# The Art of Celestial Navigation

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15 Νοε 2013 (πριν από 4 χρόνια και 5 μήνες)

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9: Instruments Used in Celestial Navigation

Presented By: MATE O

2

Course Outline

1.

Lecture 1 & 2

2.
Mean Time / Apparent Time

Lecture 3

3.
Time Zones, Zone Description, Chronometer Time

Lecture 4

4.
The Earth, Celestial, And Horizon Coordinate System

Lecture 5 & 6

5.
The Spherical Triangle

Lecture 7

6.
Circle of Equal Altitude Intercept

Lecture 8

7.
Time Diagram, Right Ascension

Lecture 9

8.
The Nautical Almanac, Finding GHA, LHA, and Dec.

Lecture 10 & 11

9.

Lecture 12

10.
Sight Reduction of the Sun, Stars, Planets, Moon

Lecture 13
-

15

11.
Plotting and Advancing / Retarding the Assumed Position

Lecture 16

12.
Calculating Time of Meridian Passage (LAN)

Lecture 17

13.
Calculating Latitude at Meridian Passage

Lecture 18

14.
Calculating Time of Sunrise/ Sunset/ Twilight

Lecture 19

15.
Star Shooting Schedule / Pub. 249

Lecture 20

16.
Azimuth and Amplitudes

Lecture 21

24

17.
Polaris

Azimuth and Latitude By

Lecture 25

18.
Star

Finder

Stars, Planets, Selecting the 3 best Stars

Lecture 26

19.
The Complete Day’s Work

Lecture 27 & 28

3

The Marine Sextant

Parts

A.
Frame

B.
Limb

C.

D.
Index Arm

E.
Tangent Screw

F.
Release Clamp

G.
Micrometer Drum

H.
Vernier

I.
Index Mirror

J.
Horizon Glass

K.

L.
Telescope

M.
Handle

4

Optical Principle Of

A Sextant

A Ray of Light That Has Undergone Two
Reflections in the Same Plane Is Twice the
Angle the Two Reflecting Surfaces Make
With Each Other.

Example:

If a Star Is 60
°

Above the Horizon

<

°

Then
<
BGC = 30
°

However, the
Graduated Arc Is Labeled With 60
°

(The
Actual
<
Above the Horizon)

< BGC = ½ < ADC

5

The Marine Sextant

6

Care of the Sextant

Always Keep Sextant in Its Case, Secure at Sea When Not in Use

If You Must Set It Down, Set It on Its Legs
-

-

Never on Its Mirrors

Do Not Drop the Sextant

Keep It Moisture Free (Use Silica Gel Inside the Case)

Keep Mirrors and Arc Clear of Debris, Wiping the Mirrors With Lens Paper and a
Small Amount of Alcohol

If Sea Water Spray Gets on the Sextant, Rinse With Fresh Water and Dry Gently
With a Soft Cotton Cloth or Lens Paper

Use Light Pressure When Cleaning

Occasionally Oil and Clean the Tangent Screw and Teeth on the Side of the Limb

When Stowing for Long Periods Use a Thin Coat of Petroleum Jelly to Protect the
Arc

If the Mirrors Need Re
-
Silvering Take the Sextant to a Sextant Repair Shop

7

Errors of the Sextant

1.
Prismatic Error

2.

3.
Centering Error

1.
Perpendicularity Error

2.
Side Error

3.
Collimation Error

4.
Index Error

Non
-

8

Testing For

Perpendicularity Error

9

Testing For

Side Error

Glass

Mirrored

Glass

Mirrored

No Side Error

Side Error

10

Alternative Method

Glass

Mirror

Glass

Mirror

No Side Error

Side Error Exists

Horizon

Horizon

11

Collimation Error

1.
Bring Two Stars 90
°

or More Apart, Together In View

2.
Tilt Sextant to See If They Remain “in coincidence”

Testing For Error Of Collimation

12

Index Error

Turn Micrometer Drum Slowly Until
Horizons Line Up.

°

00’ Will
On

Or
Off

The Arc

Horizon

Horizon

Horizon

Horizon

Hold The Sextant Vertically With
Sextant Set at 0
°

Index Error Exists

13

Hs = 29
°

42.5’

14

15

16

17

Taking the Sight

Sun
-

Moon

Limb to Choose

Swinging The Arc (Ensuring You are Tangent To Horizon)

Importance of Accurate Time

Back Sights

False Horizons

Abnormal Refraction (Dip)

18

Taking the Sight

Three Methods of Observing a Celestial Body

Bringing the Celestial Body to the Horizon

Holding the Sextant Upside Down, Bring the Horizon to the
Body

Determine in Advance the Altitude and Azimuth of the Body
and Face in the Direction of the Azimuth. The Body Should Be
Near the Predicted Altitude.

19

Taking the Sight

20

Taking the Sight

21

Taking the Sight

When Using The Sextant We Must Take Into Account The Difference Between
The Celestial Horizon And The Visible Horizon Due To Our Height Of Eye.

This Is Known As The Dip Correction And Is Applied To Sextant Altitude To
Obtain Apparent Altitude (Ha)

22

Selecting Stars

To Observe

1.
Choose the Stars And/or Planets That Will Give the Best Bearing Spread
(See Example Below)

Arcturus

0530

2.
Select Bodies With a Predicted Altitude Between 20
°

and 70
°

3.
Observe the Bodies in the East First in the Morning

Observe the Bodies in the West First in the Evening

4.
Take Sights of Brightest Stars First in the Evening

Take Sights of Brightest Stars Last in the Morning

5.
Predict Expected Altitudes for up to 7 Celestial Bodies When Preparing to
Take Celestial Sights

23

Exact Time

Use an Assistant to “Mark” the Exact Time You Observed the Body

Or, Note the Time As Quickly As Possible After Shooting the Body

(Deducting Any Seconds It Took to Look at Your Watch)

Failure to Obtain Accurate Time Will Result in Large Errors in Your Work

Exact Time

-

Accuracy of Recording the Time Is of Utmost Importance

24

The Marine

Chronometer

1.
The Marine Chronometer is a Precision Time Piece Used Aboard a Ship to
Provide Accurate Time for Celestial Observations

2.
Knowing and Maintaining a Record of the Chronometer Error Can Determine a
Chronometer Rate, Which Will Allow the Navigator to Calculate What Exact
GMT Should Be on Any Given Day

3.
Chronometer Error Should Be Checked Once a Day Against an Accurate Time
Source Such As a Time Tick on a Radio

25

Checking

Compass Error

1.
A Compass May Be Checked For Accuracy Using Celestial Bodies By
Comparing An Observed Compass Bearing Of The Celestial Body With A True
Bearing Calculated Using Celestial Navigation Procedures

2.
The Two Most Common Ways Of Checking A Compass Using Celestial
Bodies Are By Azimuths And Amplitudes

3.
Common Equipment Used To Observe Azimuths And Amplitudes

The Azimuth Circle

A Ring Designed To Fit Snugly Over A Compass Repeater To
Allow For Compass Bearings Or Azimuths To Be Taken