Mastering the Sextant: A Comprehensive Guide to Celestial Navigation

The sextant, a timeless instrument steeped in history, remains a vital tool for celestial navigation, especially in situations where modern electronic aids may fail. This guide provides a comprehensive, step-by-step approach to understanding and using a sextant effectively. Whether you’re a seasoned sailor looking to enhance your traditional navigation skills or a novice fascinated by the art of celestial navigation, this article will equip you with the knowledge to accurately determine your position using the stars, sun, moon, and planets.

I. Understanding the Sextant

A. Components of a Sextant

Before delving into the practical aspects of using a sextant, it’s crucial to familiarize yourself with its key components:

1. Frame: The structural foundation of the sextant, providing rigidity and stability.
2. Limb: The graduated arc that measures angles, typically marked in degrees and minutes of arc. It ranges from 0° to slightly beyond 120° allowing you to measure altitudes up to and slightly beyond 120° which is needed for celestial navigation.
3. Index Arm: A movable arm pivoted at the center of the sextant that sweeps along the limb. It carries the index mirror and a micrometer drum or vernier scale.
4. Index Mirror: A mirror fixed to the index arm that reflects the image of the celestial body. Its position is precisely controlled by the index arm.
5. Horizon Mirror: A half-silvered mirror fixed to the frame. It allows the observer to see both the celestial body (reflected by the index mirror) and the horizon simultaneously. Some horizon mirrors are adjustable for side error.
6. Telescope: Used to magnify the celestial body and the horizon, enhancing accuracy. Sextants often come with interchangeable telescopes for different viewing conditions.
7. Shades (Filters): Used to reduce the glare of the sun or moon, protecting the observer’s eyes. These are typically different densities and can be combined.
8. Micrometer Drum/Vernier Scale: Used for fine adjustments and precise angle readings. The micrometer drum typically displays minutes of arc, while the vernier scale provides even finer readings, down to tenths of a minute.
9. Clamp Screw: Used to lock the index arm in place for taking a reading.
10. Release Button: Used to allow the index arm to move freely along the limb.

B. Principles of Operation

The sextant operates based on the principle of double reflection. The celestial body’s light is reflected first by the index mirror and then by the horizon mirror. The observer adjusts the index arm until the reflected image of the celestial body appears to touch the horizon. The angle between the celestial body and the horizon (the altitude) is then read from the limb using the index arm’s position.

The sextant measures the angle between two objects. In celestial navigation, one of those objects is typically the visible horizon, and the other is a celestial body (Sun, Moon, star, or planet). The measured angle, after corrections, is called the altitude, or more precisely, the observed altitude (Ho).

II. Preparing for Sextant Use

A. Sextant Calibration and Error Correction

Before using a sextant, it’s essential to calibrate it and correct for any inherent errors. Common errors include:

1. Index Error: This occurs when the index mirror and horizon mirror are not perfectly perpendicular to the frame when the index arm is set to zero. To determine index error, sight the horizon or a distant object. Adjust the micrometer drum until the reflected image perfectly aligns with the direct image. The reading on the micrometer drum is the index error. Note whether it’s ‘on the arc’ (positive) or ‘off the arc’ (negative). Apply this correction to all subsequent sextant readings. Modern sextants use a small screw to adjust the horizon mirror and reduce the index error to near zero.
2. Side Error: This occurs when the horizon mirror is not perfectly parallel to the frame. It’s best corrected by adjusting a screw on the horizon mirror until the reflected and direct images of the horizon are perfectly aligned when viewing a vertical line, such as a mast or distant tower. If a visible side error is present, take sights on celestial bodies that are high in altitude.
3. Collimation Error: This occurs when the telescope is not properly aligned with the frame. This error is more difficult to correct and often requires professional adjustment. It’s generally less significant than index or side error.
4. Prismatic Error: This occurs when the shades (filters) are not perfectly parallel. This can distort the image and introduce errors. Check the shades by viewing a distant straight line (like the horizon) through each shade individually. If the line appears broken or distorted, the shade has a prismatic error. Replace the shade if necessary.

