How does a Sextant Work?

What is a Sextant? How does a Sextant work? Why is it called a Sextant? What is the Basic Principle of Sextant? What are the Errors of Sextant?

In this world, there are a lot of things that are unknown to the majority population but are still being used daily in some spheres of work, like a sextant. A sextant is an instrument that helps in measuring angles. It is an arc of one-sixth of a circle and is mainly used at the sea. This tool measures the angles between the horizon and a visible or two visible objects at the sea. It is used for celestial navigation. Are you aware of a sextant or how does a sextant work? If not, then learn about this and many other things like the basic principle of sextant in this article.

1. Why is it called Sextant?

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A sextant is an iconic navigation instrument that is used to measure the angle of elevation of celestial bodies. These celestial bodies are usually the sun or moon. By determining the angle of elevation, this device finds out one’s direction and location. In general terms, this instrument measures the angle between two objects. This instrument spans one-sixth of a circle or 60° and is thus called a sextant. Now you have learned about the reason why a sextant is called a sextant, it’s time to learn- how does a sextant work? (See What does HMS and RMS stand for?)

2. How does a Sextant Work?

In the sea, people navigate using a sextant. There is nothing complicated about it. It just measures the angle between two objects. Till now you have acquired the basic knowledge about a sextant, now learn how does a sextant work? Two mirrors are used by a sextant to overlay one object over the other. One of the mirrors is half-silvered. This allows some light to pass through this mirror. In navigation, people look at the horizon through this mirror only.

The second mirror of the sextant is linked to a movable arm. Light from an object reflects off this second mirror. For understanding purposes, let’s take the sun as the object from which light is reflected off this mirror. The arm can be moved to a certain position where the sun’s reflection shut off the first mirror and also reflects off the second mirror and through the eye-piece. When this happens, the sun which is an object gets superimposed on the horizon which is another object. After this, the angle between the two objects is read off the scale. This answer should have cleared your doubt-how does a sextant work? (See How to find House Facing Direction in Google Maps?)

3. How does a Sextant tell you Where you are?

After understanding how does a sextant work, let’s learn how a sextant can inform you where you are. A sextant is used in celestial navigation to determine longitude and latitude. It determines the angle between the horizon and a celestial body like a star, the sun, or the moon. From this angle as well as with the exact time of the day as registered by a chronometer, it is possible to determine the latitude (within a few hundred meters) using published tables. This is how a sextant tells you where you are. (See How to Identify North West East South?)

4. How do you Navigate using a Sextant?

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A sextant is a tool used to measure the altitude of celestial objects above the horizon. To understand how people navigate using a sextant, you should learn how does a sextant work. In a sextant, there are two mirrors, one of which is half-silvered. The half-silvered mirror allows some light to pass through it. During navigation, it is through this very mirror that you look at the horizon.

The other mirror is aligned on a movable arm, it allows the light from an object to reflect off it. The arm is moved to a position where the celestial object’s reflection off the second mirror also reflects off the first mirror and through the eyepiece. When this happens you see the superimposition of one object on the other. Then the angle between the two objects is read off the scale. This angle and the time when it was measured can be used to calculate a position line on an aeronautical chart or nautical chart. One can find their latitude by using a sextant to sight the sub at noon. The sextant can be held horizontally to measure the angle of two objects, like two lighthouses. This also allows for the calculation of a line of position on a chart. This is how does a sextant work and navigates. (See When can Navigation Rules be Overlooked?)

5. How Accurate is the Sextant?

Now that you know how does a sextant work and how people navigate using a sextant, it’s time to learn about the accuracy of a sextant. A sextant is a nautical protractor that measures the angle between a celestial body and the horizon with great precision. This device is useful while navigating due to its accuracy. It measures angles with precision to the nearest ten seconds. Note that a degree is divided into 60 minutes, and one minute is divided into 60 seconds. With this, you must have understood the accuracy of a sextant. (See What is a Steamboat?)

