Sextant

Today the sextant is an old piece of equipment, which has taken a back seat due to the various electronic equipment that can position the location of a ship, such as ECDIS, GPS, or Gyrocompass. However, it is still carried on board. The sextant is still an important part of navigation, being used in case of emergency in which other equipment fails.

The sextant is an instrument used to measure angles. It is so called because it is mainly a semicircle or an arc of a circle, in which it reaches up to the sixth of a circle, that is, 60°. However, because of the principle of double reflection, it can measure angles up to 120°. Its use is to measure the angle between the horizon and a visible object, in this case, the celestial bodies.

It measures the following: vertical and horizontal sextant angles and altitudes.

Part of sextant

The sextant is made up of different parts. First, there is the frame that makes up the sixth arc of a circle, to this is attached a horizontal mirror, along with the index mirror, each with its lenses against the sun. In addition, it has a telescope, a graduated scale, and a micrometer drum caliper.

Optical principle of the sextant

When a plane surface reflects a ray of light, the angle of reflection is equal to the angle of incidence. The angle between the first and last direction of a light ray that has undergone double reflection in the same plane is twice the angle that the two reflecting surfaces make concerning each other.

Mirrors allow the angle of incidence of the sun to be matched to the horizon when observing. When the sun or any star is placed on the horizon, the measurement that exists between the horizon and the celestial star is made.

Errors of the sextant

They are classified into two types of errors

1.Adjustable errors

• Perpendicular error: It is when the two glasses do not look the same. If they are aligned, the error does not exist. If they are asymmetrical, it is necessary to turn the adjustment screw on the mirror until they are aligned.
• Lateral error: This is when there is a reflection in the two mirrors. If you see two identical objects in the two mirrors there is a lateral error. This error is also corrected by adjusting the screw located behind the horizon mirror.
• Collimation error: This occurs when the telescope axis is not parallel to the plane of the instrument. Modern sextants do not have a screw to adjust the collimation.
• Index Error: This occurs when the two mirrors are not parallel to each other when the index marks 0°0.0'. It can be corrected by observing the horizon and observing the horizon, the two images should show a perfect dividing line between the two. This is corrected with the mirrors or micrometers until they coincide exactly.

2.Non-adjustable errors

• Centering error
• Screen error
• Optical error
• Wear on the frame and screw

These are errors that cannot be corrected on board but must be taken to a specialized technician for repair.

Before using the sextant

• Do not make observations at night, since moonlight gives an illusion of a good horizon that is probably false.
• When using the sextant, hold the sextant firmly and maintain a balanced position when holding it.
• Check for errors before use.
• The altitudes of the stars and planets are made during nautical twilight, as we have already discussed in another post.
• A sextant is a delicate object, as far as possible avoid using it roughly as they are expensive instruments.

Maintenance sextant

• As every instrument on board requires its respective maintenance to ensure an advanced useful life.
• Never touch the bow will stain.
• Do not hold the grip of the sextant too tightly.
• Mirrors, lenses, and screens should be cleaned with a soft cloth.
• After each use, clean the mirrors
• When not in use, put it in its respective box.
• Avoid exposure to light

The sextant is an important instrument to carry out celestial navigation, which as already mentioned allows us to position the vessel employing the celestial stars. This is done by different methods that I will discuss later.

Reference: The American Practical Navigator - Nathaniel Bowditch

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