Introduction of Sextant

Introduction of Sextant

The sextant is an ancient navigation tool that has played a critical role in shaping the history of seafaring. For centuries, this precision instrument has guided sailors across the open waters, allowing them to determine their position and plot their course with incredible accuracy. However, as modern technology has advanced, the use of the sextant has become less prevalent. In this article, we will explore the origin and evolution of the sextant, its importance in navigation, and how it continues to be used today. Whether you are a seasoned sailor or simply curious about this fascinating instrument, join us as we dive into the introduction of the sextant.

Optical Requirements of The Sextant

Optical Requirements of The Sextant

The sextant is an important navigational instrument used for determining the angle between two celestial objects, such as the sun and the horizon. It is mainly used in marine navigation and surveying applications. In order for the sextant to accurately measure these angles, it is essential to meet certain optical requirements.

1. Clear and undistorted optics: The sextant must have a clear and undistorted optical system to accurately measure the angles between celestial bodies. Any distortions or imperfections in the optical system can result in erroneous readings.

2. High precision mirrors: The primary optical components of a sextant are the two mirrors – index mirror and horizon mirror. These mirrors must be of high precision and quality to reflect the light accurately and without any distortion.

3. Parallelism of mirrors: The index and horizon mirrors must be parallel to each other to reflect the light rays in the same direction. Any deviation from parallelism can result in measurement errors.

4. Proper alignment of mirrors: The mirrors must also be properly aligned with the body of the sextant to ensure correct measurements. Misalignment of mirrors can lead to errors in angle measurements.

5. Optimum size and shape of mirrors: The size and shape of the mirrors play a crucial role in the accuracy of the sextant. The index mirror should have a larger surface area than the horizon mirror to provide a clearer view of the celestial objects.

6. Coatings on mirrors: The mirrors of the sextant are typically coated with a thin layer of aluminum or silver to enhance their reflectivity and reduce light absorption. This ensures that maximum light is reflected, providing accurate angle measurements.

7. Correctly calibrated scale: The sextant has a scale that is used to measure the angles between the celestial bodies. The scale must be precisely calibrated to ensure accurate readings.

8. Clear and legible scale: The scale on the sextant must be clear and legible to read the angles accurately. The divisions on the scale should be well-defined, and the numbering should be clear and easily readable.

9. Anti-reflective coatings on lenses: The lenses on the sextant should be coated with an anti-reflective material to reduce glare and ensure clear visibility of the celestial objects.

10. Protection from external factors: The sextant must be protected from external factors such as dust, moisture, and extreme temperatures. These factors can affect the optics of the sextant, leading to errors in angle measurements.

In conclusion, the sextant requires precise and well-maintained optics to accurately measure angles between celestial objects. Any deviation from the specified optical requirements can lead to errors in measurements, which can be detrimental in navigation and surveying applications. Therefore, regular maintenance and calibration of the sextant is essential to ensure reliable and accurate readings.

Types of Sextant

Types of Sextant

A sextant is a navigational instrument used to measure the angle between two objects, typically the horizon and a celestial body, such as the sun or stars. It has been used for centuries to determine the positioning of ships at sea, as well as for land surveying and astronomical observations.

The modern sextant was invented in the late 18th century and has gone through various advancements and modifications over time. Today, there are several types of sextants used for different purposes. Let’s take a closer look at the different types of sextants.

1. Traditional or Mariner’s Sextant
The traditional or mariner’s sextant is the most commonly known type of sextant, used by sailors and navigators since its invention. It consists of a metal frame with a graduated arc, mirrors, and a telescope. The arc is calibrated in degrees, while the mirrors are used to reflect the light from the celestial body to the observer’s eye. This type of sextant is still in use today, especially by sea captains and sailors, as it does not depend on electricity or battery power.

2. Octant
The octant is a type of sextant that was in use before the traditional sextant was invented. It is smaller and lighter than the traditional sextant and has an arc of 45 degrees, making it suitable for taking measurements at higher angles. However, the octant is not as accurate as the traditional sextant and has been replaced by it in modern times.

