Astro Navigation

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Issued May 2000

Superseding BR 45(2)

Dated May 1997

BR 45(2)

ADMIRALTY MANUAL OFNAVIGATION

VOLUME II

ASTRO NAVIGATION

By Command of the Defence Council

COMMANDER IN CHIEF FLEETCINCFLEET/FSAG/P45/2


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PREFACE

The Admiralty Manual of Navigation (BR 45) consists of seven volumes:

Volume 1 is a hard bound book (also supplied in A4 loose leaf from 2002), covering General |

Navigation and Pilotage (Position and Direction, Geodesy, Projections, Charts and Publications, |

Chartwork, Fixing, Tides and Tidal Streams, Coastal Navigation, Visual and Blind Pilotage, |

Navigational Errors, Relative Velocity, Elementary Surveys and Bridge Organisation). This |

book is available to the public from The Stationary Office. |

Volume 2 is a loose-leaf A4 book covering Astro Navigation (including Time). Chapters 1 to |

3 cover the syllabus for officers studying for the Royal Navy ‘Navigational Watch Certificate’ |

(NWC) and for the Royal Navy ‘n’ Course. (The NWC is equivalent to the certificate awarded |

by the Maritime & Coastguard Agency (MCA) to OOWs in the Merchant Service under the |

international Standardisation of Training, Certification and Watchkeeping (STCW) agreements.) |

The remainder of the book covers the detailed theory of astro-navigation for officers studying |

for the Royal Navy Specialist ‘N’ Course, but may also be of interest to ‘n’ level officers who |

wish to research the subject in greater detail. Volume 2 is not available to the general public, |

although it may be released for sale in the future. |

Volume 3 is a protectively marked loose-leaf A4 book, covering navigation equipment and |

systems (Radio Aids, Satellite Navigation, Direction Finding, Navigational Instruments, Logs |

and Echo Sounders, Gyros and Magnetic Compasses, Inertial Navigation Systems, Magnetic |

Compasses and De-Gausing, Automated Navigation and Radar Plotting Systems, Electronic |

Chart equipment). Volume 3 is not available to the general public. |

Volume 4 is a protectively marked loose-leaf A4 book covering conduct and operational methods |

at sea (Navigational Command and Conduct of RN ships, passage planning and routeing, and |operational navigation techniques that are of particular concern to the RN). Assistance |

(Lifesaving) and Salvage are also included. Volume 4 is not available to the general public. |

Volume 5 is a loose-leaf A4 book containing exercises in navigational calculations (Tides and |

Tidal Streams, Astro-Navigation, Great Circles and Rhumb Lines, Time Zones, and Relative |

Velocity). It also provides extracts from most of the tables necessary to undertake the exercise |

calculations. Volume 5 (Supplement) provides worked answers. Volumes 5 and 5 (Supplement) |

are not available to the general public, although they may be released for sale in the future. |

Volume 6 is supplied in three, loose-leaf A4 binders: the non-protectively marked Binder 1 |

covering generic principles of shiphandling (Propulsion of RN ships, Handling Ships in Narrow |Waters Manoeuvring and Handling Ships in Company, Replenishment, Towing, Shiphandling |

in Heavy Weather and Ice), and the protectively marked Binders 2 and 3 covering all aspects of |

class-specific Shiphandling Characteristics of RN Ships / Submarines and RFAs). Turning data |

quoted in Volume 6 is approximate and intended only for overview purposes. Volume 6 is |

not available to the general public, although Binder 1 may be released for sale in the future. |

Volume 7 is a protectively marked loose-leaf A4 book covering the management of a chart outfit |

(Upkeep, Navigational Warnings, Chronometers and Watches, Portable and Fixed Navigational |

Equipment, and Guidance for the Commanding Officer / Navigating Officer). Volume 7 is not |

available to the general public. |

| Note. Terms appearing in italics in newer books are defined in the ‘Glossary’ of each book. |


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PROPOSALS FOR CHANGES

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Forward copies of the above form through the usual Administrative Channels to the addressees

listed on Page ii.


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CONTENTS

Chapter 1 The Celestial Sphere - Introduction

Section 1 Basic Definitions and Structure |Section 2 The Magnitudes of Stars and Planets |

Section 3 Methods of Identifying Heavenly Bodies |

Chapter 2 Time Systems

Chapter 3 Practical Planning, Taking, Reduction and Plotting of Sights

Section 1 Introduction |

Section 2 Planning Astro Sights |Section 3 Description, Preparation and Use of Sextant |

Section 4 Reducing Sights (Processing of Sextant Readings) |

Section 5 Plotting Sights |

Annex 3A NAVPAC 2 - Extracts from HM Nautical Almanac Office NAVPAC 2 User Instructions

Chapter 4 The Celestial Sphere - Definitions, Hour Angles and the Theory of Time

Section 1 ‘Ready Reference’ List |Section 2 Hour Angles |

Section 3 Solar Time |

Section 4 Sidereal Time |

Section 5 Lunar and Planetary Time |

Chapter 5 Identification of Heavenly Bodies, Astronomical Position Lines, ObservedPosition and Sight Reduction Procedures

Section 1 Identification of Heavenly Bodies |Section 2 Astronomical Position Lines |

Section 3 Calculating Altitude, Azimuth and True Bearing |Section 4 Sight Reduction Procedures |

Section 5 Very High Altitude (Tropical) Sights |Section 6 High Latitude (Polar) Sights |

Annex 5A Description and Setting of the Star Globe

Chapter 6 Meridian Passage and PolarisSection 1 Meridian Passage |

Section 2 Polaris |

Chapter 7 The Rising and Setting of Heavenly BodiesSection 1 Requirements and Generic Definitions |

Section 2 Sunrise, Sunset and Twilights |

Section 3 Moonrise and Moonset |

Section 4 High Latitudes |

Chapter 8 Refraction, Dip and Mirage

Chapter 9 Errors in Astronomical Position Lines

Appendix 1 The Sky at Night

Appendix 2 Extracts from the Nautical Almanac (1997)

Index Index

LEP List of Effective Pages


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ACKNOWLEDGEMENTS AND COPYRIGHT

UK Hydrographic Office (UKHO)

Thanks are due to the UK Hydrographic Office (UKHO) for their permission and

assistance in reproducing data contained in this volume. This data has been derived from

material published by the UKHO and further reproduction is not permitted without the prior

written permission of CINCFLEET/PFSA and UKHO. Applications for permission should be

addressed to CINCFLEET/PFSA at the address shown on Page ii and also to the Copyright

Manager at UK Hydrographic Office, Admiralty Way, Taunton, Somerset TA1 2DN.

HM Nautical Almanac Office (HMNAO) and the Council for the Central Laboratory of the

Research Councils (CCLRC)

Thanks are due to the HM Nautical Almanac Office (HMNAO) for their assistance in

reproducing data contained in this volume. The material from the ‘Nautical Almanac’ and from|

‘NAVPAC and Compact data 2001-2005’ (published by the Stationary Office) is reproduced by|

kind permission of the Council for the Central Laboratory of the Research Councils (CCLRC).

‘NAVPAC and Compact data 2001-2005’ is also published by Willmann-Bell in the US under |

the name ‘AstroNavPC and Compact data 2001-2005’. Further reproduction of this data is not|

permitted without the prior written permission of CINCFLEET/PFSA and CCLRC. Applications

for permission should be addressed to CINCFLEET/PFSA at the address shown on Page ii and

also to HMNAO, Space Science and Technology Department, Rutherford Appleton Laboratory,

Chilton, Didcot, Oxon OX11 0QX, United Kingdom.

General

Other parts of BR 45 Volume 2 not covered by the copyright notes above are MOD

copyright and further reproduction is not permitted without the prior written permission of

CINCFLEET/PFSA at the address shown on Page ii.


