A Simplified Guide for Sextant – Uses, Principle and Errors!

As the name is interesting so is the equipment. The marine sextant is a precision navigational equipment. However, the use of it is now just a mere requirement. The sextant was invented in 1757 by John Campbell, a Royal Navy Officer for the British.


Earlier a sextant was most important equipment used for navigation. So I think there is no harm in knowing all Wh questions i.e. what, why, how, when, where and who regarding this equipment. So let me start…..

What is Sextant?

Sextant is an equipment used for measuring altitudes of heavenly bodies and angles between two objects horizontally and vertically.

Principle of Sextant

When a ray of light is reflected by a plane mirror, the angle of the incident ray is equal to the angle of the reflected ray, when the incident ray, reflected ray and the normal lie on the same plane.

When a ray of light suffers two successive reflections in the same plane by two plane mirrors, the angle between the incident ray and the reflected ray is twice the angle between the mirrors.

So the principle of double reflection is used to measure angles so therefore it can measure up to 120 degrees

The sextant is used to measure the following:

  1. Vertical Sextant Angle (VSA)
  2. Horizontal Sextant Angle (HSA)
  3. Altitudes

What are Different Parts Of A Sextant?

  1. Graduated arc– The angles are measured as per the markings on the graduated arc. The markings are from 0 to 120 degrees but can be up to 130 degrees. The arc is about 1/6th of a circle. It is the reason the navigational instrument is called a Sextant (Latin word for 1/6th is Sextans)
  2. Index arm– The movable arm that moves around the arc so as to measure angles.
  3. Index mirror– Located at the top its use is to reflect the rays of light falling on the mirror to the horizon glass it should always be perpendicular to the frame as per it is adjusted to minimize or to nullify the error we will talk later with the errors of a sextant.
  4. Horizon glass – The Horizon glass is a half view glass in front of the telescope. The image reflected from the index mirror is again reflected from the horizon glass, hence called double reflection, is directly reflected the telescope.
  5. Micrometre drum-To adjust the image on the horizon captured. It can be rotated both clockwise and anticlockwise direction.
  6. Vernier-A small scale just below the micrometre drum which is been used to measure the angle
  7. Index mirror filters– To protect the eye from the rays there are about three to four filters in sextant as per the intensity we need to apply the filters or shades.
  8. Horizon glass filters– Use is same as Index mirror filters just need to put appropriate shades to get a clear view of the object most probably its the sun we require filters because of high-intensity rays of the sun can damage the eyesight.
  9. Lighting unit– To provide adequate illumination for measuring angles.
  10. Quick-release clamp– The clamp located at the bottom of the index arm which is used to move the index arm throughout the arc.
  11. Handle– Handle is provided with the grooves for holding the sextant tight.
  12. Adjusting screw– Located at the index mirror so as to keep the index mirror perpendicular to the frame of the equipment.
  13. Telescope– The image is observed through the Telescope.

How to use a sextant?

So basically we will learn how exactly sextant is been used to calculate the angles or altitudes so as to determine the position of the observer also can be called as “shoot”

The first step is to check the condition around is suitable for taking the sight that means-

  1. Skies should be clear heavenly bodies that will be used for taking sight should be visible to naked eye.
  2.  Visibility should be good enough to see the horizon.
  3.  The position of the observer should be above sea level.
  4.  As we are going to use it on a moving vessel i.e. at sea so vessel should be steady enough for accurate readings.

Further, we should decide the object or any heavenly body of which we are going to measure angles. Before that we should first check the errors for accurate readings we will discuss errors of sextant later.

Once all errors are checked hold the sextant by its handle with your right hand firmly. See the horizon through your right eye from the telescope and adjust your position. Keep the horizon line perpendicular to the observer’s eye.

After that observe the body, for now, let’s say we are taking sight by using the sun. The sextant is designed with various shades or filters to take the altitude of the sun. Appropriate shades or filters should be applied before observing the sun.

It has a movable Index arm with its pivot point at the top of the equipment which enables the index arm to move all around the graduated arc with the help of the quick-release clamp. Quick-release clamp is generally used to lock the index arm in a particular position in the graduated arc so as to measure angles of the body.

So let’s take the altitude of the sun. Hold the sextant firmly. Observe the sun through the telescope by using appropriate shades on index mirror and horizon glass. After than use the quick release clamp by pressing the clamp to move the index arm until the image of the sun is aligned with the horizon. Sun is spherical in shape, therefore, it is measured either for its upper limb and lower limb we will discuss calculation part in another topic.

