[Answer] What is a Marine Sextant?

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An article in Marine Insight helps us understand the working principle, readings and maintenance of a Marine Sextant.

What is Sextant?

The sextant is an instrument used to measure angles. Mainly used at sea, the tool is so named because its arc is one-sixth of a circle – 60 degrees. It adheres to the principle of double reflection hence it can measure angles up to 120 degrees.

Practically speaking, the arc of the sextant is a little over 60 degrees and therefore the total angle measurable is about 130 degrees.

Sextant is an essential tool for celestial navigation and is used to measure the angle between the horizon and a visible object (or) two objects at sea.

The sextant is used to measure the following:

  • Vertical Sextant Angle (VSA)
  • Horizontal Sextant Angle (HSA)
  • Altitudes

Principle of the Sextant

When a ray of light is reflected by a plane mirror, the angle of the incident ray is equal to the angle of the reflected ray, when the incident ray, reflected ray and the normal lie on the same plane.

When a ray of light suffers two successive reflections in the same plane by two plane mirrors, the angle between the incident ray and the reflected ray is twice the angle between the mirrors.

Different parts of a Sextant

A sextant is shaped in the form of a sector (60 degrees or 1/6th of a circle). It is the reason the navigational instrument is called a Sextant (Latin word for 1/6th is Sextans). The sector-shaped part is called the frame.

A horizontal mirror is attached to the frame, along with the index mirror, shade glasses (sun shades), telescope, graduated scale and a micrometre drum gauge.

Working of a sextant

Navigation Sextant – Readings ON and OFF the arc

The normal graduations of the arc, to the left of zero, extending from 0 to 130 degrees are referred to as ON the arc. To the right of 0 degrees, the graduations extend for few degrees and are referred to as OFF the arc. When reading OFF the arc, graduations of the micrometre should be read in the reverse direction (59 as 1’, 55 as 1’ and so on).

Errors of the Sextant

The errors can be classified as

  • Adjustable Errors (adjustable onboard), and
  • Non-adjustable Errors (not adjustable onboard)

Pointers on the use of sextant

  • Always check the errors before use
  • Focus the telescope while looking at the horizon and make a mark on the circumference of the stem
  • During use, hold the sextant steady. For this, stand with feet slightly apart for balance with hands holding the sextant steady
  • While observing the altitude of a celestial body, remember to swing the sextant to the other side, The body will appear to move along the arc. Measure altitude at the lowest point on this arc
  • Stand as close as far as practicable to the centerline of the ship
  • Use appropriate dark shades while observing the sun
  • If backlash error exists remember to rotate the micrometre in one direction only
  • Altitudes of stars and planets should be taken during twilight
  • Nighttime sextant observations should be avoided so far as practicable. The strong moonlight gives the illusion of a good horizon which is most probably false
  • While observing the HSA, set the index at zero, look at the object on the right through the telescope, gradually swing the index around and finish while facing the object on the left
  • When measuring VSA, look at the top of the object, set the index at zero and look at the top of the object. VSA = height of the object in meters

Care and maintenance of a sextant

  • Do not put too much stress on the index bar when grasping a sextant
  • Never touch the arc. It will smear it. These aren’t oleophobic per se
  • Ensure that worm and rack are clean
  • Coat worm and rack with vaseline when not using it for too long
  • Mirrors, lenses and shades should be wiped clean with a soft cloth
  • After each use, gently wipe the index mirror, horizon glass
  • Put it in the box when not using it
  • Do not bump the sextant anywhere
  • Avoid exposure to sunlight
  • Keep sextant stowed away from direct sunlight, dampness, heaters or blowers

The sextant is an expensive, precision instrument which should be handled with utmost care.

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Source: Marine Insight

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