What is sextant
Bridge Equipments Nautical Science

Marine Sextant- Principle, Parts, Errors and How to Use it

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Marine Sextant

Every Mariner should know how to use Sextant and what are the errors associated with it. Even I have face difficulty in using sextant in my first ship but slowly I get comfortable with it. In this article I have covered what is Marine Sextant and It’s principle, Parts, Errors and how to use sextant. By the end of this article your concept will be cleared about sextant. Let’s get started.

What is Marine Sextant?

It is an instrument used for measuring angle between any two visible objects. It has a shape of an arc which is one sixth of a circle – 60°, this is why it is named as Sextant. It is based on the principle of double reflection so it can measure angle upto 120°. In practical the arc of the sextant is slightly more than 60° and therefore graduated upto about 130°. Sextant is mainly used for measuring altitude of celestial bodies for navigational purpose.

By sextant one can measure

  • vertical sextant angles of terrestrial objects and altitudes of celestial bodies. we can find distance off from a known vertical object.
  • Horizontal sextant angles between terrestrial objects. we can fix position even without having a compass.

Principle

Sextant is based on the principle of double reflection. when a ray of light suffers multiple reflection in the same plane by two mirrors then the angle between the original and final ray of light is twice the angle between the mirrors.

what is sextant

To prove c=2d

In Δ(M1 M2 S)

2a=2b+c ——-(1)

Ext. angle=sum of int opp. angles

In Δ(M1 M2 N)

a=b+d ——-(2)

Ext. angle=sum of int opp. angles

By multiplying equation (2) with 2

2a=2b+2d ——-(3)

From equation 1 & 3

2b+c=2b+2d

c=2d (hence proved)

Parts of Sextant

what is sextant
Credits: Joaquim Alves Gaspar/wikipedia

 

1. Frame: It is the backbone of Sextant to which all the other parts are connected with.

2. Graduated Arc: It is an arc which represents the scale reading from 0° to 120°( In actual it is above 120° and below 0° upto -5°).

3. Index Arm: It is a movable bar on which Index mirror is fitted.

4. Handle: By this one can hold the Sextant.

5. Telescope: It is fitted on the frame through which one can observe the target body.

6. Clamp: By the help of Clamp one can move the index arm. Clamp should be pressed to move the index arm and once it is released index arm will lock in it’s position.

7. Micrometer Drum: It is used to measure minutes and seconds because by index arm one can only measure degrees.

8. Index Mirror: It is mounted on the index arm. The purpose of index mirror is to reflect the observed object on to the horizon glass.

9. Horizon Glass and Mirror: It is a single piece in which half of the part is of glass through which we can see clearly and the other half of the part is of mirror. it is used to direct the reflected image form the index mirror to observer’s eyes. Hence it mounted directly in the line of sight of the telescope.

10. Shade Glass: These are movable glass placed in front of index mirror and horizon glass. It allows observer to observe bright objects like sun and reduce the flare for low altitude sights.

Errors

In Marine Sextant some errors are adjustable and some are non-adjustable.

Types of Adjustable Errors are:

1. Error of Perpendicularity: It is caused when index mirror is not perpendicular to the sextant frame. It can be identified by holding the sextant horizontally. Set the index arm about middle of the arc and look obliquely into the index mirror if the true arc and reflected arc (through the index mirror) are in line with each other then error is not present. If not, error of perpendicularity exists. To correct this error use 1st adjustment screw situated on the index mirror.

2. Side Error: It is caused when horizon glass is not perpendicular to the frame of the sextant. There are two methods to identify side error.

    • Star Method: Error Hold the sextant vertically. Set the index arm at 0° and look at a heavenly body then turn the micrometer drum one way or the other doing so the reflected image of the body will move up and down while doing this if the reflected image is not coinciding with the real image, side error is present.
    • Horizon Method: Error of sextantHold the sextant horizontally. Set the index arm at 0° and look at the horizon. If the reflected image of the horizon is not in the line with real image, side error is present.

To correct this error use 2nd adjustment screw situated on the centre line of the horizon mirror.

3. Index Error: It is caused when horizon glass is not parallel to index mirror when the sextant is at 0°. There are three methods to identify index error.

    • Horizon method: Error Hold the sextant vertically. Set the index arm at 0° and look at the horizon. The reflected and direct image should appear in the same line. If they are vertically displaced that means index error is present.
    • Star or planet method: ErrorHold the sextant vertically. Set the index arm at 0° and look at the star or planet. The reflected and direct image should coincide. If they are not coinciding, turn the micrometer until they coincide then if the reading is more than 0°, error is on the arc and if the reading is less than 0°, error is off the arc.
    • Sun Method: What is sextantHold the sextant vertically. Set the index arm at 0° and look at the sun using proper shades. Now turn the micrometer on the arc till the upper limb of the reflected sun touches the lower limb of the true sun then note down the reading (on the arc). Now again look at the sun, turn the micrometer off the arc till the lower limb of the reflected sun touches the upper limb of the true sun then note down the reading (off the arc). If both the readings are same no index error is present. If not, calculate the difference between them and half the calculated value, this the value of Index error and named as the higher reading.

To correct this error use 3rd adjustment screw situated at the edge of horizon mirror. Normally if the Index error is not much we can apply the error to get the correct reading.

4. Collimation Error: It is caused when axis of telescope is not parallel to the frame of Sextant. To correct this error adjust the screws at the junction of the frame of the sextant.

Types of Non-adjustable Errors are:

1. Graduation Error: It is caused when there are fault in design of the sextant like marking the scale of the arc or micrometer drum, machining the arc, cutting the limbs gears.

2. Prismatic Error: It is caused when the planes of a mirror is not exactly parallel.

3. Centering Error: It is caused when the index arm is not pivoted at the exact centre of the curvature of the arc.

4. Shade Error: It is caused when two shades are not exactly parallel to each other.

Now that you all are familiar with Marine Sextant. The most important thing is how to use one. Their are many methods to take sights but we will learn the simplest one.

Discovery

How to use Sextant

  • First check the sextant for errors like error of perpendicularity, side error and index error.
  • If their is any, made the required adjustments.
  • Make sure you have a pen and paper standby for writing the readings and time of observation.
  • Set the Index arm and micrometer drum at zero.
  • Now hold the sextant with your right hand and make sure your left hand is free so you can adjust the index arm or micrometer drum.
  • Choose any Star or Sun for the observation and look the Star or Sun through the telescope with appropriate shades.
  • Slowly move micrometer drum to separate reflected image form true image.
  • Bring the body to the horizon by slowly moving the sextant downward simultaneously with the index arm moving forward.
  • When the body touches the horizon use micrometer drum for precision.
  • Swing the sextant in an arc while adjusting the micrometer drum. You will see the body is also moving in an arc touching the horizon. Adjust until the body touches the horizon at very bottom of the arc.
  • pen down to the exact time of the observation and reading of the sextant.
  • Now show some skills which you have learnt in your academy.

Related Article: what is ECDIS?

Now a days we use sextant very rarely some ships do not even have sextant onboard. But it is very important to have a good understanding of the sextant. Every mariner should know how to use it.

Still have any doubt? Ask us in the comment section.

Source: The Marine Sextant by Capt. H. Subramaniam and Bridge Equipment & Watch Keeping by Capt. Aley Nabi Rizvi

 

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