Micrometre drum is connected to the index arm with vernier scale below the drum. Micrometre drum is used to adjust the position of the image on the horizon. In general, it is used for minor adjustment to make the body perfectly aligned with the horizon.

Once the sun is aligned for its lower or upper limb on the horizon we can then measure the angle. So in a similar manner, we can measure the altitudes of various bodies or object.

Taking altitudes during night time is the same as we would take in day time many sextants are provided with self-illuminating light for reading the angles at night. Always remember sextant cannot be used if the horizon is not visible. Reading the sextant angles will be discussed further.

Reading the Sextant

Reading the Sextant means to identify the angle from the equipment. Once the altitude or angles are taken from sextant, we need to then identify the angles by a graduated arc from the position of index arm with micrometre drum and vernier scale which is just below the micrometre drum. Refer to the featured image of this article to understand it in a better way.

Following is the image we will use to read the angle. As it is an angle it will be in degrees, minutes and seconds. Degrees are measured from the graduated arc, minutes and seconds are from the micrometre drum with the help of vernier scale. So lets read the sextant.


In the above image, the marking shown on the graduated arc is 70 degrees. The micrometre drum marking is aligned with 0 of vernier scale on 44 minutes. 45 marking of micrometre drum is perfectly aligned with 2 of vernier scale so seconds will be read as .2 minutes. So the angle is 70 degree 44.2 minutes. In this way, we need to read sextant of whichever angle you obtained.

Errors of the Sextant

So the most common topic which is been asked both in the oral and written examination of second mate the errors of the sextant. So these errors are been classified as Adjustable and Non-Adjustable errors.

Adjustable Errors

Perpendicularity error:-

When the index mirror is not perpendicular to the plane of the instrument then this error exists. So to check whether such error exists to bring the index arm to the middle of the arc with the help of quick-release clamp and holding the sextant horizontally, keep the arc away from you, look obliquely into the index mirror till the arc of the sextant and its reflection on the index mirror are seen simultaneously. If in alignment, the error does not exist. If not then the error exists so to minimize or to remove the error turn the adjustment screw at the back of the index mirror until they are aligned.

Side error:-

When the horizon glass is not being perpendicular to the plane of the instrument such error exist. Clamp the index arm at 0 degrees. Hold the sextant vertically and look at the heavenly body. Turn the micrometre one way and then the other, while looking at the body. The reflected image of the body will move above and below the direct image and should pass exactly over it. If the reflected image passes to the left or right of the direct image, side error exists. This error can be removed by turning the second adjustment screw which is the top screw behind the horizon glass until the true and reflected horizons appear in the same line.

Index error:-

This is caused if the Index mirror and the Horizon glass are not exactly parallel to each other when the index arm is set at 0 degrees. Basically, this is the difference between the optical zero of the sextant and its graduated zero also termed as OFF the arc if the optical zero lies to the right of the graduated zero and termed ON the arc if the optical zero lies to the left of the graduated zero.

So for obtaining the index error of a sextant, clamp the index at 0 degrees and holding the sextant vertical look at the horizon. The reflected image and the direct image should appear in a perfect line. If not then the Index error exists so to obtain the reading of index error turn the micrometre until they coincide exactly. The reading of the micrometre will be ON or OFF the arc which gives the Index error. Same can be done with heavenly bodies if the horizon is not visible to obtain Index error.

Error of Collimation:-

This is due to the axis of the telescope not being parallel to the plane of the instrument. Nowadays the modern sextants have its telescope attached to the sextant in such a manner that it cannot tilt. Therefore not provided with any screws for adjusting.

Non-Adjustable Errors

Graduation Error:-

Due to the inaccurate graduation of the main scale on the arc or of the micrometre/vernier.

Centring Error:-

If the pivot of the index arm is not situated at the geometric centre of the arc. This can be caused due to a manufacturing defect or due to careless handling.

Shade Error:-

The shades should be so mounted that their glass surfaces are normal to the rays of light passing through them. If not, it would result in distortion. The greater number of shades used, the greater the chances of distortion.  

Optical Errors:-

It is caused by prismatic errors of the mirrors or aberrations in the telescope lens.

In case of any doubt, Let me know in the comment section below. Keep reading awesome stuff!

Suggested read: What does Boxing the Compass mean? Explained with Images!

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