SEXTANT HÀNG HẢI VÀ CÁC SAI SỐ

Side Error continued:- Using the Star method Set the sextant to zero Hold the sextant vertically and view a well defined star through the eyepiece The true and reflected images should be

The Sextant

What it is and how it works.

The parts of a Sextant

What is a Sextant ?

A sextant is a measuring instrument generally used to measure the angle of elevation of a

celestial object above the horizon.

Making this measurement is known as sighting

Making this measurement is known as sighting

the object, shooting the object or taking a sight

The angle, and the time when it was measured, can be used to calculate a position line on a

nautical chart.

What is a Sextant?

Held horizontally, the sextant can be used to measure the angle between any two objects, such as between two

lighthouses, which will also allow for

calculation of a line of position on a

chart.

What is a Sextant?

The scale of a sextant has a length of 1⁄6th of

a full circle (60°); hence the sextant's name

( sext ā ns, -antis is the Latin word for "one

An Octant

 A Quintant

How to handle a Sextant

• Please remember, especially for MCA ‘Orals’ examinations that the Sextant is a delicate

instrument and must be handled in a particular manner

• MCA examiners will be looking for this if they ask you questions relating to the Sextant

The Principle of the Sextant

• When a ray of light

is reflected twice in

the same plane by

two mirrors……

• the angle between

the first and last

directions of the ray

• is twice the angle

between the

mirrors

A Sextant at Work

 First Error =

Perpendicularity:- The index mirror must be perpendicular to the

plane of the instrument

 To find out if this error is present , set the index bar near the middle of the arc

 Hold the sextant horizontally with the arc away from you

 View the true arc, and also the reflected arc in the index

mirror



Perpendicularity:- The true & reflected arcs should appear as one

 The true & reflected arcs should appear as one

unbroken & continuous line

 Correct any error by means of the first adjustment screw (1)

on the back of the Index morror

What you should see

What you should NOT

see

What you should NOT

see

 Second Error = Side Error

:- The horizon mirror must be perpendicular to the plane

 Side Error

(continued):- Using the Star method

Set the sextant to zero

Hold the sextant vertically and view a well defined star

through the eyepiece

The true and reflected images should be in the same vertical plane

Side Error

(continued):-
Using the Star method

This is what should be seen

This is what should be seen

when no side error exists

If it does exist, this is what

will be seen

 Side Error

(continued):- Using the Horizon method

Set the sextant to zero

Set the sextant to zero

Hold the sextant near to horizontal and view a well defined section of the horizon through the eyepiece

The true and reflected images should be in the same

continuous line

Side Error

(continued):-
Using the Horizon method

This is what should be seen

This is what should be seen when no side error exists

If it does exist, this is what will be seen

Side Error

(continued):-This error is corrected by using the

2nd adjustment screw on the 2

2 adjustment screw on the

horizon mirror

The 2nd adjustment screw is

furthest from the plane of the

instrument

2

3

Sextant Errors

 Index Error

 The horizon mirror and the index mirror must be parallel when the sextant reads 00 0 00.0’

 There are three methods by which we can find out if

this error exists

:- Star method

 Horizon method

 Sun method

Sextant Errors

 Index Error

(continued):- Using the Star method

Set the sextant to zero

Hold the sextant vertically and view a well defined star through the eyepiece

The true and reflected images should be superimposed

Sextant Errors

 Index Error

(continued):- Correcting using the Star method

This is what should be seen

when no index error exists

If it does exist, this is what will

be seen

Sextant Errors

 Index Error

(continued):- Correcting using the Horizon method

Set the sextant to zero

Set the sextant to zero

Hold the sextant vertically and view a well defined section of the horizon through the eyepiece

The true and reflected images should form a straight line

Sextant Errors

 Index Error

(continued):- Correcting using the Horizon method

This is what should be seen when

no index error exists

If it does exist, this is what will

be seen

Sextant Errors

 Index Error

(continued):- Correcting using the Sun method

Set the sextant to zero, hold the

sextant vertically and, using the

appropriate shades view the sun

If there is no error the true &

reflected images of the sun will be

superimposed on each other like this

Sextant Errors

 Index Error

(continued):- Correcting using the Sun method

Nonetheless, to confirm the

presence or not of index error the

following procedure is carried

out:-
With the sextant set to zero rotate

the micrometer clockwise until the

true & reflected images are just

touching and take a note of the

reading “on” the arc

Sextant Errors

 Index Error

(continued):- Correcting using the Sun method

Then, reverse the images and

take a note of the reading “off” the

arc

Halve the difference between the

readings and name the correction

“on” or “off” the arc according to

the largest reading

 Reading “on” the arc = 32.6’

 Reading “off” the arc = 31.4’

 If the error is “On The Arc” then the error is subtracted

 If the error is “Off The Arc” then it is added

INDEX ERROR

 N.B Adjusting the 3 rd Adjustment screw may well re-introduce side error which will then need to be corrected

 This may then re-introduce index error

 Consequently, if Index Error is small it is generally easier to apply the error rather than trying to

correct it.

Errors which cannot be corrected