Document your sextant’s index error and apply it to all observations. Regularly check the sextant’s calibration, especially after rough handling.

B. Essential Tools and Resources

In addition to the sextant, you’ll need the following tools and resources:

1. Nautical Almanac: Provides the Greenwich Hour Angle (GHA) and declination of celestial bodies for each day and hour. Critical for determining your position. Modern nautical almanacs also include sight reduction tables.
2. Sight Reduction Tables (e.g., Pub. No. 229): Simplifies the calculations involved in determining your position. These tables provide pre-calculated altitudes and azimuths based on assumed positions.
3. Timepiece: An accurate timepiece (chronometer or accurate watch) is essential for determining the Greenwich Mean Time (GMT) of your observations. Even a few seconds of error can result in significant positional errors. Keep the timepiece synchronized with UTC/GMT.
4. Navigation Tables: Provide various conversion factors and mathematical functions used in navigation calculations.
5. Pencils, Paper, and Calculator: For recording observations and performing calculations.
6. Horizon: A clear, unobstructed view of the horizon is crucial for accurate sextant observations. If a natural horizon is not available, an artificial horizon (a tray of reflective liquid) can be used.

C. Understanding Celestial Coordinates

To use a sextant effectively, you need to understand celestial coordinates:

1. Greenwich Hour Angle (GHA): The angular distance, measured westward, from the Greenwich meridian to the celestial body’s meridian.
2. Declination (Dec): The angular distance of a celestial body north or south of the celestial equator.
3. Local Hour Angle (LHA): The angular distance, measured westward, from the observer’s meridian to the celestial body’s meridian. LHA = GHA + Longitude (West Longitude is added, East Longitude is subtracted).
4. Altitude (Alt): The angular distance of a celestial body above the horizon. This is what the sextant measures (after corrections).
5. Azimuth (Zn): The horizontal direction of a celestial body from the observer, measured clockwise from North.

III. Taking a Sextant Sight: Step-by-Step

A. Preparing for the Observation

1. Select a Celestial Body: Choose a bright star, planet, the sun, or the moon. The sun is the easiest to observe during the day. Stars are best observed at twilight.
2. Determine the Approximate Time of Observation: Use the Nautical Almanac to plan your observation for a time when the celestial body will be well above the horizon (ideally between 20° and 60° altitude) and not too close to the meridian. Sights taken near the meridian require very accurate time keeping and are more sensitive to small errors.
3. Record the Time: Note the exact time (GMT/UTC) of the observation to the nearest second. Accuracy is paramount.
4. Estimate Your Position (Dead Reckoning): Use your last known position and course/speed information to estimate your current latitude and longitude. This estimated position will be used in sight reduction calculations.

B. Performing the Sextant Observation

1. Adjust the Sextant: Set the index arm to zero and check for index error. Apply any necessary corrections. Select the appropriate shade(s) to protect your eyes, especially when observing the sun.
2. Find the Celestial Body: Using the sextant telescope, locate the celestial body. You may need to pre-set the index arm to an approximate altitude based on your estimated position and the Nautical Almanac.
3. Bring the Celestial Body to the Horizon: Hold the sextant vertically and sweep the index arm until the reflected image of the celestial body appears in the horizon mirror. Gently rock the sextant back and forth to ensure you’re sighting the lowest point of the body touching the horizon.
4. Make Fine Adjustments: Use the micrometer drum to make fine adjustments until the bottom edge of the sun (lower limb) or the center of a star or planet appears to just touch the horizon. For the sun or moon, you’ll typically observe the lower limb to avoid looking directly at the brightest part of the body.
5. Take the Reading: Once you’re satisfied with the alignment, clamp the index arm and carefully read the angle from the limb and micrometer drum. Record this observed altitude (Hs).
6. Repeat the Observation: Take several sights in quick succession (3-5 sights) to minimize random errors. Average the observed altitudes (Hs) to obtain a more accurate reading.

