Introduction of Sextant


Introduction of Sextant

The sextant is a tool that has been instrumental in navigation and geography for centuries, revolutionizing the way sailors and explorers determined their position on the open seas. It has played a crucial role in shaping world history and has since evolved into a vital instrument in various fields, including aviation, surveying, and space exploration. In this article, we will delve into the origins and evolution of the sextant, its mechanism and use, and its continued relevance in today’s modern world. So, let us embark on a journey of discovery and explore the introduction of the sextant.

Optical Requirements of The Sextant

Optical Requirements of The Sextant

The sextant is a traditional navigation instrument that has been used for centuries by sailors and navigators to determine their position at sea. It works by measuring the angle between the horizon and a celestial object, such as the sun or a star.

In order for the sextant to accurately measure these angles, it must meet certain optical requirements. These requirements ensure that the instrument is precise and reliable, allowing for accurate navigation.

1. Clear and Distortion-Free Mirrors: The sextant uses two mirrors to reflect light and create the angle measurement. These mirrors must be perfectly flat and polished to avoid any distortion in the reflected image. Any imperfections or distortions in the mirrors can result in inaccurate readings.

2. High-Quality Index Glass: The index glass is a small prism that is used to split the light beam in two, allowing the user to view both the horizon and celestial object at the same time. The glass must have a high refractive index, which means it bends light more than regular glass, in order to split the light beam accurately.

3. Precise Vernier Scale: The sextant has a scale on which the angle measurement is read. This scale is made up of small divisions called the vernier scale. The scale must be precisely engraved or etched to ensure accurate readings.

4. Properly Aligned Telescopes: The sextant has two telescopes, one for viewing the horizon and one for viewing the celestial object. These telescopes must be accurately aligned with the mirrors and the index glass to avoid any parallax error, which can lead to inaccurate readings.

5. Focusing Mechanism: The sextant has a focusing mechanism in both the telescopes to allow for a clear and sharp image. This is especially important when viewing celestial objects, which are usually very small and far away.

6. Sturdy and Durable Construction: The sextant is a delicate instrument that needs to be handled with care. It must be made of high-quality materials and have a sturdy construction to withstand the vibrations and movements of a ship at sea.

In conclusion, the optical requirements of the sextant are essential for accurate navigation. Any deviations or imperfections in these requirements can result in erroneous readings, which can have serious consequences for the safety of the navigator and their vessel. Therefore, it is crucial for the sextant to meet these requirements to ensure reliable and precise navigation.

Types of Sextant

Types of Sextant

A sextant is a navigation instrument used to measure the angle between any two visible objects. It is commonly used in marine and aviation navigation to determine the position of the vessel or aircraft. There are different types of sextants based on their design, purpose, and accuracy. Here are the three main types of sextants:

1. Traditional Sextant: The traditional sextant, also known as the marine sextant, is the most commonly used type. It consists of a rigid metal frame with a graduated scale, a telescopic sight for sighting the object, and two mirrors. One mirror is half-silvered, which reflects the image of the celestial body, and the other mirror is fully reflective, which reflects the image of the horizon. The angle between the two mirrors when the objects are aligned is read on the scale, providing the angle of elevation or depression of the celestial object.

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2. Bubble Sextant: A bubble sextant is a modern version of the traditional sextant. It uses a bubble-level to determine the horizon instead of the reflected horizon in a traditional sextant. It is more accurate and reliable than the traditional sextant since the angle of the bubble is not affected by the sea waves. It is mainly used for aviation purposes, where a stable determination of the horizon is necessary.

3. Octant: An octant is an early version of the sextant, which was first used in the 18th century. It consists of a brass frame with a 45-degree angle scale, a sighting telescope, and two mirrors. Unlike the traditional sextant, it uses an octant-shaped frame, which makes it more compact and portable. However, the octant has limited accuracy and is not commonly used today.

