What is a Sextant? How to Use a Sextant?

What is a Sextant?

Table of Contents

Introduction - What is a Sextant?

A sextant is a special navigation instrument used to measure the angular distance between thehorizon and a celestial body (for example, the Sun, the Moon, and more). Measuring the altitude of a celestialbodyis called taking a sight.


The term sextant derives from Latin sextans, sextant - “sixth part”, fromsextus- “sixth”. The arc of this instrument is ⅙ of a circle or 60 degrees, hence the name.

Today, people widely use electronic navigation systems, however, many ships have a sextant on board. One of theundeniable benefits of this instrument is that it doesn’t require neither electricity nor other kinds ofpower. It is worth mentioning that even today, celestial navigation is used by private yachtsmen, forexample.

Of course, GPS technology is irreplaceable in most cases, however, using a sextant can be your alternativenavigational instrument, it is wise to have a reliable backup.

Sextants have been used in navigation for over 200 years.

The first models back in the day were cumbersome. Look at the picture below.

The first models back in the day were cumbersome.

This is an early sextant produced by John Bird in the 18th century. This instrument was not lightweight to carryaround. In fact, it required using a support.

Later, more improved variants were produced. Nowadays, sextants are more accurate, they have a better design andareeasier to use as well.


Sextant Construction

To understand how a thing works, it is useful to know what it consists of.

Sextant Construction

The foundation of a sextant is a frame. It can be made of metal or plastic. The frame holds allelements of this instrument together. Quality sextants have a solid frame for high levels of accuracy.

foundation of a sextant

At the bottom of the frame, there is an arc with a scale, which runs from 0 to 120 degrees. Butasextant itself covers only 60 degrees. How is it possible? Due to the double reflection principle. But moreaboutthat a bit later.

bottom of the sextant

The index arm is attached to the frame. It is the main moving part of the instrument. It is usedfortaking measurements.

index arm of sextant

The lower end of the arm is lined up with the scale for reading the measurements.

lined up of sextant

At the bottom of the index arm, there is a Vernier scale & a micrometer drum.Along with the main scale on the arc, they are used to get precise measurements.

Vernier scale & micrometer drum

A clamp is designed to release the scale for the index arm to move freely. It is necessary for taking a sight.

A clamp of sextant

The mirror attached to the index arm is called the index mirror. It is rigidly attached so itmovesas the arm is moved. Note that the scale at the bottom of the frame is related to the angle at which the indexarmand, as a result, the index mirror is used.

the index mirror of sextan

There is another mirror - the horizon mirror. It is attached rigidly to the frame. Partially, itisa mirror, and partially - it is transparent. Why so? For you to see the horizon and the reflection of thecelestialbody at the same time. Some sextants have a semi-silvered mirror instead.

horizon mirror of sextant

Opposite the horizon mirror, there is a telescope. It lines up with the horizon mirror for youtosee down the sights & see straight through it.

telescope of sextant

Pay attention to the angle of the horizon mirror. When you are looking down the telescope, the reflection in thehorizon mirror is the index mirror.

sextant horizon mirror

In front of each mirror, there is a set of shades. They will come in handy for eye protection ifyouneed to take a sight of the Sun, for example. If you don’t use these shades, you can harm your eyes.

sextant shades

How Does a Sextant Work?

Now that we figured out what the main parts of a sextant are, let’s see how it works.

A sextant works on the principle of double reflection as we mentioned earlier. When a ray oflightis reflected from 2 mirrors successively, the angle between the incident ray (the one that hits the mirror) andthereflected ray is twice the angle between the mirrors.

how a sextant works

Look at the diagrams below.

how a sextant works

When the sextant is set at 0 degrees, looking at the horizon, both mirrors are looking at the same spot and theimagein both parts of the horizon mirror should be identical. There is no deflection in the beam of light.

how a sextant works

To deflect a beam, rotate the index arm. The image in the reflected part of the horizon mirror will change.Theangle along the scale corresponds with the angle between the objects you are looking at.

Thanks to the principle of double reflection, using a sextant, it is possible to measure celestial bodies twiceashigh as the index mirror can move.

Using a Sextant - A Step-By-Step Guide

Let’s move on to a few simple steps of using a sextant. This instrument may look intimidating, butactually, itis pretty easy to use.

To measure the altitude of a celestial body, follow the instructions below.


Step #1 - Set your sextant to zero.

The index arm and micrometer drum should be set to 0 degrees.

Set your sextant to zero

Step #2 - Make sure that the actual horizon is level with the horizon you see in the mirror.

If the image is distorted and the horizon line is not continuous, see the Error Corrections section.


Step #3 - Set the shades.

Adjust the horizon and index shades (they can be found in front of each mirror).

Set the sextant shades


Step #1 - Place the celestial body in the center of the image.

(Hold your sextant upright when taking a sight).