C. Applying Corrections to the Observed Altitude (Hs)

The observed altitude (Hs) must be corrected to obtain the true altitude (Ho). These corrections account for various factors that affect the accuracy of the observation.

1. Index Error Correction: Apply the index error correction, as determined during sextant calibration. If the index error is ‘on the arc’ (positive), add it to Hs. If it’s ‘off the arc’ (negative), subtract it.
2. Dip Correction: This correction accounts for the height of the observer’s eye above sea level. The higher your eye, the further away the horizon appears, resulting in a slightly lower observed altitude. Use a dip correction table (found in the Nautical Almanac) based on your height of eye. Always subtract the dip correction from Hs.
3. Refraction Correction: This correction accounts for the bending of light as it passes through the Earth’s atmosphere. Atmospheric refraction causes celestial bodies to appear slightly higher than they actually are. Use a refraction correction table (found in the Nautical Almanac) based on the observed altitude. Always subtract the refraction correction from Hs.
4. Parallax Correction: This correction accounts for the difference in the observer’s position on the Earth’s surface compared to the Earth’s center. Parallax is significant for the Moon, and smaller for the Sun and planets. It’s negligible for stars. Use a parallax correction table (found in the Nautical Almanac) based on the celestial body’s horizontal parallax and observed altitude. Add the parallax correction to Hs.
5. Semi-diameter Correction: When observing the Sun or Moon, you typically observe the lower limb (bottom edge). This correction accounts for the angular radius of the Sun or Moon to determine the altitude of its center. Use a semi-diameter correction value (found in the Nautical Almanac) based on the date. Add the semi-diameter correction to Hs for the Sun’s lower limb. For the Moon, the semi-diameter correction and horizontal parallax corrections are often combined into a single correction.

After applying all corrections, you’ll obtain the true altitude (Ho).

IV. Determining Your Position: Sight Reduction

A. Assumed Position (AP)

Sight reduction involves comparing the observed altitude (Ho) with a calculated altitude (Hc). The calculated altitude is based on an assumed position (AP) close to your estimated (dead reckoning) position. Choose an AP with whole degrees of latitude and longitude to simplify calculations.

B. Calculating the Calculated Altitude (Hc) and Azimuth (Zn)

Use Sight Reduction Tables (e.g., Pub. No. 229) to determine the calculated altitude (Hc) and azimuth (Zn). These tables require the following inputs:

1. Latitude of the Assumed Position (AP Lat):
2. Local Hour Angle (LHA): Calculate LHA using the formula: LHA = GHA + Longitude (West Longitude is added, East Longitude is subtracted). Remember that GHA is obtained from the Nautical Almanac, corrected for the time of the observation.
3. Declination (Dec): Obtain the declination of the celestial body from the Nautical Almanac, corrected for the time of the observation.

Enter the Sight Reduction Table with these values. The table will provide the calculated altitude (Hc), the azimuth angle (Z), and a difference value (d) that may need to be applied to Hc. The azimuth angle (Z) needs to be converted to true azimuth (Zn) using the following rules:

• If AP Lat is North and LHA is less than 180°, Zn = Z
• If AP Lat is North and LHA is greater than 180°, Zn = 360° – Z
• If AP Lat is South and LHA is less than 180°, Zn = 180° – Z
• If AP Lat is South and LHA is greater than 180°, Zn = 180° + Z

C. Calculating the Altitude Intercept (a)

The altitude intercept (a) is the difference between the observed altitude (Ho) and the calculated altitude (Hc):

a = Ho – Hc

The altitude intercept is measured in nautical miles. If Ho is greater than Hc, the intercept is plotted towards the celestial body (towards Zn). If Ho is less than Hc, the intercept is plotted away from the celestial body (opposite Zn).

