How to use a sextant for navigation

Using a sextant for navigation may not seem important now that sailors have access to GPS; however, learning how to use this traditional piece of navigational equipment certainly comes with its benefits.

In this guide, we discuss what a sextant is and how it works. We’ll look at how to operate a sextant, the different options and how much sextants cost, plus the benefits of learning how to use one.

What is a sextant?

A sextant is a navigational instrument used to measure the angle between two visible objects, such as the angle between a celestial object like the sun, moon, or a star and the horizon. This measurement helps sailors determine their position, which is important when successfully navigating at sea.
How does a sextant work

A sextant works by using a system of mirrors and a graduated arc to measure the angle between two objects, typically a celestial body, like the sun or a star, and the horizon.

A sextant has many important parts that make navigation calculations possible, including:

• Arc – A curved scale marked in degrees, usually 0° to 120°
• Index arm – A movable arm that rotates along the arc
• Index mirror – This is attached to the index arm and moves with it
• Horizon mirror – Enables you to see the reflected image of your chosen celestial body and the horizon at the same time
• Telescope – Provides accurate sighting
• Filters – Reduce glare, which is particularly important when looking at the sun
• Micrometre drum – Allows for fine-angle adjustments and precise readings

How to operate a sextant

While being able to accurately use a sextant takes practice, the steps for operating this piece of nautical equipment are quite simple.

• Sight the horizon – Look through the telescope and line up the horizon in the view.
• Bring down the celestial object – Move the index arm, which adjusts the index mirror. This reflects the image of the celestial body off the horizon mirror into your line of sight.
• Align the images – Both the reflected image of the celestial body and the direct view of the horizon will be visible at the same time. Adjust the index arm until the reflected image of the celestial object just touches the horizon line.
• Read the angle – When the two images are aligned, read the angle from the arc where the index arm points. This is the altitude of the celestial object above the horizon. Make a note of the angle and the time.
• Calculate your position – Work out your position at sea using the angle and time you have written down.

Are sextants still used today?

Sextants are still used today, although not as much as they were in the past due to the introduction of GPS and satellite navigation. These new navigational options mean that sextants are no longer essential for most sailors, but knowing how to use one can be very handy should there be an error with GPS while at sea.

Not only are they a great backup should your GPS fail while sailing, but they also help you to build on your skill set and enable you to embrace an important part of sailing history and tradition.

How much does a sextant cost?

The cost of a sextant varies depending on the type you’d like to buy. There are options for all budgets, from affordable plastic sextants which can cost as little as £60 to premium metal sextants which can cost as much as £1400. A cheaper plastic option can be useful for first-time users learning how sextants work, while a higher-end metal sextant is best for precise readings. There are also mid-range plastic sextants on the market, which have better functionality than cheaper options but are still not as accurate as more costly metal sextants.

How accurate is a sextant?

As an instrument, a sextant is very accurate. However, its precision depends on the quality of the sextant, user skill, and observation conditions. For example, a cheap plastic sextant is great for practising but isn’t as accurate as a high-end metal option. Stable, clear conditions are also important, as waves or haze can lead to poor visibility and inaccurate readings. The more practice a sailor has with using a sextant, the more accurate their readings will be.

How hard is it to learn to use a sextant?

Learning to use a sextant isn’t necessarily hard, but becoming good at it and taking accurate readings takes practice. You need to learn how to adjust the mirrors and take accurate sight, read and correct the angle, and use the information gathered to work out your position.
While learning the basics of using a sextant may only take a couple of days, becoming confident and accurate requires ongoing practice – especially if you’d like to be able to calculate your position using a sextant in an emergency, should your GPS fail while at sea.

The benefits of learning to use a sextant

There are many benefits of learning how to use a sextant, including:

• Navigating without GPS – Sextants don’t need power, satellites, or electronics, making them a great backup when modern navigational systems fail.
• Building discipline and focus – The more practical sailing skills you have, the more focused and self-reliant you can be should things go wrong.
• Being prepared for an emergency – When you know how to use a sextant, there’s no need to panic should you lose access to your GPS while at sea.
• Gaining technical maths experience – Learning to use a sextant provides experience of using spherical trigonometry and angular measurement.
• Understanding traditional navigation methods – Using a sextant is a timeless skill that will help connect you to sailors and navigators of the past.
• Learning a unique life skill – Being able to work out your position from the sun, moon, and stars is a unique life skill that will set you apart from others and provide a great topic of conversation.

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