What Is A Sextant? Explained By A Ship’s Officer

The sextant is a navigational instrument used to measure angles. In celestial navigation, it measures the angle between the horizon and a celestial body (the sun, moon, planets or stars), and in terrestrial navigation, it measures the angle between two charted objects (lighthouses, piers, etc.).

Close up of officer using a sextant

The sextant is essentially a very accurate, scientific version of a protractor. It measures angles.

Protractors measure the angles between lines on a sheet of paper to the closest degree. The sextant can measure angles between distant objects, accurate to the closest 0.002°.

The main difference is that a sextant uses mirrors to measure the angle between different beams of light.

When you angle a sextant’s mirrors correctly, you can visually overlay two objects. In celestial navigation, you would overlay a star with the horizon. In terrestrial navigation, you might overlay a lighthouse with a pier.

As the sextant uses the Double Reflection Principle, the angle between its mirrors is directly proportional to the angle between the objects.

Using the micrometer, you can read a very precise angle straight from the sextant itself.

Labelled illustration of a sextant
The angle between the mirrors is controlled by the index arm, and read using the micrometer.

Having established that the sextant is basically just a very accurate, scientific protractor, we can start to learn a little more about exactly what it is.

Types of sextant

In modern navigation, there are two types of sextants: plastic nautical sextants, and metal nautical sextants.

In the past, there were different sextants used for different tasks so you had three further sub-categories: Nautical Sextants, Box Sextants, and Sounding Sextants.

Today, however, Box Sextants and Sounding Sextants are no longer used because their functions can be accomplished by other means, or performed by the standard nautical sextant. Consequently, the only versions of those still available are historical metallic versions.

This essentially only leaves us with a choice between a plastic nautical sextant, metal nautical sextant, a historical metal box sextant, or a historical metal sounding sextant.

Plastic sextants

A plastic sextant is a nautical sextant with plastic used as its main construction material. They are a relatively recent development, made possible by advancements in the precision and durability of plastic manufacturing techniques.

Commonly, they will be priced lower than their metal counterparts because they are easier and quicker to manufacture.

Their disadvantage, however, is that plastic is still less durable than metal, so you can expect a plastic model to suffer greater deterioration over time.

Despite that, I actually recommend a plastic sextant over a metal sextant for most users. They offer enough accuracy for most use cases, and their lower price point keeps them affordable for more people.

If you are considering buying a plastic sextant, you should read my round-up article looking at your different options: Plastic Sextants: Which One Is Best?

Metal sextants

A metal sextant is a nautical sextant with metal used as its main construction material. Traditionally, sextants were all made from metal because it was possible to manufacture accurate metal instruments much earlier than plastic versions.

Due to the additional resources and work required in the manufacture of metal sextants, you will find most models priced significantly higher than their plastic counterparts.

The additional investment is offset by the gains in durability and subsequent gains in accuracy over time that are possible with a metal sextant.

For users needing to guarantee a higher amount of accuracy, or planning to use their sextant frequently, I will always recommend a metal model. There is no doubt that they are more accurate and durable than plastic versions, so if you would benefit from those then they are worth the additional expense.

Box sextants

Box sextants were a type of metal sextant, sometimes referred to as a pocket sextant. As the name suggests, they were much smaller than nautical sextants.

The idea behind a box sextant is that it is small enough to be carried in your pocket. Its box cover would protect the delicate mechanism when it was carried inside your pocket.

They were popular in the past when all navigators would carry their own sextant as it meant they always had one to hand when required.

Nowadays, it is more common to only have one or two sextants for an entire ship. As such, nautical sextants are always chosen instead, so the box sextant has fallen out of common usage.

Sounding sextants

Sounding sextants are very similar to nautical sextants, except they were designed to be used by hydrographers.

They get their name because they were used to confirm the position of lead-line soundings that were being taken during a survey of the coastline. Accurately plotting your position gave the sounding an accurate position on the chart.

Nowadays, most soundings are taken by single or multi-beam sonars, with positions continuously plotted through the use of GPS.

Additionally, the small differences between sounding sextants and nautical sextants meant that it was not economically viable to continue producing them for the shrinking market.

How does a sextant work?

Diagram illustrating a sextant measuring the altitude of the sun
Sextant measuring the altitude of the sun.

A sextant works by adjusting the angle between two successive mirrors to superimpose the image of two objects over each other. The angular distance between two objects can then be found by reading the angle between the two mirrors using the sextant principle (described below).

The exact workings of the sextant are quite complicated, so are best illustrated visually. Fortunately, I have written a complete guide if you are interested in the details: How Sextants Work: An Illustrated Guide.

The sextant principle

The principle that the sextant operates under is known as the sextant principle, or the Principle of Double Reflection: “When a ray of light is reflected from two mirrors in succession, the angle between the incident ray and reflected ray is twice the angle between the mirrors.”

Diagram illustrating the double reflection principle
The Principle of Double Reflection

In a sextant, the horizon mirror is equivalent to the first mirror and the index mirror is equivalent to the second mirror. The angle, A, is controlled using the micrometer on the end of the index arm, which is fixed rigidly to the index mirror.