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CHAPTER 1

THE CELESTIAL SPHERE - INTRODUCTION

CONTENTS

SECTION 1 - BASIC DEFINITIONS AND STRUCTURE

Para

The Celestial Sphere . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0101

Angular Distance Between Stars . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0102

Apparent Path of the Sun in the Celestial Sphere . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0103

The First Point of Aries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0104

Declination and Parallels of Declination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0105

Hour Angles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0106

Sunrise and Sunset . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0107Twilight . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0108

Geographic Position of a Heavenly Body . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0109

Great Circles and Small Circles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0110

Meridian . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0111

Greenwich Meridian . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0112

Rhumb Lines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0113

Observer’s Zenith (Z) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0114

Celestial Horizon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0115

Visible Horizon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0116

Azimuth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0117

Altitude (of a Heavenly Body) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0118

Vertical Circles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0119

SECTION 2 - THE MAGNITUDES OF STARS AND PLANETS

The Solar and Stellar Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0120

Stellar Magnitudes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0121

SECTION 3 - METHODS OF IDENTIFYING HEAVENLY BODIES

The Identification of Heavenly Bodies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0130

Use of Computers for Identification of Heavenly Bodies . . . . . . . . . . . . . . . . . . . . . . . 0131

Description of the Star Finder and Identifier (NP 323) . . . . . . . . . . . . . . . . . . . . . . . . . 0132

The Nautical Almanac Planet Diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0133


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CHAPTER 1

THE CELESTIAL SPHERE - INTRODUCTION

SECTION 1 - BASIC DEFINITIONS AND STRUCTURE

0101. The Celestial Sphere

To an observer on Earth, the sky has the appearance of an inverted bowl, so that the stars

and other heavenly bodies, irrespective of their actual distance from the Earth, appear to be

situated on the inside of a sphere of immense radius described about the Earth as centre. This is

called the Celestial Sphere (Fig 1-1). The Earth’s axis, if produced, would cut the Celestial

Sphere at the Celestial Poles (P, P’). The Earth’s equator, if produced, would cut the Celestial

Sphere at the Celestial Equator (Q, Q’).

Fig 1-1. Celestial Sphere, Celestial Poles and Celestial Equator

0102. Angular Distance Between the Stars

The appearance of the stars on the Celestial Sphere conveys no idea of their actual

distances from the Earth. Two stars chosen at random may actually be at vastly different

distances from earth, but as both are deemed to reside on the surface of the Celestial Sphere, the

only practical method of measuring their relative positions is to measure the angle between them.

This angle is known as an Angular Distance. As the stars are immensely far away, the Angular

Distances of stars remain virtually constant within the ordinary limits of time. The position of

a heavenly body on the celestial sphere can be defined by two Angular Distances - ‘Declination’

and ‘Hour Angle’ which are explained more fully at Paras 0105-0106 and Chapter 4.


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0103. Apparent Path of the Sun in the Celestial Sphere

a. The Ecliptic. The Earth describes an elliptical orbit around the Sun which takes

one year to complete. The apparent path of the Sun in the Celestial Sphere is known as

The Ecliptic. It is a Great Circle, and makes an angle of 23° 27' (23½°) with the|

Celestial Equator because the Earth’s axis of rotation is tilted by that amount from the

perpendicular to the plane of the Earth’s orbit (Fig 1-2). The angle between the plane of

the Celestial Equator and that of the Ecliptic is known as the Obliquity of the Ecliptic.

Fig 1-2. Celestial Equator, Plane of the Ecliptic and First Point of Aries

b. Seasons, Tropics, Solstices and Equinoxes. The existence of the Earth’s 23° 27'tilt is of fundamental importance to life on earth, as it defines the limits of the tropics,

causes the seasons to change and the length of daylight to vary during the year (outside

the equatorial region where very little change takes place). The extent of the Sun’s

apparent movement can be established by plotting the Latitude of positions on Earth

where the noon sun is directly overhead at some time during the year (Fig 1-3) . The Sun

is directly over the Equator at the Spring Equinox (21 March), moves north to Latitude

23½° at the Summer Solstice ( 21 June), back to the Equator at the Autumn Equinox (23

September), moves south to Latitude 23½° at the Winter Solstice (22 December) and

back to the Equator at the Spring Equinox (21 March). The seasonal changes caused by

this apparent movement of the sun through the year have a profound effect on ocean

currents, weather systems and overall climate. Many biological ecosystems in the world

depend on these seasonal changes for their existence (Fig 1-4).


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Fig 1-3. Latitude of Positions on Earth where the Noon Sun is Directly Overhead |

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Fig 1-4. Change of Seasons during the Year, Associated with Sun’s Movement |

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0104. The First Point of Aries

To measure Angular Distances, a fixed point in space is needed as a datum; a star locatedwhere the Ecliptic cuts the Celestial Equator would be ideal for this. When the early Greek

astronomers started to make observations, the Ecliptic cut the Celestial Equator at the Spring

Equinox (21st March) in the vicinity of the constellation of Aries; one star on the edge of the

constellation, known as the First Point of Aries ( ), was perfectly aligned and so was selected

as this datum (Fig 1-2). Over time, due to slow Precession of the earth’s tilt (see Para 0544f for

a full explanation of Precession), there has been a backward movement of the point of

intersection of the Ecliptic and the Celestial Equator . As a result, Aries has ‘apparently moved

away’ from this position. However, the name ‘ First Point of Aries’(normally abbreviated to |

‘ Aries or ’) for the spring intersection of the Ecliptic and the Celestial Equator has been |

retained as the datum for calculations and tables ever since, even though no star now occupies

this position. The position of the autumn intersection of the Celestial Equator and the Ecliptic

(23rd September) is known as the First Point of Libra.


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0105. Declination and Parallels of Declination

Declination corresponds to terrestrial Latitude projected onto the Celestial Sphere and

is the Angular Distance of the heavenly body north or south of the Celestial Equator (Fig 1-6).

A Parallel of Declination corresponds to a terrestrial parallel of Latitude and is a Small Circle

on the Celestial Sphere, with its plane parallel to the plane of the Celestial Equator .

Note 1-1. Although the concept of projecting terrestrial Latitude onto the Celestial Sphere is

a very good description, ‘Declination’ should never be described as ‘Celestial Latitude’ because

this term is used by astronomers to measure an Angular Distance, referenced to the Ecliptic

rather than the Celestial Equator. ‘Celestial Latitude’ has no use in the navigational problem.

0106. Hour Angles

Hour Angles loosely correspond to terrestrial Longitude projected onto the Celestial

Sphere, but the analogy is complicated by the easterly rotation of the Earth which continually

changes some Angular Distances with time. It was because of this fundamental link to time that

the term Hour Angles was used to describe this measurement. There are several variants of Hour

Angle which, depending upon which two bodies are to be referenced for measurement, can beadded or subtracted to calculate the required Angular Distance. Further details of these are at

Chapter 4 but do not concern students studying for the Royal Navy NWC (Navigational

Watchkeeping Certificate) except familiarity with the titles and where to look up the data if using

the Star Finder and Identifier (Paras 01312 and 0324) or The Nautical Almanac Planet Diagram

(Para 0133). A brief summary of these terms is as follows:

a. Sidereal Hour Angle (SHA). The Sidereal Hour Angle (SHA) is almost static for

stars and is tabulated once per 3 days for stars and planets in The Nautical Almanac.

b. Right Ascension (RA). Right Ascension (RA) is the same as SHA except measured

eastwards (rather than westwards as in SHA). Thus RA = 360° - SHA.

c. Greenwich Hour Angles (GHA). The Greenwich Hour Angle (GHA) of the First

Point Aries ( ) and the GHAs of the Sun, Moon and Planets are tabulated hour-by-hour

(and can be established to the second using Increment Tables) in The Nautical Almanac.

d. Local Hour Angle (LHA). The Local Hour Angle (LHA) is GHA of the body +/-

the observers’s Longitude.

Note 1-2. Although the concept of projecting terrestrial Longitude onto the Celestial Sphere

is a useful analogy, ‘Hour Angles’ should never be described as ‘Celestial Longitude’ because

this term is used by astronomers to measure an Angular Distance, referenced to the Ecliptic

rather than the Celestial Equator. ‘Celestial Longitude’ has no use in the navigational problem.

0107. Sunrise and Sunset

a. Visible Sunrise or Sunset. Visible Sunrise or Sunset occurs when the Sun’s Upper

Limb (UL) appears on the Visible Horizon (ie. the Apparent Altitude of the Sun (UL) is

0° 00'). The times of Visible Sunrise and Sunset for Latitudes 60°S to 72°N are

displayed on right hand pages of The Nautical Almanac. These times, which are given

to the nearest minute, are the UT of the Sunrise / Sunset on the Greenwich Meridian for

the middle day of the three days covered by each double page.

b. True (Theoretical) Sunrise or Sunset. True (Theoretical) Sunrise or Sunset occurswhen the Sun’s centre is on the Celestial Horizon, but due to Atmospheric Refraction the

Sun’s Lower Limb appears to be one Semi-Diameter above the Visible Horizon.