In addition to these main types of sextants, there are also specialized sextants for specific purposes, such as the artificial horizon sextant, which measures the angle between the sun and a man-made horizon, and the digital sextant, which displays the angle electronically instead of using a scale. These specialized sextants are mainly used for research purposes or in extreme conditions where traditional sextants may not be accurate.

In conclusion, there are different types of sextants, each with its unique features and purposes. While the traditional sextant is commonly used for marine navigation, the bubble sextant and octant have their own advantages and uses. With the advancement of technology, new types of sextants are being developed, making navigation more accurate and efficient.

Measurement of Horizontal Angle with Box Sextant

Measurement of Horizontal Angle with Box Sextant

The measurement of horizontal angle is a crucial aspect in the field of civil engineering. It is used to determine the direction of a line or object in relation to a reference point. This measurement is important in various tasks such as surveying, construction layout, and navigation.

One of the tools commonly used by civil engineers to measure horizontal angles is the box sextant. This instrument is a precision optical device that uses the principle of a sextant to measure the angle between two objects. The box sextant consists of a frame or box with two mirrors and a sighting mechanism.

The measurement of horizontal angles with a box sextant follows a simple principle. It involves aligning the instrument with the desired object and the reference point, and then reading the angle from the scale on the instrument. The process of taking horizontal angle measurement with a box sextant can be broken down into the following steps:

1. Mounting the instrument: The first step is to mount the box sextant on a stable and level surface. This ensures accurate readings and prevents any errors due to instrument tilt.

2. Aligning the mirrors: The box sextant has two mirrors, the index mirror, and the horizon mirror. The horizon mirror should be adjusted to align with the horizontal plane, while the index mirror should be set at a 45-degree angle to the horizon.

3. Sighting the reference point: The engineer should locate the reference point and align the horizon mirror with it. The horizon mirror has a small hole in the center, which is used to sight the object or reference point.

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4. Sighting the object: The next step is to align the index mirror with the desired object. The two mirrors should appear as a single object in the index mirror when the instrument is correctly aligned.

5. Reading the angle: Once the instrument is correctly aligned, the engineer can read the angle from the scale on the instrument. The scale is marked in degrees, and the reading can be taken from the point where the line of sight intersects the scale.

6. Recording the measurement: It is essential to record the measurement accurately. This includes noting down the angle as well as the direction of the object in relation to the reference point.

The accuracy of the horizontal angle measurement with a box sextant depends on various factors such as the stability of the instrument, the skill of the operator, and environmental conditions. To ensure the utmost precision, it is crucial to take multiple readings and average them out.

In conclusion, the measurement of horizontal angle with a box sextant is a simple yet important aspect of civil engineering. It provides a quick and accurate way to determine the direction of a line or object and plays a critical role in various engineering tasks. With proper training and practice, civil engineers can use this instrument to obtain precise horizontal angle measurements for their projects.

Measurement of Vertical Angle with Sextant

Measurement of Vertical Angle with Sextant

Measurement of vertical angle with a sextant is an important technique used in surveying and navigation. A vertical angle is the angle between a horizontal line and a line of sight to an object located above or below the observer. It is used to determine the height or elevation of an object or to measure the inclination of a slope.

A sextant is a navigational instrument that is used to measure the angle between two objects, such as the horizon and a celestial body, to determine a ship’s position at sea. It consists of a graduated arc or scale, a mirror, two sighting arms, and an index arm. The scale is marked in degrees, minutes, and seconds, and can measure angles up to 120 degrees.

To measure a vertical angle with a sextant, the instrument is held vertically with the sighting arms aligned with the object being measured. The horizon is then sighted through the horizon mirror and the object is sighted through the index mirror. The index arm is then moved until the two images in the mirrors are aligned. The angle is then read from the scale on the arc.

