What Is a Nautical Sextant and How to Use It for Navigation

A sextant is one of the most elegant and reliable navigation instruments ever created. For centuries, mariners have used sextants to determine their position at sea by measuring the angle between celestial bodies and the horizon. Even in an age of GPS and electronic chartplotters, the sextant remains an essential backup tool that requires no power, satellites, or external signals to function.

Understanding how a sextant works and how to use one is increasingly valuable as mariners recognize the vulnerability of electronic systems to interference and failure.

What Is a Sextant? Definition and Purpose

A nautical sextant is a doubly reflecting navigation instrument that measures the angular distance between two visible objects, typically a celestial body (sun, moon, star, or planet) and the visible horizon. The name derives from the Latin word "sextus," meaning one-sixth, because the instrument's arc spans 60 degrees, or one-sixth of a circle.

Primary Purpose

The primary purpose of a sextant is to measure the angle between a celestial body and the horizon. Combined with precise time and reference tables from a nautical almanac, these measurements allow navigators to calculate their exact latitude and longitude through a process called celestial navigation.

How this creates position data:

1. Measure the angle between the celestial body and the horizon (sextant altitude)

2. Record exact observation time (chronometer accurate to seconds)

3. Apply corrections for atmospheric refraction, observer height, and instrument errors.

4. Consult the nautical almanac for the celestial body's declination and Greenwich Hour Angle.

5. Use sight reduction tables or formulas to calculate latitude and longitude

This elegant system has remained fundamentally unchanged for over 250 years because the underlying principles are as reliable today as when first developed.

How Does a Sextant Work? The Mechanical Principles

A sextant operates on the principle of double reflection. Understanding this principle is essential for proper use.

The Double Reflection Principle

When a ray of light reflects off a plane mirror, the angle of the incident ray equals the angle of the reflected ray. A sextant uses two mirrors positioned at a specific angle to each other. When light reflects off both mirrors successively, the angle between the incident ray and the final reflected ray is twice the angle between the mirrors themselves.

This mathematical principle allows a sextant to measure angles up to 120 degrees using an arc that spans only 60 degrees,s creating a mechanical advantage that makes the instrument both compact and highly precise.

Key Components and How They Work Together

Frame: The sector-shaped backbone, typically brass or aluminum for durability and corrosion resistance.

Index Mirror: Attached to the movable index arm, reflects light from the celestial body. As you move the index arm along the arc, you adjust the angle of this mirror.

Horizon Mirror: A half-silvered mirror positioned perpendicular to the frame. The unsilvered half allows direct viewing of the horizon; the silvered half reflects the celestial body image from the index mirror.

Telescope: Magnifies the view for clearer sighting. When looking through the telescope, you see both the direct horizon and the reflected celestial body image simultaneously.

Arc and Vernier Scale: Marked in degrees (0-130) with minute subdivisions for precise angle readings.

Micrometer Drum: Enables fine adjustments of the index arm, with accuracy to 0.1 minutes of arc.

Shade Glasses: Protect your eyes when observing the sun, with multiple density options.

What You See When Using a Sextant

When you look through the sextant telescope at the horizon:

• The direct image shows the actual horizon through the clear portion of the horizon mirror

• The reflected image shows the celestial body, bounced from the index mirror to the silvered portion of the horizon mirror.

As you adjust the index arm, you control the angle between these two images until the celestial body appears to rest on the horizon line. This visual alignment indicates your precise measurement angle.

How to Operate a Sextant: Step-by-Step Instructions

Step 1: Check for Index Error

Before taking any observation, verify your sextant's index error, the difference between the instrument's optical zero and its graduated zero.

Procedure:

• Set the index arm to zero

• Hold the sextant vertically and look at the horizon

• The direct and reflected horizon images should align perfectly

• If they don't align, note the difference (either ON or OFF the arc)

• Apply this correction to all subsequent readings

Step 2: Select and Sight Your Celestial Body

Choose a celestial body appropriate for your position and time:

• Sun: Daytime observations (use shade glasses)

• Stars and planets: Twilight observations when both the horizon and stars are visible

• Moon: Can be observed day or night when above the horizon

Hold the sextant vertically and point it toward your selected body.