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0108. Twilight

Twilight is the period of the day when the Sun is between 0° and 18° below the Celestial

Horizon. During Twilight , although the Sun is below the Celestial Horizon, the observer is still

receiving light reflected and scattered by the upper atmosphere.

a. Civil Twilight (CT). The times of Morning Civil Twilight (MCT) and Evening

Civil Twilight (ECT) are tabulated in The Nautical Almanac for the moment when the

Sun’s centre is 6° below the Celestial Horizon. The times are shown in chronological

order and the terms ‘ Morning ’ and ‘ Evening ’ are omitted. This is roughly the time at

which the horizon becomes clear (morning) or becomes indistinct (evening).

b. Nautical Twilight (NT). The times of Morning Nautical Twilight (MNT) and

Evening Nautical Twilight (ENT) are tabulated in The Nautical Almanac for the moment

when the Sun’s centre is 12° below the Celestial Horizon. The terms ‘ Morning ’ and

‘ Evening ’ are omitted as the times are in chronological order. Morning and evening stars

are usually taken between the times of Civil Twilight (CT) and Nautical Twilight (NT).

c. Astronomical Twilight. The time of Astronomical Twilight (AT) is the moment

when the Sun’s centre is 18° below the Celestial Horizon. Whilst the Sun’s centre is 18°

or greater below the Celestial Horizon, ‘Total Darkness’ (with respect to the Sun) is

deemed to exist and observations by astronomers may usefully take place. The times of

Astronomical Twilight (AT) have no significance in solving the astro- navigation problem

and so AT times are not tabulated in The Nautical Almanac.

0109. Geographic Position of a Heavenly Body

The Geographic Position of a heavenly body is the position where a line drawn from the

body to the centre of the Earth, cuts the Earth’s surface. To an observer at the Geographic

Position, the heavenly body would appear to be directly overhead, ie. at the Observer’s Zenith(Z).

0110. Great Circles and Small Circles

Great Circles and Small Circles are defined and discussed in BR 45 Volume 1. For the

convenience of readers their definitions are repeated here:

• Great Circle. The intersection of a spherical surface and a plane which passes

through the centre of the sphere is known as a Great Circle. It is the shortest

distance between two points on the surface of a sphere.

• Small Circle. The intersection of a spherical surface and a plane which does NOT pass through the centre of the sphere is known as a Small Circle.

0111. Meridian

A Meridian is a semi - Great Circle on the Earth’s surface which also passes through

both Poles.

0112. Greenwich Meridian

The Greenwich Meridian is also known as the Prime Meridian, and passes through

Greenwich. It is the starting point (0°) for the measurement of Longitude, East and West from

this Meridian.


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0113. Rhumb Lines

Rhumb Lines are defined and discussed in BR 45 (1). For the convenience of readers the

Rhumb Line’s definition is repeated here:

Rhumb Line. A line on the Earth’s surface which cuts Meridians (of Longitude) and

Parallels (of Latitude) at the same angle is known as a Rhumb Line. It appears on Mercator Charts as a straight line and equates to the (True) compass course steered. It

is NOT always the shortest distance between two points on the surface of a sphere. (See

BR 45(1) for information on Meridians, Parallels and Mercator Charts.)

0114. Observer’s Zenith (Z)

The Observer’s Zenith (Z) is the point where a straight line from the Earth’s centre

passing through the observer’s terrestrial position cuts the Celestial Sphere, and may be described

(loosely) as the point on the Celestial Sphere directly above the observer. The Declination of this

point (Z) on the Celestial Sphere is equal to the observer’s Latitude.

0115. Celestial Horizon

The Celestial Horizon is a Great Circle on the Celestial Sphere, every point of which is

90° from the Observer’s Zenith (Z). It corresponds to the projection of the terrestrial horizon onto

the Celestial Sphere, but without the errors associated with atmospheric optical refraction at the

Visible Horizon.

0116. Visible Horizon

The Visible Horizon is position on the Earth’s surface where a straight line drawn from

an observer, at a given Height of Eye, meets the Earth’s surface as a tangent to that surface.

0117. Azimuth Azimuth may be regarded (loosely) as the True Bearing when using tables in The Nautical

Almanac. More precise definitions may be found at Paras 0535 and 0536.

0118. Altitude (of a Heavenly Body)

Altitude is (loosely) described as the angle between a ‘horizon’ and the heavenly body,

but normally has to be qualified as Sextant Altitude, Apparent Altitude, Observed (True) Altitude

or Calculated (Tabulated) Altitude depending which ‘horizon’ is used and which corrections are

applied.

• Sextant Altitude. Sextant Altitude of a heavenly body is the angle measured by a

sextant between the Visible Horizon and the body on a Vertical Circl e towards theObserver’s Zenith(Z) and must be corrected before use.

• Apparent Altitude. Apparent Altitude of a heavenly body is Sextant Altitude|

corrected for Index Error and Height of Eye (Dip).

• Observed (True) Altitude. Observed (True) Altitude is Apparent Altitude corrected

for atmospheric refraction errors. See Para 0348d.

• Calculated (Tabulated) Altitude. See Para 0531.

1119. Vertical Circles

All Great Circles passing through the Observer’s Zenith (Z) are necessarily perpendicular to the Celestial Horizon and are known as Vertical Circles.


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SECTION 2 - THE MAGNITUDES OF STARS AND PLANETS

0120. Solar and Stellar Systems

The Earth rotates on its axis to the east, and thus the bodies in the Celestial Sphere

appear to rotate westward (ie. rise in the east and set in the west).

a. Planets. The planets reflect light from the Sun and only Venus, Mars, Jupiter and

Saturn are sufficiently bright for navigational use. There are at least 1,500 other small

satellites and asteroids orbiting the Sun but none of these are relevant for navigational

use. The ‘navigational planets’ move across the backdrop of stars in the Celestial Sphere

within a band of about 5° from the Ecliptic. The speed and volatility with which they

move is irregular due to their widely changing ranges from the earth and care is needed

to identify them. Further details of the ‘Navigational Planets’ are at Appendix 1.

b. Stars. The stars transmit their own light from an immense distance and because of

this distance remain in a fixed pattern in the sky. Of the 4,850 stars visible to the nakedeye, only Polaris and the 57 other stars tabulated in The Nautical Almanac are

sufficiently bright for navigational use. Further details of the ‘Navigational Stars’are at

Appendix 1.

0121. Stellar Magnitudes

Hipparchus (2nd century BC) and Ptolemy (2nd century AD), arbitrarily graded stars and

planets into six magnitudes according to their brightness. Heavenly bodies of the first magnitude

were among the brightest in the sky and sixth magnitude were those just visible to the naked eye.

The discovery by Sir John Herschel in 1830 that a first-magnitude star was about one hundred

times brighter than a sixth-magnitude star, and that the brightness each magnitude of star varied

to the next magnitude by a factor of about 2.5 (the fifth root of 100) caused the Ptolemaic grading

to be modified slightly. Stars are now classified by brightness according to the definition that:

A first-magnitude star is one from which the earth receives exactly one hundred times as

much light as it received from a sixth-magnitude star.

By this definition, the intervening magnitudes between 1 and 6 are found from a

logarithmic scale, so that, if ‘a’ is the numerical index of the quantity of light received:

a6 : a 100 : 1

ie. a5 = 100

a = 2.51

With numerically small magnitudes indicating the brightest objects, any object 2.51 times

brighter than a first-magnitude star must have a magnitude of 0 and any object brighter than this

must have a negative magnitude. Sirius is of magnitude -1.46, Venus at its brightest can be -4.4,

the Sun’s magnitude (as seen from earth) is 26.7, and the Moon when full is 12.5.

With brightness varying by a factor of 2.51 between each magnitude, it is simple to

calculate the relative brightness of heavenly bodies from the magnitude information given inThe

Nautical Almanac: simply multiply 2.51 by power of the difference between magnitudes.

Egs. Vega (0.1), Aldebaran (1.1) Vega: 2.51(1.1-0.1) = 2.51(1) = 2.51 times brighter

Canopus (-0.9), Aldebaran (1.1) Canopus: 2.51(1.1-(-0.9)) = 2.51(2) = 6.3 times brighter

Sirius (-1.6), Regulus (1.3) Sirius: 2.51(1.1-(-1-6)) = 2.51(2.9) = 14.4 times brighter

0122-0129. Spare


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SECTION 3 - METHODS OF IDENTIFYING HEAVENLY BODIES

0130. The Identification of Heavenly Bodies

In the practice of astro-navigation, ‘star’ sights are usually taken at Morning Twilight and

Evening Twilight when the Visible Horizon and only a few bright stars/planets are visible at the

same time. This means that the background of constellations are not visible to assist the

navigator in star/planet identification, although an early start for ‘morning stars’ can overcome

this difficulty. However, in the main, other methods of identification must be used.