One important factor to consider when using a sextant for measuring vertical angles is correction for sextant index error. The index error is the difference between the reading on the arc and the true angle. This error can be caused by instrument imperfections or incorrect alignment of the mirrors. To correct for this, the observer must measure the angle twice, once with the index arm set at zero and then with the index arm set at 180 degrees. The average of the two readings is taken as the correct angle.

Another factor to consider is the dip of the horizon. The dip of the horizon is the angle between the true horizontal and the horizontal line of sight caused by the observer’s eye being above sea level. This can be corrected for by subtracting the dip angle from the measured vertical angle.

In addition, the observer must also take into account the refraction of light, which causes an apparent shift in the position of an object when viewed through the atmosphere. This affects the accuracy of the measurements, especially when measuring small angles.

Measurement of vertical angles with a sextant is often used in marine navigation and land surveying. In navigation, it is used to determine the height of objects such as lighthouses and landmarks to aid in ship navigation. In surveying, it is used to measure the elevation of specific points on the ground or to determine the slope of a terrain.

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In conclusion, measurement of vertical angle with a sextant is a useful and accurate technique for determining the height and slope of objects. While it requires knowledge of various correction factors and careful observation, it remains an important tool in the fields of navigation and surveying.

Permanent Adjustment of a Sextant

Permanent Adjustment of a Sextant

A sextant is a navigational instrument that is used to measure the angle between two objects, typically the horizon and a celestial body like the sun or stars. These measurements are crucial for determining the position of a vessel or an aircraft. However, due to various factors, such as temperature changes and wear and tear, the accuracy of a sextant can be affected. To ensure precise and reliable measurements, a process known as permanent adjustment is carried out on a sextant.

Permanent adjustment is a process that involves making fine-tuned corrections to a sextant to eliminate any errors and ensure accurate measurements. This adjustment is usually done by trained professionals and requires specialized tools and equipment.

The first step in permanent adjustment is to check the condition of the sextant and ensure that all the parts are functioning correctly. The mirrors, index arm, and micrometer drum, which are essential components of a sextant, must be in good condition and properly aligned. Any signs of rust, dust, or damage must be addressed before proceeding with the adjustment.

The next step is to calibrate the sextant. This involves checking the accuracy of the instrument by comparing it to a known reference, such as a precision angle calibrator. The angle of reflection between the mirrors and the index arm is measured, and any discrepancies are noted. This helps determine the extent of adjustment needed.

The main part of permanent adjustment involves adjusting the mechanical components of the sextant, such as the horizon glass, micrometer drum, index mirror, and index arm. These adjustments are made using specialized tools, including a sextant collimator, micrometer reading rod, and pitch and yaw test tools. The goal is to align all the components precisely, making sure that the angles measured are as accurate as possible.

Temperature changes can also affect the accuracy of a sextant. To overcome this, a thermal test is conducted where the instrument is exposed to different temperatures while still mounted on a precision angle calibrator. If the readings are consistent despite the temperature changes, then the sextant is deemed successfully adjusted.

Once the permanent adjustment is completed, the sextant is subjected to a final evaluation to ensure that all the necessary adjustments have been made. The instrument is then certified as accurate and ready for use.

In conclusion, permanent adjustment is a crucial process to ensure the reliability and accuracy of a sextant. It involves a series of checks and adjustments to correct any errors and ensure precise measurements. A properly adjusted sextant is vital for navigational purposes, making permanent adjustment an essential aspect of a civil engineer’s job.


In conclusion, the introduction of the sextant revolutionized navigation by providing a more accurate and efficient way to determine a ship’s position at sea. This intricate instrument has a long history, dating back to ancient civilizations, and has been continuously improved upon over time. Its use has helped to facilitate global exploration and trade, leading to advancements in technology and scientific knowledge. While advancements in technology have made the sextant less commonly used today, its legacy lives on and continues to be appreciated by navigators, historians, and enthusiasts alike. The sextant remains a symbol of human ingenuity and determination to navigate the vast and often unpredictable oceans with precision and skill.


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