D. Plotting the Line of Position (LOP)

The line of position (LOP) is a line on which the observer is located. To plot the LOP:

1. Plot the Assumed Position (AP) on a chart.
2. Draw a line from the AP in the direction of the azimuth (Zn).
3. Measure the altitude intercept (a) along the azimuth line. If Ho is greater than Hc, measure the intercept towards the celestial body. If Ho is less than Hc, measure the intercept away from the celestial body.
4. Draw a line perpendicular to the azimuth line at the end of the intercept. This is the line of position (LOP).

E. Determining Your Position

A single LOP indicates a line on which you are located. To determine your exact position, you need at least two LOPs. The intersection of two LOPs gives you a fix (your position). If you have three or more LOPs, they will ideally intersect at a single point. However, due to observational errors, they may form a small triangle. In this case, the center of the triangle is taken as your best estimate of your position.

When taking multiple sights, it’s important to account for the vessel’s movement between observations. Advance the LOPs to a common time by moving them along the course line a distance equal to the distance traveled in the time interval. The course line should be the course made good during the time interval.

V. Tips for Improving Accuracy

• Practice Regularly: Proficiency with a sextant requires practice. Regularly take sights, even when you have access to electronic navigation aids.
• Maintain Your Sextant: Keep your sextant clean and properly calibrated. Store it in a protective case when not in use.
• Use a Stable Platform: A stable platform is essential for accurate observations. On a small boat, try to take sights when the boat is relatively steady.
• Minimize Errors: Be meticulous in your observations and calculations. Double-check your work to avoid mistakes.
• Use Averaging Techniques: Take multiple sights and average the results to reduce random errors.
• Consider Atmospheric Conditions: Atmospheric conditions can affect the accuracy of sextant observations. Avoid taking sights in hazy or unstable conditions.
• Learn from Experienced Navigators: Seek guidance from experienced celestial navigators. Attend workshops or courses to improve your skills.
• Utilize Software and Online Resources: Many software programs and online resources can assist with sight reduction calculations and other aspects of celestial navigation. These tools can help verify your manual calculations and improve your understanding of the process.
• Understand the Limitations: Celestial navigation is not a perfect science. Be aware of the limitations of the method and use it in conjunction with other navigation techniques.

VI. Advanced Techniques

A. Star Identification

Identifying stars can be challenging, especially at twilight. Star finders and star charts can be helpful in locating and identifying stars. Practice identifying bright stars to improve your skills.

B. Lunar Observations

Lunar observations can be used for celestial navigation, but they are more complex than solar or stellar observations due to the Moon’s rapid motion and relatively large parallax. The Nautical Almanac provides detailed information for lunar observations.

C. Artificial Horizon

When a natural horizon is not available (e.g., due to fog or land obstruction), an artificial horizon can be used. An artificial horizon is typically a tray filled with a dark, reflective liquid (such as oil or mercury – though mercury is generally avoided due to its toxicity). The angle measured with an artificial horizon is double the altitude, so you must divide the observed angle by two before applying corrections.

VII. The Enduring Value of Celestial Navigation

In an age of GPS and electronic navigation, celestial navigation may seem like an antiquated skill. However, it remains a valuable tool for sailors and navigators for several reasons:

• Redundancy: Celestial navigation provides a backup navigation method in case of electronic failures.
• Independence: Celestial navigation does not rely on external signals or infrastructure, making it immune to jamming or other disruptions.
• Self-Reliance: Mastering celestial navigation fosters self-reliance and a deeper understanding of navigation principles.
• Historical Significance: Celestial navigation is a skill with a rich history, connecting navigators to a long tradition of exploration and discovery.

By mastering the sextant and the principles of celestial navigation, you can enhance your navigation skills, increase your self-reliance, and gain a deeper appreciation for the art and science of navigating by the stars.

This guide provides a solid foundation for learning celestial navigation. Further study and practice are essential for developing proficiency. Consult additional resources, such as textbooks, online courses, and experienced navigators, to continue your learning journey. Happy navigating!