3. Artificial Horizon Sextant
An artificial horizon sextant, also known as a gyro sextant, is used to measure the angle between two objects without relying on the horizon. It consists of a gyroscopic stabilizer that maintains a level frame of reference, allowing for accurate celestial observations even on a rolling or pitching vessel. This type of sextant is commonly used by aircraft pilots, as well as for land surveying.

4. Bubble Sextant
A bubble sextant is a lightweight and portable version of the traditional sextant with a built-in spirit level for taking measurements. It is commonly used by small aircraft or yachtsmen who may not have enough space for a traditional sextant. However, it is not as accurate as the traditional sextant and is mostly used as a backup.

5. Electronic Sextant
The electronic sextant is a modern version of the traditional sextant that uses electronic sensors and a display to measure angles. It is widely used in commercial ships due to its accuracy and ease of use. However, it is not commonly used in small boats or aircraft due to its high cost and reliance on battery power.

In conclusion, sextants come in different types and variations to suit various needs and purposes. While the traditional sextant remains a reliable and widely used instrument, advancements in technology have led to the development of more accurate and convenient types of sextants. Regardless of the type, sextants play a crucial role in navigation and are an essential tool for any sailor or navigator.

Measurement of Horizontal Angle with Box Sextant

Measurement of Horizontal Angle with Box Sextant

Box sextant is a precision instrument used to measure horizontal angles. It is commonly used in civil engineering and surveying applications to determine the position of objects relative to the horizon.

The instrument consists of a telescope mounted on a graduated arc with a reflecting prism at one end. The prism reflects the image of a celestial body, usually the Sun or a star, and directs it towards the observer’s eye through the telescope. The graduated arc can be rotated to align the prism with the object being measured.

To measure the horizontal angle, the observer first aligns the telescope and the prism with the object of interest. Then, using the micrometer knob, the observer adjusts the arc to bisect the image of the object in the telescope. The reading on the graduated arc gives the horizontal angle between the object and the observer’s position.

The accuracy of the measurement is mainly dependent on three factors – the quality of the instrument, the skill of the observer, and the atmospheric conditions. The instrument must be properly calibrated, and the readings should be taken by skilled observers to minimize error. Additionally, adverse weather conditions such as mist, fog, or haze can affect the accuracy of the measurement.

Box sextants are often used to measure horizontal angles in highway and railway construction, as well as in land surveying for topographical mapping. They are also used in ship navigation for determining the vessel’s position on a chart.

One of the advantages of using a box sextant is its portability and ease of use. It can be taken to remote locations, and its simple design makes it relatively easy to operate. However, it also has some limitations, such as the need for a clear line of sight to the object, and the fact that it can only measure angles up to 180 degrees.

In conclusion, the measurement of horizontal angles with a box sextant is an essential tool for civil engineers to determine the position of objects in relation to the horizontal plane. With proper calibration and skilled observation, this instrument can provide accurate results and aid in various engineering and surveying applications.

Measurement of Vertical Angle with Sextant

Measurement of Vertical Angle with Sextant

The measurement of vertical angle with a sextant is an essential tool for civil engineers to determine heights and elevations of various points in engineering surveys and construction projects. It is a common method used in geodetic and topographic surveys to accurately determine the vertical angles between two points.

A sextant is a navigational instrument that is traditionally used by sailors to determine their position by measuring the angles between celestial objects and the horizon. In civil engineering, the sextant is used to measure the vertical angles between points on the ground and the horizon, typically either the natural horizon or a man-made one.

The process of measuring vertical angles with a sextant involves aligning the instrument’s telescope with the two points of interest and taking readings of the angle between them. The sextant works by using an index mirror and a horizon glass to reflect the readings of the angles between the two points.

Before taking any measurements, it is crucial to ensure that the sextant is correctly calibrated and aligned with the horizon. The instrument should be placed on a stable surface, and the horizon should be as flat and clear as possible for accurate readings.

To start the measurement, the observer will use the index arm to align the sextant with the upper point and the horizon. Once the instrument is aligned, the degree of the angle will be read on the sextant’s scale. Next, the lower point will be aligned with the horizon, and the second reading will be taken.