6. What is the Basic Principle of Sextant?

Along with how does a sextant works, note the basic principle of sextant given as follows:

  • When a ray of light is reflected by a plane mirror, the angle of the reflected ray is the same as the angle of the incident ray, when the incident ray, the reflected ray, and the normal lie in the same place.
  • The angle between the incident ray and the reflected ray is twice the angle between the mirrors when a ray of light suffers two successive reflections in the same plane by two mirrors.

7. Why is a Sextant 60 Degrees?

A sextant is a tool used to measure angles between objects. It’s mostly used at the sea. It is named so because it encompasses one-sixth of a circle or 60 Degrees. It adheres to the principle of double reflection and it is thus 60 degrees so that it can measure angles up to 120 degrees. (See How many Degrees in a Third of a Full Turn are there?)

8. Is a Sextant still used today?

A sextant is a device used for navigation in the sea and this device is in use even today. The navigating officers on ships have regular routines to keep themselves familiar with the work of a sextant. Although the world has gyro compasses, radio communications, and Global Positioning Satellites, the sextant is still used today as it works without any sort of power or electricity. So, when everything else fails a sextant can still work. (See What does GPS stand for?)

9. How do you Identify and Remove Errors from a Sextant?

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In a sextant, you can only remove adjustable errors. Let’s look at the types of adjustable errors and see how you can identify and remove errors from a sextant:

  • Perpendicularity Error: It occurs because of the index glass not being perpendicular to the plane of the instrument. You can identify this error by clamping the index bar at about the middle of the arc and holding the sextant horizontally. Keep the arc away from you and look obliquely into the index mirror, see simultaneously. If they appear in alignment then there isn’t a perpendicularity error present. If they aren’t aligned then turn the first adjustment screw at the back of the index glass until they appear in alignment.
  • Side Error: It occurs when the horizon glass is not perpendicular to the plane of the instrument. You can identify this error by clamping the index bar at zero, holding the sextant horizontally, and observing the horizon through the telescope. If there is alignment between the true horizon and its reflection in the mirror half of the horizon glass, then there is no side error. But if there is no alignment then there is a side error. To remove this error, you need the turn the second adjustment screw until the true and reflected horizons appear on the same line.
  • Index Error: It occurs when the index glass and the horizon glass are not parallel to each other. This happens when the index bar is at zero. You can find the index error, during the daytime, by using the horizon. You need to clamp the index bar at zero, hold the sextant vertically and view the horizon through the telescope. The index error is not present if the true horizon and its reflection appear in the same line. If they appear displaced vertically then turn the micrometer drum until they are in the same line. The micrometer reading is the index error. This error is shown off the arc if the micrometer reading is less than zero and on the arc if the micrometer reading is more than zero. You can remove this error by clamping the index bar at zero, looking through the telescope, and turning the third adjustment screw until the true horizon and its reflection appear in the alignment.

10. What are the Errors of Sextant?

After learning how to identify and remove errors from a sextant, let’s learn more about the errors of a sextant. There are two types of errors in a sextant namely adjustable and non-adjustable errors. These errors are discussed in detail in the following pointers: 

Non-adjustable error:

  • Shade Error- This occurs when the two surfaces of the colored shades are not parallel to each other.
  • Graduation Error- This occurs due to the inaccurate graduation on the main scale on or of the vernier/micrometer.
  • Optical Errors- The prismatic errors of the mirrors or aberrations in the telescope lenses can cause these errors.
  • Centering Error- This occurs when the pivot of the index bar is not coincident with the center of the circle of which the arc is a part. If there is wear on the rack and worm which forms the micrometer movement, then it leads to inconsistent errors.

Adjustable Error:

  • Perpendicularity Error- This error occurs when the index glass is not perpendicular to the plane of the instrument.
  • Side Error- This error occurs when the horizon glass is not perpendicular to the plane of the instrument.
  • Index Error- It occurs when the index glass and the horizon glass are not parallel to each other. This happens when the index bar is at zero.
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