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A sextant is an instrument used for measuring the angle between two visible objects. Both horizontal and vertical angles can be measured using a sextant. It contains two mirrors which are arranged in such a way that the observer can sight both the objects at the same time. It is mainly used for navigational purposes in sea routes.

Principle of a Sextant

The principle of a sextant is when the ray of light is reflected from two mirrors in succession in the same plane, Then the angle between the incident and reflected ray is two times the angle between the mirrors.

Types of Sextants

There are three types of sextants :

  1. Nautical Sextant
  2. Box Sextant
  3. Sounding Sextant

1. Nautical Sextant

Nautical sextant, also called as vernier sextant or marine sextant, is an instrument mainly used to determine latitude and longitude by measuring angles between two objects. Celestial objects such as sun, moon, and stars are sighted using a nautical sextant, and angular measurements between them and the horizon can be determined.

Nautical Sextant
Fig 1: Nautical Sextant

Nautical sextant comprises of two mirrors namely, index mirror and horizon mirror and a graduated arc which is about 1/6th of the circle. The radius of the arc is about 15 to 20 cm. The arc is graduated with 0 to 140 degrees and a vernier of readings up to 20″ is attached to index arm of a sextant.

Uses of Nautical Sextant

  • It is used for navigational purposes on ships and boats.
  • It is used for astronomical purposes to take angular measurements between celestial bodies.
  • The angles can be measured in any plane i.e either in the plane of a telescope or in the plane of an object.
  • Vertical angles measured can be reduced into horizontal angles.

2. Box Sextant

Box sextant is a small pocket instrument which looks like a sextant enclosed in a box and is 75mm in diameter. Similar to the nautical instrument, it is also used for measuring both the horizontal and vertical angles. Box sextant is a very small and handy instrument which is easy to carry. It is also used in ships for celestial navigation, and it also works well even if the ship or boat is moving.

Box Sextant
Fig 2: Box Sextant

Uses of Box Sextant

  • Box sextant can be used as an optical square by setting vernier to 90o.
  • Box sextant can be used to measure angles in chain surveying.
  • It is used to check the angles measured by other surveying instruments.
  • Radiation in traversing can be done using box sextant.

3. Sounding Sextant

Sounding sextant is more similar to the nautical instrument and the only difference is that it has a larger index glass compared to that of the nautical sextant. Sounding is basically the measurement of the depth of the water bodies. Sounding is located by different methods and one of those methods is to locate it by measuring angles from shore or from the boat and for these angular measurements sounding sextant is used.

To locate sounding in hydrographic surveys, small boats are recommended but these boats may undulate while measuring angles. In such cases, sounding sextant gives more accurate values than a normal nautical sextant.

Sounding Sextant
Fig 3: Sounding Sextant

Read also: Sounding in Hydrographic Surveying – Methods of Locating Sounding

About Sadanandam AnupojuVerified

Sadananda is a Civil Engineer and is an Author, Editor and Partner of The Constructor since 2016.