0131. Use of Computers for Identification of Heavenly Bodies

a. History. A variety of computer programs for star and planet identification became

available from 1980, and they also carried out rhumb line / great circle passage planning

and astro-navigation calculations. Between 1980 and 1996 the Hewlett Packard HP41CV

Hand-held Calculator was used in the Royal Navy for this purpose but was replaced in

1996 by a PC program produced by The Nautical Almanac Office called ‘Compact Datafor Navigation and Astronomy 1996-2000' (short title NAVPAC 1), for star/planet

identification, rhumb line / great circle passage planning and astro-navigation

calculations.

b. NAVPAC 1. This program was effective and accurate, but the user interface was

labour intensive and rather awkward to use. The program ran under MSDOS and could

be operated on the simplest of PCs (minimum IBM 286 or equivalent). The ephemeral

data in the program expires on 31 December 2000 and it may not be used after this date.

It is to be replaced for Royal Navy use by NAVPAC 2 in mid-2000.

c. NAVPAC 2. NAVPAC 2 replaces the earlier (NAVPAC 1) version for the post-

2000 epoch. The program is Windows-based and needs a PC operating on a minimum

of Windows 95 and may also be used under later Windows systems (98, NT etc).

NAVPAC 2 incorporates a much improved user interface and has an extended

functionality. It is also capable of making calculations for dates prior to the year 2001

and so may be used for the worked exercises contained in BR 45(5). Operating

instructions for NAVPAC 2 are contained at Annex A to Chapter 3.

d. Command Support System. The inclusion of NAVPAC 2 into the Command

Support System in major warships is under consideration.

0132. Description of the Star Finder and Identifier (NP 323)

The Star Finder is carried by all warships and affords a simple and speedy means of

identifying stars and planets. It is also independent of power supplies and the availability of

NAVPAC / computer facilities. It consists of a double-sided 30cm x 30cm cardboard star-chart

(Fig 1-5a) and eight transparent templates for Latitudes 10°, 20°, 30°, 40°, 50°, 60° and 75° (Fig

1-5b) respectively. One side of the star-chart is for use in the northern hemisphere and the other

for use in the southern hemisphere, although both have an overlap to allow equatorial stars to be

identified. The 57 navigational stars are printed on the star-chart, and on the templates show rings

of Altitude and curves of Azimuth. The edge of the star-chart is marked in LHA Aries for

alignment with the Meridian of the grids. Full instructions for use are printed on the star-chart

and are designed to allow a user with no prior experience of the Star Finder to obtain immediate

results.


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0133. The Nautical Almanac Planet Diagram

The Nautical Almanac Planet Diagram shows the local mean time of Meridian Passage

(see Para 0325 for explanation of Meridian Passage) of the Sun and the five planets Mercury,

Venus, Mars, Jupiter and Saturn in graphical form, together with lines showing the Local Mean

Time (LMT) (see Para 0435a for explanation of LMT ) of Meridian Passage of even-hour circles

of Right Ascension (for every 30° of SHA). The horizontal argument on the page is date, and the

vertical argument is LMT . A band on either side of the time of transit of the Sun is shaded to

indicate the bodies within this area on a particular date which are too close to the Sun for

observation. The lines joining the times of transit of the five planets are drawn in a distinctive

manner to avoid confusion. The diagram is mainly intended for planning purposes when a star

globe is not available and by entering with the date alone gives the following information:

a. Observable. The diagram shows whether a planet is observable on that day or whether

it is too close to the Sun (within the shaded area).

b. Meridian Passage. The diagram shows the local mean time of Meridian Passagefor each planet. The time of Meridian Passage of a star may be found by inspection if

its SHA is known. Users may also plot an SHA / date line corresponding to any particular

star if desired.

c. Morning and Evening Stars. The diagram shows that when Meridian Passage is

at about 24h the planet is observable from Evening Twilight (in the east ), through the

night until Morning Twilight (in the west). When Meridian Passage falls just below the

shaded area (ie before 11h) , it is visible low in the east during Morning Twilight . When

Meridian Passage falls just above the shaded area (ie after 13 h), it is visible low in the

west during Evening Twilight . In broad terms, a body in the bottom half of the diagram

is a morning star, and one in the top half is an evening star.

d. Confusion with Other Planets. The diagram shows whether other planets are in the

immediate vicinity, when care must be taken to avoid confusion.


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CHAPTER 2

TIME SYSTEMS

CONTENTS

ParaUniform Time System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0201Standard Legal Time and Summer Time/Daylight Saving Time (DST) . . . . . . . . . . . . . 0202Standard Legal Time - Regional Designators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0203Use of Standard Time and Zone Time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0204Conversion between UT and LMT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0205International Date Line . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0206Clock Zone Changes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0207Zone Times of RVs and ETAs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0208Universal Time (UT1 or abbreviated to UT) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0209

Greenwich Mean Time (GMT) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0210Co-ordinated Universal Time (UTC) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0211


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CHAPTER 2

TIME SYSTEMS

0201. Uniform Time System

The world is divided into 24 Standard Time Zones. ‘Standard Time Zone’ is the generic |term for all Time Zones within the Uniform Time System, both on land and sea. Each zone is 15°wide and each zone is numbered and lettered. The Greenwich Meridian is the centre of Zone 0 |and also the centre of the system. Zones to the east of Zone 0 are numbered 1, 2 etc., andthose to the west +1, +2 etc. The 12th zone is divided by the International Date Line ( IDL), the

part to the west being 12 and that to the east +12. The zone number indicates the number of hours by which Standard (or Zone) Time must be decreased or increased to obtain Universal Time UT (previously known as GMT - see Para 0210). Time Zones may also be indicated byletters; UT is Z (zero) and the zones to the east are lettered A to M (omitting J) and those to thewest N to Y. The Standard (or Zone )Time appropriate to Longitude (see Fig 2-1 and Fig 2-2) isusually referred to as ‘Zone Time’ and is the Time Zone normally kept at sea.

0202. Standard Legal Time and Summer Time / Daylight Saving Time (DST)On land, countries may modify the Standard (or Zone)Time to suit local needs. The Time

Zone kept on land is decided by national laws and is known as Standard Legal Time (or ‘ Legal Time’). The ALRS Vol 2 (NP 282) gives the Standard Legal Time in each territory (see Fig 2-1and Fig 2-2). Within NP 282 a negative prefix denotes that Legal Time is ahead of UT and

positive behind it; details are given if there is a seasonal change from the Standard Legal Timeto Daylight Saving Time (DST) (Summer Time); an asterisk indicates that a territory is not expectedto observe DST in the current year; DST dates followed by the letter ‘E’ are estimates. The changefrom Standard Legal Time to DST is normally effected before 0300 (Local Time) and the changefrom DST to Standard Legal Time after 2200 (Local Time). Certain Islamic countries that

observe DST may revert to their Standard Legal Time during the 29 days of Ramadan. The listis corrected in Section VI of the Weekly Edition of Admiralty Notices to Mariners. Standard

Legal Time (sometimes abbreviated to ‘ Legal Time’) is the Time Zone kept on land.

0203. Standard Legal Time - Regional DesignatorsIn countries extending over large east-west distances (eg USA), different Standard Legal

Times may be kept in separate geographical areas within a country. Such variations may havetheir own regional designators. Regional designators may also be used to describe collectivelya common Standard Time adopted by a number of countries. The table below lists the regionaldesignators for Standard Time with their abbreviations and relationship to Universal Time UT .A negative prefix denotes Standard Times in advance of UT ; a positive prefix those behind UT ,

as shown at Table 2-1.Table 2-1. Standard Time Designators |

Designator Abbreviation Standard Time

Atlantic Standard Time (Canada)Central European TimeCentral Standard Time (Canada and USA)Eastern Standard Time (Canada and USA)Mountain Standard Time (Canada and USA)

Newfoundland Standard Time (Canada)Pacific Standard Time (Canada and USA)Yukon Standard Time (USA)

AST

CSTESTMST NSTPSTYST

+0401+06+05 |+07

+03½+08+09


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Fig 2-2. Standard Time Zone Chart of Europe and North Africa


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0204. Use of Standard Time and Zone Time

UT is used as the standard Time Zone for worldwide reference books such as The Nautical Almanac, is the Time Zone in which Ship’s Chronometers and Deck Watches are keptand is also used for signal message Date-Time-Groups (DTGs). UT was previously known asGMT (see Para 0210). It should be noted that Tides Tables, which are specific to local areas,

normally provide information in Standard Legal Time (See Para 0202) but care must be exercisedwhen any Daylight Saving Time (DST) is in force.

