What is Sextant?

Introduction: Navigating the Seas the Old School Way

If you work at sea or have a passion for maritime history, you’ve likely heard of the sextant. But have you ever wondered how this iconic navigation instrument actually works, or why it’s still relevant today, even in the age of GPS? Let’s dive into the details of the sextant—what it is, how it’s used, and why every seafarer should respect this timeless navigational tool.

What Exactly is a Sextant?

A sextant is a precision instrument used by mariners to measure the angle between two visible objects. Most commonly, it’s employed to determine the angle between a celestial body (like the sun, moon, or a star) and the horizon. This measurement is crucial in celestial navigation, allowing us to figure out our position at sea, especially when far from shore and out of range of terrestrial navigation aids.

Think of the sextant as an essential bridge between the sky and the ship, letting sailors plot their course using the oldest, most reliable GPS: the stars.

Who Uses a Sextant?

The sextant isn’t just for nostalgic captains or maritime historians. Many professional mariners, including Masters and Navigation Officers, still learn and use sextants as part of their core navigation training. Cadets and even seasoned officers know that while electronic navigation aids—like ECDIS and modern GPS—are incredible, they aren’t immune to failures or satellite outages. A well-trained seafarer should never be left adrift simply because the screen went blank.

If you’re pursuing your license or working on deck, chances are you’ll have to demonstrate your ability to use a sextant during your career. Even the STCW Convention (Standards of Training, Certification & Watchkeeping for Seafarers) recognizes celestial navigation as a vital skill.

Why is the Sextant So Important?

You might ask: with all our satellite tech, do we really need a sextant? The answer is yes—and here’s why:

• Reliability: Sextants don’t need batteries, signals, or power. They just work, rain or shine.
• Backup navigation: In case of electronic failure, the sextant is your failsafe for open-ocean position fixing.
• Professional competency: Knowing how to use a sextant is a hallmark of a well-trained navigator and required by maritime authorities.
• Connection to tradition: There’s a true sense of pride and seamanship in practicing skills used by sailors for centuries.

On my first long voyage across the Indian Ocean, our GPS blacked out during a squall. With all eyes on the navigation team, we pulled out the sextant, took a sun sight, and calculated our noon position. That day, the crew respected the “old school” way and understood why redundancy matters at sea.

How Does a Sextant Work?

At its core, the sextant is all about measuring angles with accuracy. Let’s break it down:

• Structure: The main part of a sextant is a shaped arc (usually covering 60°—hence the Latin word “sextans,” for a sixth of a circle). Attached to this is a movable arm called the index arm.
• Mirrors: The instrument uses two mirrors—a fixed “horizon mirror” and a movable “index mirror.” When you look through the telescope, you can bring the image of a celestial body down to the horizon by adjusting the index arm.
• Reading the Angle: Once the sun or star “touches” the horizon through the mirrors, you read the angle off the arc. Accuracy is key—some sextants let you read angles to an accuracy of 0.2 minutes of arc.

If you’re new to it, it can be a bit tricky aligning everything just right while the ship is rolling. But with practice, it becomes second nature. You’ll hear old-timers say it’s a bit like riding a bicycle—the skill stays with you.

Steps of Using a Sextant

Let’s simplify the steps:

• Stand facing the horizon with the sextant in hand.
• Look through the telescope, adjust the index arm until the celestial body is just touching the horizon.
• Take the reading from the scale (arc and micrometer drum).
• Apply corrections, like index error or dip.
• Record the time of the observation precisely (usually with a chronometer).
• Use the Nautical Almanac and sight reduction tables to fix your position.

When and Where is the Sextant Used?

Sextants are vital whenever ships are out of sight of land and reliant on celestial navigation. You’ll most often see them used:

• During ocean crossings
• On training vessels and at maritime academies
• For regulatory exams and drills
• As a regular backup to electronic navigation

Key times for observations are typically around sunrise, sunset, and local noon. These are optimum periods for clear observations of multiple celestial bodies. On my last transatlantic, our navigation team would start their mornings with a cup of coffee and a sun sight—just to brush up on the basics.

What Are the Main Parts of a Sextant?

Understanding the anatomy of a sextant will make it far less intimidating. Here are the essential components:

• Frame: The skeleton or backbone of the instrument.
• Arc: The graduated scale (usually 0° to 120° or more).
• Index Arm: Moves across the arc and carries the index mirror.
• Index Mirror: Mounted on the index arm; reflects the ray onto the horizon mirror.
• Horizon Mirror: Partly silvered, lets you see both the horizon and the reflected image simultaneously.
• Telescopes: Aids in sighting and accuracy.
• Shades: Filter out intense sunlight during sun sights.
• Micrometer Drum: For precise angle measurement.

Getting hands-on with each of these functions helps develop your “sextant muscle memory.”

How Do You Take Care of a Sextant?

A sextant is a precise and expensive piece of equipment. Proper care is essential:

• Always clean lenses and mirrors gently with lens tissue.
• Store the sextant in its case, protected from moisture and salt air.
• Check the instrument’s calibration (index error) frequently and adjust as needed.
• Avoid banging or dropping—it’s not as robust as your steel toolbox!

Regular maintenance ensures that when you need it, the sextant is ready for action.

Modern Relevance: Sextant vs. Electronic Navigation

While today’s ships are loaded with high-tech navigational gear, international authorities like the International Maritime Organization (IMO) still require proficiency with traditional navigation methods, including sextant use. Why? Because:

• Electronic systems can fail or be jammed.
• Batteries can run out when you least want them to.
• Seafarers always need a robust backup plan to ensure ship safety and compliance.

Many flag states and classification societies echo this sentiment in their training and examination requirements.

Conclusion: The Skilled Seafarer’s Companion

A sextant might look like an artifact from a maritime museum, but it remains an essential tool for every serious navigator. Mastering the sextant isn’t just ticking a regulatory box—it’s part of the pride and professionalism of life at sea. Whether you’re a cadet or a seasoned Master, take every opportunity to hone your skills with this timeless instrument.

So, next time you’re on the bridge, why not pick up the sextant and practice a sun sight? After all, navigation isn’t just about knowing your position—it’s about keeping sharp, being prepared, and respecting the seafarer’s craft.

Learn more about navigation standards and training at the official IMO STCW Convention and IMO Navigation Safety pages.

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