1A. Get the celestial body in the middle of the horizon mirror. (If the image is distorted andthereal & reflected objects are separated, see the Error Corrections section).


1B. Bring your sextant down aiming it towards the horizon. Move the index arm holding it by theclamp and keep the celestial body in the center of the horizon mirror. The horizon should be in the center aswell.


1C. Then, bring the celestial body to the center of the image.


1D. Adjust the index arm to bring the celestial body in line with the horizon. The bottom of thebody in the mirror should touch the horizon. Use the micrometer for final adjustments.


NB Sometimes, to see the horizon line better, you can remove one of the horizon shades. However, be careful! Make sure you remove the right shade for the bright light of the celestial body not to harm your eyes.

In a nautical almanac (a publication describing the positions of a selection of celestial bodies), there is information about an upper limb & a lower limb measurement for you to easily line up the top an the bottom of the object.

Step #2 - Check whether your sextant was upright when you took a sight.

Sway the sextant from side to side, keeping it pointed straight ahead. Thus you are checking if you held yoursextantupright when taking a sight. It you did, looking through the telescope of your sextant, you will see that thelowestpart of the celestial body touches the horizon. Otherwise, you will see that the object drops below the horizon.Inthis case, just use the Vernier scale to make the lowest point of the object touch the horizon line.


Step #1 - Read off the measurements from the main scale + the Vernier scale.

The main scale on the arc shows degrees, whereas the Vernier scale (you can find it next to the micrometer knob)shows minutes (1 minute is 1/60 of a degree).

Sextant Erros Correction

Step #2 - Write down the measurements.

Sextant Erros Correction

Error Corrections

Sometimes, there are errors of a sextant and it is important to a) identify the problem and b) know how to fixit.Pay attention to three correctable sextant errors.

Perpendicularity error - the index mirror is not perpendicular to the plane of yoursextant.

Set the index arm at, for example, 30 degrees. Look straight across the plane of the sextant so the arc is seenbothdirectly and reflected the index mirror.

Sextant Erros Correction

The line of the real arc and the reflected arc should be continuous and flat as well.

In case there is a perpendicularity error, the real arc and the reflected arc are not continuous anymore.

Sextant Erros Correction


Use the adjustment screw located at the top of the index mirror. Turn the screw until the real and reflectedviews ofthe arc make a straight line. Now the index mirror is perpendicular to the plane of the sextant.

Side error- the horizon mirror is not perpendicular to the plane ofyoursextant.

Set the instrument to 0. For it, just set the index arm and micrometer drum to 0. If you look at an objectthroughthe telescope of the sextant and see that the real object and the reflected object are separated, there is asideerror.

Sextant Side Error

The adjustment screws found on the horizon mirror will help you fix both

the side error and the index error at the same time. Keep reading.

Index error- the index mirror and the horizon mirror are not paralleltoeach other when your sextant is set to zero.

Sextant Index Error

Look at the horizon through the telescope of your sextant. If the horizon is not one straight continuous line,thereis an index error.

If you have both the side error and the index error, the images you see in the telescope will be displacedverticallyand horizontally.

sextant index and sextant error

Set your sextant to zero. Look at the horizon through the telescope of the sextant. Then, slightly tilt yourhead. Ifyou have a side error, the real & reflected horizon lines will move apart. At 45 degrees, a step is the sideerror & the index error combined.

Set your sextant to zero

Solution (the side error)

To fix the side error, use the side error screw (the one that is located further from the frame).

fix the side error of sextant

Turn the screw so that the horizon line is straight & continuous.

fix the side error of sextant

Bring the instrument to its normal angle. You may see that the horizon line is distorted.Ithappensbecause of the index error.

fix the side error of sextant

Solution (the index error)

To fix the index error, use the index error adjustment screw (the one that is closer to the frame).

fix the index error of sextant

Turn it until you see that the horizon line is continuous again.

fix the index error of sextant

Tilt your head again to check if the side error is present. Repeat the steps above until both of the errors arefixed.

How to Maintain a Sextant?

Since a sextant is a precision instrument, make sure that you handle it carefully. Otherwise, you may getincorrectresults and you don’t want that, right? If you have a sextant, there are several simple rules to follow tokeep it in good condition & extend its service life.

  • don’t put too much stress on the index arm;
  • make sure your instrument is clean and dry after you use it, wipe all the lenses, mirrors, and shades using a piece of soft cloth;
  • store your sextant in the box when it is not in use;
  • avoid dropping the instrument;
  • limit its time in the sun, keep it away from blowers, heaters, etc.

How to Maintain a Sextant


Hopefully, now you understand what a sextant is and how to use it. Don’t forget to maintain it properly sotheinstrument will serve you for a long time. If you don’t have a sextant or you want to replace your oldinstrument, on Mega Depot, you can choose from a selection of high-quality sextants and replacement parts.

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