0205. Conversion between UT and LMTApplying the Uniform Time System (Para 0201), the following rules may be established:

If a Longitude is West, ADD the time equivalent of the Longitude when changing

from Local Mean Time to UT (and vice versa - SUBTRACT if changing from UT to LMT ).

If a Longitude is East, SUBTRACT the time equivalent of the Longitude when

changing from Local Mean Time to UT (and vice versa - ADD if changing from UT to LMT ).

Examples 2-1 and 2-2. What are the LMT equivalents if UT is 23 hours 31 minutes 25 secondson 14 September, (1) at 48° West, and (2) at 22½° East. Note that this is changing from UT to

LMT.

Example 2-1: At 48° West Example 2-2: At 22½° East

Date Hrs Mins Secs Date Hrs Mins Secs

14 Sep UT 23 31 25 14 Sep UT 23 31 25 Long W. (-) 03 12 00 Long E. (+) 01 30 00

14 Sep LMT (48°W) 20 19 25 15 Sep LMT (22½°E) 01 01 25

Examples 2-1 and 2-2. Converting UT (previously known as GMT ) to LMT ( Note that the date has also changed in Example 2-2 at 22½° East )

0206. International Date Line

a. Reason for the International Date Line. Inspection of Fig 2-1 will show that a

traveller leaving UK and heading east to make a trip around the world would advanceclocks by 1 hour on passing each successive Meridian 15° further east from Greenwich,in accordance with the Standard (or Zone)Time arrangements of the Uniform TimeSystem (Para 0201). If this process were to continue until the traveller circumnavigatedthe world and reached UK again, 24 hours would have been added to the traveller’s clock and calendar, and thus the traveller would believe it to be the same time as kept in UK

but 1 day later (this fact was the key to the plot in Jules Verne’s famous book, ‘Around the World in 80 Days’ which was later made into a classic film). To avoid this difficulty,it has been agreed worldwide that at approximately 180° East, on crossing the

International Date Line, travellers would advance or retard calenders by 1 day (retardwhen eastbound, advance when westbound) and simultaneously apply the new Time

Zone (-12hr to +12hr or vice-versa) to the new date.


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0209. Universal Time (UT1 or abbreviated to UT)

Universal Time (UT1 or UT) is the Mean Solar Time (MST) (see Chapter 4 for definitionof MST ) of the Prime Meridian obtained from direct astronomical observation and corrected for the effects of small movements of the Earth relative to the axis of rotation (polar variation). Sincethese time scales correspond directly with the angular position of the Earth around its axis of

diurnal rotation, they are used for astronomical navigation and form the time argument in The Nautical Almanac.

0210. Greenwich Mean Time (GMT)GMT may be regarded as the general equivalent of UT / UT1.

0211. Co-ordinated Universal Time (UTC)

a. Requirement. Co-ordinated Universal Time (UTC) has been developed to meet theneeds of scientific users for a precise scale of time interval, and those of navigators,surveyors and others who require a timescale directly related to the Earth’s rotation (like

UT1).

b. UTC - TAI - UT1 Linkage. UTC corresponds exactly in rate with International Atomic Time (TAI). TAI is based on atomic clocks and is independent of the Earth’s rotationand UTC differs from it by an integral number of seconds. The UTC scale is adjusted by theinsertion or deletion of seconds (positive or negative leap seconds) to ensure that thedeparture of UTC from UT1 does not exceed +/- 0.9 seconds. Further details of these timesystems may be found in Radio Time Signals section of the Admiralty List of Radio SignalsVolume 2 (NP 282).

c. Time Signal Broadcasting Stations. Operational details of stations broadcasting time

signals are listed in Radio Time Signals section of the Admiralty List of Radio SignalsVolume 2 (NP 282) and they broadcast in the UTC time scale unless otherwise indicated.Leap seconds are notified in advance as corrections in a Table in the Radio Time Signalssection of NP 282. Changes to this Table are notified in Section VI of the Weekly Editionof Notices to Mariners.

d. GPS Time-Transfer. GPS provides a very accurate source for time-transfer and may be the most convenient source of UT for time checks (see Paras 0326c.2 and 0340), and toestablish any error in the chronometer time and any Deck Watch Error (DWE).


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CHAPTER 3

PRACTICAL PLANNING, TAKING, REDUCTION AND PLOTTING OF SIGHTS

CONTENTS

SECTION 1 - INTRODUCTIONPara

Assumptions Made and Scope of the Chapter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0301

SECTION 2 - PLANNING ASTRO-SIGHTS

Ship’s DR / EP Position for Sights . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0320NAVPAC 2: Calculating SS / SR, CT, NT (RiseSet Pages) . . . . . . . . . . . . . . . . . . . . 0321The Nautical Almanac - Calculating SS / SR, CT, NT . . . . . . . . . . . . . . . . . . . . . . . 0322NAVPAC 2: Prediction of a Body’s Azimuth (Bearing) and Altitude (FindIt Page) . 0323The Star Finder - Prediction of a Heavenly Body’s Altitude and Azimuth (Bearing) . 0324

The Nautical Almanac - Calculating Time of Sun’s Meridian Passage . . . . . . . . . . . 0325NAVPAC 2: Calculating Time of Sun’s Meridian Passage (FindIt Page) . . . . . . . . . . 0326Other Organisational and Material Preparations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0327

SECTION 3 - DESCRIPTION, PREPARATION AND USE OF SEXTANT

Sextant - Principle of Operation and Origin of Name . . . . . . . . . . . . . . . . . . . . . . . . . . 0330Description of Sextant . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0331Measurements ‘On’ and ‘Off’ the Arc . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0332Positioning and Marking of the Index Bar and Arc . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0333Viewing and Collar / Telescope Adjustments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0334

The Sextant Telescopes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0335Sextant Errors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0336Sextant Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0337Care of a Sextant . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0338Using a Sextant . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0339

SECTION 4 - REDUCING SIGHTS (PROCESSING OF SEXTANT READINGS)

NAVPAC 2: Assumptions and Overall Arrangement of ‘Sights’ Sub-Programs. . . . . 0340NAVPAC 2: Options Page . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0341NAVPAC 2: Sights-Fix Page . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0342NAVPAC 2: Sights-Legs Page . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0343NAVPAC 2: Sights-Astronomical Observations Page (using Stars / Planets) . . . . . . . 0344NAVPAC 2: Sights-Astronomical Observations Page (using Sun / Moon Planets) . . 0345NAVPAC 2: Sights-Results, Sights-Log and Sights-Position Line Plot Pages . . . . . . 0346NAVPAC 2: Summary of Printing, Saving and Loading Facilities . . . . . . . . . . . . . . 0347The Nautical Almanac - Meridian Passage, Polaris and Altitude Corrections . . . . . . 0348NAVPAC 2: Almanac Facilities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0349

SECTION 5 - PLOTTING SIGHTS

NAVPAC 2: Plotting of Astronomical Position Lines . . . . . . . . . . . . . . . . . . . . . . . . . 0350Manual Plotting Astronomical Position Lines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0351

ANNEXES

Annex A: Extract of HM Nautical Almanac Office NAVPAC 2 User Instructions


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CHAPTER 3

PRACTICAL PLANNING, TAKING, REDUCING AND PLOTTING OF SIGHTS

SECTION 1 - INTRODUCTION

0301. Assumptions Made and Scope of the Chapter

a. Navigational Watchkeeping Certificate (NWC). This chapter provides a practical

guide for planning, taking, reducing and plotting of astro-sights for readers studying for

the (Navigational Watchkeeping Certificate (NWC).

b. Assumptions.

• Chapter 3 assumes that NAVPAC 2 is available on-screen and can be

worked through, step by step, with the instructions in the chapter; it is

not intended that the NAVPAC 2 elements of Chapter 3 should be read

in isolation.

• It is assumed that NAVPAC 2 will be used to carry out the majority of

calculations.

c. Scope.

• Although NAVPAC 2 is the primary method for solving calculations, some

simple procedures using The Nautical Almanac are also covered.

• Although solutions of Great Circle and Rhumb Line sailings are contained in

NAVPAC 2 they are not included in Chapter 3; explanation of these sailings

are at BR 45 Volume 1 Chapters 2 and 5 and the NAVPAC 2 user’s manualat Annex 3A includes instructions for making these calculations.

• Astro-theory is covered at Chapters 4-9.