You simply adjust the angle, A, until the desired object is clearly visible, at altitude 2A.

To save you from performing additional calculations, the sextant’s arc shows you the readings equivalent to 2A. Essentially, the angle you read from the sextant is twice the angle that you have deflected the index mirror.

This is why a sextant’s arc spans only 60°, yet it can measure angles up to 120°.

Different ways you can use a sextant

The most well-known use of the sextant is in celestial navigation, but there are a surprising number of other ways in which you can use one to help you navigate.

We have already established that a sextant is just a tool for measuring angles. For an experienced navigator, there is a whole range of different ways you can use those angles to help you find your way.

All techniques rely on the same basic operation of the sextant, which I summarise in 6 steps:

  1. Plan the time of your sights
  2. Pick up your sextant correctly
  3. Correct your sextant for the correctable errors
  4. Locate the body you are measuring using the telescope
  5. Bring the body in line with the horizon, or other reference position
  6. Read the altitude from the sextant’s arc and micrometer

Pro Tip: For a complete explanation of these steps to use your sextant, check out: How To Use A Sextant: A Step By Step Guide.

Once you master getting angles using your sextant, it is just a case of implementing it in the way you choose.

Measuring the altitude of celestial bodies for celestial navigation

When you use your sextant to measure the altitude of celestial bodies above the horizon, you can use the readings to obtain a celestial position fix.

The basic principle of the fix is that you make a guess of where you think you are and calculate the precise altitude of stars from that position.

You then measure the true altitude of the stars using your sextant.

If the stars are exactly where you think they should be, it proves that the guess of your position was correct.

If the true altitude of stars is different to the calculated altitudes, you can use the difference to plot a line of position from your best guess.

It does not matter how accurate your guess was, the idea is that comparing the true altitude of stars to their calculated altitude allows you to correct your guess and find your real position.

Measuring the angular height of a charted object to obtain a range

A Vertical Sextant Angle is used to measure the angular height of a charted object with a known height so that you can calculate how far from the object you are.

For example, if you know that a lighthouse has a height of 10m, and you measure its angular height using a sextant and find that it is 1°10.2’, then you can use trigonometry to find the range is 49m.

Height / Tan(Angle) = Distance

10m / Tan(1°10.2’) = 49.0m

You could then plot a line of position on your chart by drawing a ring 49m away from the lighthouse.

Measuring the angular distance between two charted objects to obtain a line of position

A Horizontal Sextant Angle is used to measure the horizontal angular distance between two charted objects so that you can plot a line of position on a chart.

For example, if you measure the angle between a church spire and a lighthouse to be precisely 50°, you can plot a curved line of position around the objects to get a line of position.

The easiest way to do this is to draw two lines on tracing paper, 50° apart. You then overlay it on the chart and make sure both lines continuously touch the charted objects.

You’ll be able to slide the tracing paper around, and your line of the position will be the one following the intersection of the lines as you move it around.

Observing a charted object to monitor a clearing range

In a similar way to finding a range from a charted object, you can also calculate a safe clearing range from danger and use a Vertical Sextant Angle to keep yourself clear.

For example, if you know you want to keep at least 50m off the lighthouse in the previous section, you can set your sextant at 1°10.2’ and observe the lighthouse. If you observe it and the angle is less than 1°10.2’, you know you are further away.

If the angle reaches 1°10.2’, you know you are now 49m away, placing your boat in the previously established dangerous area.

Monitoring the sun at midday to calculate the time of local noon

Local noon occurs when the sun is at its highest point in the sky, viewed from your location.

Glancing up at the sun, it is impossible to detect the exact time that midday occurs.

With a sextant, however, you can take such precise readings that you can observe the sun around midday and monitor it.

All the while it continues rising, you know that it is not yet midday. The moment it stops rising, you can record the time as local noon.

Interestingly, you can compare the time of local noon to the time of noon at Greenwich to determine your longitude. Assuming your watch is accurate, it is possible to get a very accurate longitude using this method.

Buying a sextant

Now that you know what a sextant is, you may be considering purchasing one yourself.

Read More: for a complete guide on buying a sextant, check out: Choosing The Perfect Sextant: Which One Is Best.

As with everything, there are countless options to choose from.

First, you should consider whether you want to use your sextant for navigation or whether you want it to be purely ornamental.

There are a lot of ornamental models out there that cost a lot less than usable ones. That is fine if you are after an ornamental version, but be wary if you are looking for one that you can use yourself.

Next, consider how much you want to use your sextant. If you will be using it a lot, for many years, you may consider a metal version. Otherwise, a more affordable plastic one will do just fine.

In fact, I actually recommend a plastic one for most people, especially for beginners. I explain why in this article: Which Sextant Is Best For A Beginner?

Who invented the sextant?

Around 1730, John Hadley and Thomas Godfrey are both credited with first using the Double Reflection Principle in a navigational application. They both independently invented the octant within a few years of each other, displacing the previously dominant quadrant.

Almost 30 years later, in 1759 a British Royal Naval officer, John Campbell is credited with suggesting modifications to the octant by extending its arc to 60° and using brass for its construction. In doing so, his modifications led to the first sextant as we know it today.