The vertical angle can then be calculated by subtracting the two readings. It is important to take multiple measurements and calculate the average to ensure accuracy.

There are two different methods for measuring vertical angles with a sextant – the direct and the reflected method. In the direct method, the observer looks directly through the eyepiece to take the readings, while in the reflected method, the readings are taken by sighting the reflected images of the two points on the horizon.

The accuracy of the vertical angle measurement with a sextant depends on various factors, such as the instrument’s quality, the observer’s skills, and the atmospheric conditions. To improve accuracy, it is essential to reduce external factors such as wind, vibration, and temperature variations during the measurement process.

In conclusion, the measurement of vertical angles with a sextant is an important technique in civil engineering, and it requires precision and accuracy to obtain reliable results. It is a valuable tool for determining site elevations, slope angles, and for creating accurate topographic maps, making it an essential instrument for any civil engineer.

Permanent Adjustment of a Sextant

Permanent Adjustment of a Sextant

A sextant is an essential tool used by civil engineers for surveying and navigation purposes. It is a hand-held instrument that measures angles between two distant points, such as the horizon and a celestial object, to determine the position and angle of a fixed object in relation to the observer’s eye. To ensure accurate and precise measurements, a sextant needs to be properly adjusted or calibrated before use. This process is known as permanent adjustment of a sextant.

Permanent adjustment is a one-time process that is carried out during the manufacture of the sextant and can also be done by a trained technician. It involves setting the instrument’s mirrors and lenses at a proper angle and position, ensuring that they are parallel and perpendicular to each other. This alignment is crucial as any error in the instrument’s optical system can lead to inaccurate measurements.

The following are the steps involved in the permanent adjustment of a sextant:

1. Vertical adjustment: The first step is to ensure that the telescope and horizon mirror are set at the same horizontal plane. This is done by placing the sextant on a flat surface and aligning the mirrors by adjusting the screws provided.

2. Horizontal adjustment: The next step is to check the horizontal alignment of the index and horizon mirrors. This is done by rotating the instrument and observing a distant object through the telescope. If the image shifts, the mirrors need to be adjusted using the screws provided until the image remains stationary.

3. Collimation adjustment: This step involves making sure that the optical axis of the sextant is in line with the mechanical axis. This is done by rotating the instrument and observing a distant object through the telescope. Any deviation from the center of the reticle signifies a misalignment that can be corrected by adjusting the screws provided.

4. Index mirror adjustment: The index mirror is used to reflect the image of the celestial object into the telescope. It needs to be perpendicular to the horizon mirror for accurate measurements. This is done by rotating the instrument and adjusting the index mirror until the intersection point of the reflected image and the horizon mirror is at the center of the field of view.

5. Horizon mirror adjustment: The horizon mirror is used to reflect the image of the horizon into the telescope. It needs to be parallel to the index mirror for accurate measurements. This is done by adjusting the screws provided and rotating the instrument until the horizon line is parallel to the horizontal cross-hair in the eyepiece.

6. Inclination adjustment: In some sextants, there is a bubble level to ensure the instrument is held at a horizontal level. This is checked by rotating the instrument and observing the bubble. If the bubble moves, the sextant needs to be adjusted by rotating the screws provided.

Once these steps are completed, the sextant is permanently adjusted, and the instrument is ready to be used for accurate and precise measurements. It is essential to regularly check and maintain the adjustment of the sextant to ensure accurate results. Any change in the instrument’s alignment can lead to errors in measurements, affecting the overall accuracy of the survey.


In conclusion, the introduction of the sextant revolutionized navigation at sea, allowing sailors to determine their position based on the stars and the horizon. This ancient instrument, dating back to the 1700s, continues to be used today in various forms and is an essential tool for marine navigation. With the advancement of technology, sextants may seem outdated, but they still serve as a backup and provide a sense of tradition and skill in the modern world. As we look to the future of navigation, let us not forget the significance and impact that the sextant has had on the history of maritime travel.

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