0302-0319. Spare

SECTION 2 - PLANNING ASTRO-SIGHTS

0320. Ship’s DR / EP Position for Sights

a. Star Sights. The starting point for all astro-sights is to establish an approximate DR

/ EP position from the Bridge chart for the time of the planned observation. It is only possible to take star sights between Civil and Nautical Twilight , when both the horizon

and the brightest stars/planets are visible. This will require the calculation of Morning

Nautical Twilight (MNT), Morning Civil Twilight (MCT) and Sunrise (SR) or Sunset

(SS), Evening Civil Twilight (ECT) and Evening Nautical Twilight (ENT) as appropriate.

b. Sun Sights. The Sun’s position in the sky is normally self evident and calculation

to predict this is not required except for its Meridian Passage ( Mer Pass). NAVPAC 2

does not specifically calculate the time of Mer Pass but an iterative process in the FindIt

page will allow the user to predict precisely when the Sun will cross the Observer’s

Meridian (ie due North or South of the observer). The Nautical Almanac may also beused to calculate for the time of Mer Pass. If the ship’s Longitude is changing rapidly

both of these calculations may involve extensive iterative processes.


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0321. NAVPAC 2: Calculating SS / SR, CT, NT (RiseSet Pages)

a. NAVPAC Home Page. On starting NAVPAC 2, the user is taken to a top-level

menu page referred hereafter as the ‘ Home’ page. To calculate SS / SR, CT or NT times,

click on the button on the Home page

(see Fig 3.1) or key in ‘ Alt T ’. This brings NAVPAC 2 to the ‘ RiseSet ’ page.

Note 3-1 . NAVPAC 2 provides keyboard shortcuts throughout the program by the use of ‘Alt’

and the key for letter underlined on the menu buttons (Eg. See Fig 3-1 below).

Fig 3-1. NAVPAC 2 ‘ Home’ Page.

b. RiseSet Page. A RiseSet page, with a variety of dialogue boxes (Fig 3-2 opposite)

for data input allows the user to select a DR / EP position , or

, , or ), , , , the observer’s

of eye above sea level, the required, the predictions required and theheavenly for which rising and setting data is needed. The should

be set to 1 (unless the ship will remain in the same position for more than 1 day), and

only the ‘Sun’, ‘Civil Twilight’ and ‘Nautical Twilight’ should be selected in the

option. When all details are complete, click the button and the RiseSet - Results

page will be displayed.

c. RiseSet-Results Page. The RiseSet-Results page (Fig 3-3) may be printed or saved

(see Paras 0323e/f and 0347). The line showing MNT for morning stars or ECT for

evening stars should be clicked and the or buttons

should be clicked in order to save the time calculated for use in later. Irrespective of any

set, if saved, the appropriate UT will be transferred to subsequent NAVPAC 2 menus. Click the button to return to the Home page.


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0322. The Nautical Almanac - Calculating SS / SR, CT, NT

a. Latitude Time. Using the ship’s DR / EP position from the Bridge chart, from The

Nautical Almanac obtain the ‘Latitude Time’ for the nearest Latitude on the mid-date for

the page in question for ENT / ECT / SS or SR / MCT / MNT . Note that The Nautical

Almanac displays this information in chronological order and so does not display the prefix ‘ Evening ’ or ‘ Morning ’ with CT or NT . Because the tables only provide times at

intervals of 5° of Latitude, interpolation may have to take place. This is undertaken

either by mental arithmetic or by using ‘TABLE 1 - FOR LATITUDE ’ at the end of the

yellow pages at the back of The Nautical Almanac.

b. Date Interpolation. Should the date not be the central date on the on The Nautical

Almanac double page, then interpolation by mental arithmetic will require to be

undertaken between the pages before or after the required date.

c. Longitude. The result of the data extraction and interpolation at Paras 0322a and0322b above is the UT of ENT / ECT / SS or SR / MCT / MNT on the Greenwich Meridian.

If the ship's position is not on the Greenwich Meridian, ie either East or West of the 0o line

of Longitude, a correction must be subtracted or added. Converting Longitude to Time is

undertaken either by mental arithmetic or by using the ‘CONVERSION OF ARC TO TIME ’

table at the start of the yellow pages in The Nautical Almanac. A useful way to remember

whether to add or subtract is given by the rhymes:

East is Least - MINUS

West is Best - PLUS

Note 3-2. The Nautical Almanac Table II (at end of yellow pages) is for additional Moon

corrections, and is NOT for SR/SS corrections.

d. UT (GMT). If the data from The Nautical Almanac has been extracted / interpolated

correctly and the observer’s Longitude applied, the result will be the UT of ENT / ECT /

SS or SR / MCT / MNT as appropriate at the observer’s DR / EP position. If desired the

Time Zone may be applied to obtain Local Mean Time (LMT) (see Para 0205).

e. Summary and Example 3-1. The calculation is summarised below with an example

of SS (interpolated from The Nautical Almanac) at 1800, at 25° East, in Time Zone B(-2).

Worked examples of rising and setting calculations, and answers are contained in BR 45

(5), pages 1B-2 to 1B-3.

Interpolated SS (or SR/CT/NT) from NA 1800

Longitude (W+ or E-) (25°E) -0140

Local Mean Time UT(GMT) 1620Z

Zone(-2) (+ = subtract) (- = add) +0200

Zone Time 1820B

Example 3-1. Summary of SS/SR/CT/NT Calculations

f. Further Iterations. If the time of the DR / EP position from the Bridge Chart was notclose to the time subsequently calculated for MNT (for morning stars) or ECT (for evening

stars), a further iteration of the calculation may be required to refine the answer.


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0323. NAVPAC 2: Prediction of a Body’s Azimuth (Bearing) and Altitude (FindIt Page)

a. Use. The prediction of a heavenly body’s bearing and altitude is usually associated

with taking morning or evening stars, as the position of the sun (and sometimes the moon)

in the sky during the day is self evident. Note that NAVPAC 2 uses the astronomical term

‘navigational body’ throughout, instead of the traditional maritime usage ‘heavenly body’.

b. Transfer of Times and Positions. Assuming that the times of ENT / ECT / SS or SR

/ MCT / MNT have been calculated in NAVPAC 2 and the appropriate time saved ( MNT

for morning stars or ECT for evening stars), then NAVPAC 2 will transfer that information

to the next NAVPAC 2 menu (Para 0321). Otherwise it must be entered manually into the

new menu in the next part of the program.

c. FindIt Page. From the Home page of NAVPAC 2, click

or key ‘ Alt F’ ; this brings the screen to the FindIt page (Fig 3-4). Confirm that

, (UT), ( / ) and ( / ) have transferred correctlyinto the dialogue boxes, or if not, correct them. Then:

• In the dialogue box, select an of to (not as

shown set to the default in Fig 3-4 below) and an of to ,

if they have not already been set.

• In the dialogue box, select .

• Check all data is correct and click on the button; the FindIt-Results

page will then be displayed.

Fig 3-4. NAVPAC 2 ‘FindIt’ Page.


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d. FindIt-Results. The heavenly bodies visible on the date and time within the

parameters selected will now be displayed on the FindIt-Results page as a 360° plot and

a list (Fig 3-5). By clicking on or and then double-clicking

any star on the plot, the appropriate item will be identified on the list (and vice-versa).

Bodies which have been identified (double-clicked) will have their details transferred

through to the Sights-Astronomical Observations page.

Fig 3-5. NAVPAC 2 ‘FindIt-Results’ Page.

e. Printing FindIt-Results. The FindIt-Results page may be printed by clicking the

button or on the direct-print button (see Para 0347a for a general explanation

of printing). A printout of both the plot and list will be needed for taking stars. When

printed, unlike the screen (Fig 3-5) , the plot will include the name of each body (Fig 3-6a).

The list of heavenly body details is provided on a separate sheet (Fig 3-6b).

f. Saving FindIt-Results. The FindIt-Results page may be saved to a file using the

button. Clicking on the button brings up the standard NAVPAC Saving - Loading page (see Fig 3-18). See Paras 0347b/c for general explanations of saving and

loading.

0324. The Star Finder - Prediction of Heavenly Body’s Altitude and Azimuth (Bearing)

The ‘Star Finder and Identifier’ is described in full at Para 0132. Full instructions for use

are printed on the star-chart (shown at Figs 1-5a and 1-5b) and are designed to allow a user with no

prior experience of the Star Finder to obtain immediate results. In summary, by placing one of the

8 transparent templates over the star-chart underlay the Altitude and Azimuth (Bearing) of the

heavenly bodies may be read of the template. The ‘Star Finder and Identifier’ provides a quick,

cheap method of identifying heavenly bodies and is independent of power supplies. However, it is

less accurate than NAVPAC 2.


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c. Deck Watch Time, Deck Watch Error and Mistakes.

