum a nonsense, just add a quick 360 where it counts and all will be well.

Case 3

East longitude: GHA a smaller value than the longitude (expressed in 360 notation).

A glance at the diagram makes this one obvious, remembering always that LHA is the angular distance of the body from the observer, moving to the westward (clockwise on the diagram). In this case LHA = GHA + longitude east.

Example

What is the LHA of the Sun at 03 h 15 m 22 s on 1st May? Your longitude is 110°E.

GHA 03 h 225° 42'.9 + Increment 15 m 22 s 3° 50'.5 GHA Sun 229° 33'.4 + Longitude east 110° LHA Sun 339° 33'.4

Case 4

East longitude: GHA a greater value than longitude (expressed in 360° notation).

This is easier than a first glance at the diagram might suggest. You are looking for the angular distance to the westward between the observer and the Sun or star. One way to do this is to work your longitude into 360° notation and subtract it from the GHA, but the easiest method is to add up the GHA and the longitude expressed conventionally as degrees east (of Greenwich). The sum of the two will be greater than 360° which is a nonsense, but if you subtract 360° from the result, you will have the right answer.

(Continues...)

Excerpted from Celestial Navigationby Tom Cunliffe Copyright © 2010 by John Wiley & Sons, Ltd. Excerpted by permission of John Wiley & Sons. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
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Edition details

• Publisher: Fernhurst Books Ltd.
• Publication date: 2010
• Language: English
• ISBN 10: 0470666331
• ISBN 13: 9780470666333
• Binding: Paperback
• Edition number: 3
• Number of pages: 80