Step 3: Bring the Body to the Horizon

Move the index arm along the arc, watching through the telescope:

• The reflected image of the celestial body will move through the field of view

• Continue adjusting until the lower edge of the celestial body touches the horizon line

• Use the micrometer drum for fine adjustments (smallest visible increments)

Step 4: Rock the Sextant

Gently swing the sextant in a small arc around its vertical axis approximately ±5 degrees. The celestial body should appear to swing like a pendulum across the horizon.

Why this matters: The lowest point of the arc's swing gives the true altitude reading. This rocking motion compensates for your vessel's motion and ensures you're measuring the precise moment when the body touches the horizon.

Step 5: Record Your Observation

Note two critical values immediately:

1. Exact time to the nearest second (using a chronometer, not a wristwatch)

2. Sextant altitude from the arc and micrometer drum readings

Step 6: Apply Corrections

Raw sextant readings require several corrections before use in position calculations:

• Index error: Add or subtract as determined in Step 1

• Dip correction: Accounts for your height of eye above sea level (approximately -3 minutes per meter of height)

• Refraction correction: Atmospheric bending of light (approximately -34 minutes at horizon)

• Semi-diameter correction: When observing the sun or the moon (approximately +16 minutes for the sun)

These corrections yield Observed Altitude (Ho), which you then compare to Computed Altitude (Hc) to determine your position.

Why Sextants Remain Essential in Modern Maritime

Regulatory Requirements

The International Maritime Organization requires commercial vessels to carry working sextants and maintain trained personnel capable of celestial navigation. SOLAS regulations and USCG requirements explicitly mandate this backup navigation capability.

Vulnerability of Electronic Systems

GPS systems face documented vulnerability to:

• Jamming: Intentional signal disruption

• Spoofing: False GPS signals mimicking genuine satellites

• System failures: Equipment malfunction or software errors

• Cyberattacks: Deliberate interference with navigation systems

Maritime industry studies document increasing GPS interference incidents. In these situations, celestial navigation via sextant provides the only independent position verification method.

Accuracy Comparison

A skilled navigator using a sextant can achieve accuracy within 0.25 nautical miles (approximately 460 meters). While less precise than GPS (which provides meter-level accuracy), this accuracy is entirely sufficient for safe landfall and coastal navigation in situations where backup navigation is most critical.

Mastering Celestial Navigation

Learning to use a sextant connects you to centuries of maritime tradition while providing an independent, reliable navigation capability. Start by practicing observations on stable ground before attempting measurements on a pitching deck. Take multiple observations and average results for improved accuracy.

Quality equipment matters. Investin a well-maintained sextant and a current nautical almanac. Consider formal celestial navigation training, which the U.S. Coast Guard requires for officers in charge of navigational watch on vessels 500 gross tons or larger.

American Nautical Services provides comprehensive navigation resources , including sextants, nautical almanacs, sight reduction tables, and celestial navigation guides, supporting mariners committed to this timeless skill.

FAQs

Q: What is a sextant used for in modern navigation?

A: A sextant serves as the primary backup navigation tool on commercial vessels. When GPS fails due to equipment malfunction, power loss, signal interference, or cyberattack, navigators determine position using celestial observations. Many offshore sailors maintain sextant skills for long ocean passages.

Q: How accurate is sextant navigation compared to GPS?

A: A highly skilled navigator can achieve accuracy within 0.25 nautical miles using a sextant. GPS typically provides accuracy within several meters. While sextant navigation is less precise, the difference is negligible for safe passage and coastal navigation.

Q: Do I need special training to use a sextant?

A: While basic sextant operation can be learned independently, formal training improves accuracy and confidence significantly. Navigation courses teach sight reduction methods, proper correction procedures, and position calculation techniques that require considerable practice to master.

Q: Are sextants still required on ships?

A: Yes. International maritime regulations require commercial vessels to carry functional sextants and maintain crew members trained in celestial navigation. Recreational sailors making ocean crossings are advised to carry a sextant as emergency backup equipment.

Related Resources

• Navigation Kits and Instruments

• Nautical Almanac and Tables

• SOLAS Navigation Requirements

• Maritime Navigation Fundamentals

American Nautical Services – Supporting celestial navigation education and maritime excellence since 1977. Official provider of sextants, nautical instruments, and navigation publications for professional mariners and maritime students.