(1) GMT. The Deck Watch Time (DWT) should always be UT .

(2) Deck Watch Error (DWE). The difference between DWT and UT is the Deck

Watch Error (DWE) and must be known precisely. DWT must be manually correctedfor DWE before inputting the resultant time (UT ) into NAVPAC 2.

(3) Analogue Clocks. Deck Watches are 12-hour analogue clocks and care must be

taken not to confuse 0600 with 1800 and thus inject a 12 hour error into the NAVPAC

2 calculation. If this is done it should become evident by the excessive size of the

intercepts and/or a refusal by NAVPAC 2 to compute a sensible observed position.

(4) Errors in Recording Time. The second and minute hands of the Deck Watch

should be aligned precisely, so that there can be no possibility of an error when the

reading the minute hand. If times are being taken by an assistant, it is advisable for the minute hand to be checked by the NO as well. Times should be read to the nearest

second. It is useful to be able to count in seconds so that, if there is no one else

available to take times, the NO can count the seconds until it is possible to read the

Deck Watch.

d. Mustering in Good Time. Both the NO and the NO’s Assistant should be up on the

Bridge in plenty of time for stars, particularly in the morning. For morning stars there

should be time to adjust to night vision to help spot the best stars while they are really

bright against a dark sky. As a general rule, the astro team should be on the Bridge ready

to go for taking stars just after Sunset for evening stars and by Nautical Twilight for

morning stars. In the tropics the periods of twilight are much shorter than in temperate Latitudes and an even earlier start is often prudent.

e. Rough Weather. Taking star shots on a stormy morning or evening from a lively

Bridge Roof, with spray flying and patches of cloud skudding past the stars giving only a

few seconds for a snatched observation can be a challenging experience. The NO and the

NO’s Assistant need to be correctly dressed as wet clothes and cold hands make accurate

Sextant work much harder. Similarly, the Sextant mirrors and lenses need to be protected

from spray; if they become wet the Sextant rapidly becomes impossible to use accurately

and any clumsy attempts to wipe the mirrors clean will probably introduce unknown errors

into an otherwise ‘zeroed’ Sextant . Having a suitably sized towel ready and keeping theSextant covered with it until immediately before raising it to the eye often solves the

problem in such conditions. If the Sextant does get wet, a damp chamois leather or a small,

clean, dry, soft, absorbent, lintless cloth should be immediately available to dry it quickly

and carefully before the next sight. Afterwards the Sextant will need careful cleaning and

oiling.

0328-0329. Spare


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SECTION 3 - DESCRIPTION, PREPARATION AND USE OF SEXTANT

0330. Sextant - Principle of Operation and Origin of Name

If a ray of light is reflected twice in the same plane by two plane mirrors, the angle between

the first and the last ray is twice the angle between the mirrors. Thus the Sextant has a graduated

Arc of about 1/6th of a circle’s circumference (60°- hence the name), but the arc is graduated to 120°.

0331. Description of Sextant

The micrometer Sextant in Royal Navy service is illustrated at Fig 3-7 and consists of

elements built around the Main Frame. The bottom edge of the Main Frame is the Arc, which has

its geometric centre at the top of the Main Frame. An adjustable Collar is fitted on the rear edge

of the Main Frame into which a removable Telescope is fitted. The Index Bar, which can rotate

about the geometric centre of the Arc, is hinged at the top of the Main Frame and has a Clamp at

the bottom; an Index Mark and Micrometer Drum are fitted at the Clamp end. The Horizon Glass,

which is half-silvered and half-clear, is mounted on the front of the Main Frame. Various Shades

are fitted to filter the Sun’s rays and a Reading Lamp for observing the scales is also fitted.

Fig 3-7. The Marine Sextant


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0332. Measurements ‘On’ and ‘Off’ the ArcThe Arc is graduated in degrees of observed altitude, and so arranged that when the Index

Glass is parallel to the Horizon Glass, the Index Mark on the Index Bar should point to the zero onthe Arc scale. Angles read on the main part of the scale part are said to be ‘On’ the Arc. The

graduations are continued over a small arc on the other side of the zero; this is called the ‘ Arc of

Excess’ and angles read on this part are said to be ‘Off ’’ the Arc. When establishing the Index Error of the Sextant (see Para 0336), if the Micrometer Drum reads ‘Off ’’ the Arc, this error must beadded to subsequent Sextant readings and if ‘On’ the Arc the error must be subtracted. The sign of

the correction can easily be remembered by the rhyme:

“When its ‘Off’ its on (+), and when its ‘On’ its off (-)”

0333. Positioning and Marking of the Index Bar and Arc

The Index Bar can be set to any position on the Arc by means of the Clamp; this releasesor engages a worm thread in the teeth of a rack that extends along the entire periphery of the Arc.When clamped, the Index Bar’s motion along the Arc can be controlled in either direction by

turning the Micrometer Drum which rotates the worm in the rack. With this arrangement it is theworm and the rack that govern the accuracy of the setting. One rotation of the Micrometer Drummoves the Index Bar one degree along the Arc. When reading the Sextant , the engravings on the

Arc are read against the Index Bar to the nearest whole degree, while the Micrometer Drum

provides the intermediate reading for minutes. In Fig 3-7 the Sextant may be seen to read 34° 58.1'.

0334. Viewing and Collar / Telescope Adjustments

a. Telescope and Mirror Alignment. The Telescope is fitted in the Collar so that itsaxis makes the same angle with the plane of the Horizon Glass as the latter makes with theline joining the centres of the Index Glass and Horizon Glass, thus ensuring the Sextant is

capable of accurate altitude readings of heavenly bodies.

b. Telescope Adjustment Facility. The Collar can be moved nearer or further from the Main Frame by means of a Milled Head beneath the frame. In the normal (mid) positionof the Collar , the optical centre of the Telescope is aligned with the silvered/unsilvered

boundary of the Horizon Glass and equal parts of the silvered and unsilvered halves of thehorizon glass should be visible; the Telescope should normally be aligned to this position.

c. Telescope Adjustment Effect. The action of moving the Collar and Telescope nearer or further from the Main Frame regulates the brilliance of the reflected image, which will

greatest when the Telescope is nearest to the Main Frame. As the Telescope is movedaway from the Main Frame, less of the silvered part of the Horizon Glass appears in the

field and the reflected image is less bright. This action can be useful when anexperienced user wishes to regulate the relative brilliance of the horizon and the

reflected heavenly body but is not recommended for inexperienced users who willfind the Sextant very difficult to handle when configured away from the mid-setting.

d. Shades and Reading Lamp. Sets of neutral density Shades are mounted in front of both the Index Glass and Horizon Glass for use when observing the Sun. Three legs anda handle are fitted on the other side of the Main Frame to that shown in Fig 3-7. Thehandle contains a battery and switch for operating the swivel-mounted Reading Lamp.


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0335. The Sextant Telescopes

The Sextant is provided with two telescopes: a ‘Star Telescope’ and a ‘Sun Telescope’.

a. Star Telescope. The (short, fat) Star Telescope (shown in Fig 3-7) is an ‘erecting’

telescope which shows objects the right way up; it has a large object lens with a low

magnification.The ‘ Star Telescope’ is designed for taking star-sights and for observingterrestrial objects (ie. vertical and horizontal Sextant angles) but should also be used

by inexperienced Sextant users for taking sun-sights.

b. Sun Telescope. The (long, thin) Sun Telescope is an ‘inverting’ telescope which

shows objects upside down; it has a small object lens with a high magnification. It is

designed for taking sun-sights in good conditions only, as it is hard to hold steady. It has

two eyepieces, one of which has higher magnification than the other. Each eyepiece is

fitted with cross-wires at its focus (to define the line of ‘collimation’, which is the line

joining the focus to the centre of the object-glass). The eyepiece of higher power has two

cross-wires and the lower power eyepiece has four. These cross-wires are fragile and can be destroyed by careless cleaning. The high-power eyepiece is designed for use when the

horizon is bright and the ship is very steady. Experienced Sextant users can achieve a

higher degree of accuracy with the Sun Telescope than with the Star Telescope. However,

inexperienced Sextant users will find great difficulty in using the Sun Telescope at

first and should wait until manual dexterity has been achieved with the Star Telescope

before graduating to the Sun Telescope.

0336. Sextant Errors

Apart from a lack of manual dexterity in using theSextant (which is overcome by practice),

the greatest single cause of inaccurate sights is the presence of unknown errors in the Sextant . There

are 3 adjustable errors which must be corrected or determined by the user and also 2 non-adjustableerrors which if significant will require the Sextant to be returned for workshop repair. The

adjustable errors must be adjusted or established for each sight in the following order:

a. Perpendicularity. This is the perpendicular (90°) alignment of the Index Glass to

the plane of the Arc and thus to the Sextant . To check Perpendicularity, remove the

Telescope and set the Index Bar to about 60° (roughly the middle of the Arc). Hold the

instrument horizontal at arm’s length with the Index Glass nearest to oneself and look into

the Index Glass as nearly as possible along the plane of the Arc in order to see the reflected

image of the Arc at the edge of the Index Glass mirror, in line with the actual Arc observed

directly. The Index Bar may need to be moved slightly to allow this to be seen. If thereflected image of the Arc is not absolutely aligned with the directly observed part of the

Arc, bring the two in line by adjusting the small screw in the centre of the Index Glass

frame. This adjustment is critical and must be carried out before any others.

b. Side Error. Side Error is a variation from the perpendicular alignment of the Horizon

Glass to the plane of the Arc and thus to the Sextant . Side Error adjustment cannot be

carried out successfully unless Perpendicularity of the Index Glass (see Para 0336a above)

has already been correctly set. Once the presence of Side Error has been established (see

sub-paras below), it can be removed by turning one of the two adjusting screws on the

Horizon Glass. Side Error may be established as shown below and the screw used to

correct it may be remembered by the linkage of the word ‘side’: Side Error may be removed by adjusting the screw on the side of the Horizon Glass.


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(1) To check for Side Error , fit a Telescope (the Sun Telescope provides the most

accurate results but the Star Telescope may be preferred by inexperienced Sextant

users due to the difficulty of holding the Sun Telescope sufficiently steady).

(2) With the chosen Telescope fitted, hold the Sextant in the vertical plane ( ie as

normal) and look at a well-defined distant object such as a medium-bright star andmove the Index Bar across the zero of the Arc.

(3) As the Index Bar passes the zero of the Arc (+/- any Index Error), the reflected

image should be exactly superimposed over the direct image of the star. (Very bright

objects such as Venus or Saturn should be avoided, as it will be found their very size

and extreme brightness make them awkward to use for this purpose).

(4) If two images sit level, but to the left and right of each other, Side Error is

present and adjustment can be made (as above) until the images are superimposed.

c. Index error. Index Error is a variation from the parallel alignment of the plane of the

Horizon Glass to the plane of the Index Glasswhen the Index Bar is set to the zero position

on the Arc. If Index Error is zero, when the Sextant is pointed at a well-defined distant

object (such as a medium-bright star) it should show exactly 0° 00.0' on the Arc scale when

the direct and reflected images of a distant heavenly body are coincident. This seldom

occurs in practice because the two glasses are rarely adjusted so well that they are exactly

parallel at this point. When this difference occurs, the zero on the scale is therefore not the

true zero of the instrument and a small correction has to be made (see Para 0332). Index

Error can be determined by 4 methods, and once its presence has been established (see

sub-paras below), it can be removed by turning one of the two adjusting screws on the

Horizon Glass. If the Index Error is less than 3.0' of arc it may be left and allowed for mathematically (see Para 0332). If the Index Error is larger than 3.0' of arc it should be

removed or reduced by turning the adjustment screw at the bottom of the Horizon Glass.

If the method of recalling the correct adjustment screw for Side Error is remembered (see

Para 0336b), it is simple to ensure the other screw is used for Index Error .

(1) By Observing the Diameter of the Sun On and Off the Arc. To check for

Index Error set the Sextant to about 0° 30', fit shades and adjust the Micrometer Drum

to make the edges of the two images of the Sun touch (Fig 3-8a). Note the ‘On’ the

Arc reading. Reverse the images (Fig 3-8b) and note the ‘Off’ the Arc reading. To

obtain the Index Error halve the difference in readings and note the resultant signfrom the larger reading. If Index Error exists either correct it (see Para 0336c above)

or make a note its amount and whether it is ‘On’ or ‘Off’ the Arc (see Para 0332).

Fig 3-8a. Index Error ‘On’ the Arc Fig 3-8b. Index Error ‘Off’ the Arc


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(2) Difficulties in Observing the Diameter of the Sun On and Off the Arc. To

check Index Error by the Sun On and Off the Arc method (Para 0336c(1) above)

is a particularly awkward procedure. For accurate results the adjacent images have to

be sighted exactly under each other (ie At the maximum tangential reading). The

slightest error in this vertical alignment will induce an additional accidental error as

shown by Fig 3-8c and Fig 3-8d.

Fig 3-8c. Correct Alignment of Images Fig 3-8d. Incorrect Alignment of Images

Note 3-3. The Sun’s semi-diameter given for the day in The Nautical Almanac will provide a check

on accuracy - the Sextant readings On and Off the Arc added together should equal four times

the semi-diameter of the Sun.

WARNING

NEVER OBSERVE THE SUN WITHOUT FIRST

FITTING SEXTANT / TELESCOPE SHADES.

(3) By Observing a Star . The best method of checking for Index Error is to set the

Index Bar a few minutes of arc to one side of zero, then bring the two images of a star

together so that they are coincident. If any error exists either correct it (see Para 0336c

above) or make a note of its amount and whether it is ‘On’ or ‘Off’ the Arc (see Para

0332). The choice of telescope is similar to Side Error procedure (see Para 0336b(1)).

(4) By observing the horizon (or other distant terrestrial object). This is a variation

on the ‘star’ method but is the least reliable method of checking for Index Error. The

reflected horizon (or distant object) is brought in line with the directly observed

horizon (or distant object). The accuracy of this method depends on having a clearly

defined, sharp horizon or a sharply defined distant object; it is much preferable to

observe a heavenly body if one is available. Having aligned horizons/objects as

carefully as possible, if any error exists either correct it (see Para 0336c above) and

or make a note its amount and whether it is ‘On’ or ‘Off’ the Arc (see Para 0332).

d. Collimation Error. Collimation Error is an variation from the parallel alignment

of the axis of the Telescope to the plane of the instrument. Collimation Error should

be checked periodically but cannot normally be corrected outside a specialist

workshop and correction should not be attempted by users. It is a difficult error to

establish (see Para 0336e below) and should only be attempted in good conditions and

with the utmost care.


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e. Collimation Error Check. After having corrected the Sextant for Perpendicularity,

Side Error and Index Error, check the Sextant for Collimation Error as follows:

(1) To check for Collimation Error , ship the inverting telescope with the wires

parallel to the plane of the instrument. Then choose two heavenly bodies not less

than 90 apart, and bring them into accurate contact on one wire of the telescope.

(2) Then move the telescope until the bodies are on the other wire. If they are not

still in contact, there is Collimation Error and the Sextant should be returned.

f. Backlash in a Micrometer Sextant. The Micrometer Drum might wear over time

and develop an error due to backlash. The amount of backlash may be determined by

setting the Index Bar a few minutes of arc to one side of zero (as for Index Error checks

on a star), rotating the Micrometer Drum clockwise to bring a star into coincidence and

then repeating this, but turning anti-clockwise. The difference in the two readings will

reveal any backlash. It should be negligible in operational Sextant s in ships but may existin those used regularly by students for training. If there is sufficient backlash to justify

making a correction, either make two observations by bringing the drum from opposite

directions and mean the result, or habitually turn the Micrometer Drum from one

direction and apply any backlash established as a ± correction to the Sextant Altitude.

g. Micrometer Drum Friction Clutch. In a micrometer Sextant , if the Index Error

adjustment screw on the Horizon Glass (see Para 0336c) has reached the extent of its

travel, the index setting may also be adjusted by releasing the friction clutch of the

Micrometer Drum. The friction clutch should then be reset lightly in conjunction with

the Index Error adjustment screw, and by trial and error, the index setting reduced and

set close to zero. The clutch should then be tightened again carefully and firmly. The needto carry out this procedure is very rare and it must be done with particular care.

0337. Sextant Calibration

Marine Sextant s are calibrated when first supplied and on completion of repair or

refurbishment, either by a MoD Agency or a contractor. A calibration certificate (or certificates),

located in the Sextant box list any small residual errors due to prismatic errors in the mirrors and

shade glasses, and aberrations in the lenses of the telescopes. These corrections do not normally

exceed a maximum if 0.8' of arc on any part of the Arc, and may be applied to Sextant readings

for absolute accuracy. However, in most Royal Navy Sextant s these errors are so small as to be

almost negligible. Once calibrated, these characteristics should not change if the Sextant isstored. When in regular use for astronomical observations, the Sextant ’s general performance

( Perpendi

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