Understanding Marine Sextant – Principle, Readings and Maintenance

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:

  1. Vertical Sextant Angle (VSA)
  2. Horizontal Sextant Angle (HSA)
  3. Altitudes

Principle of the Sextant

  1. 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
  2. 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.

parts of sextant

Here is a great image from wikipedia explaining different parts of sextant.

How Does A Sextant Works And How To Use It?

Watch this video to understand how to use 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

1. Adjustable Errors (adjustable onboard), and

2. Non-adjustable Errors (not adjustable onboard)

Adjustable Errors Of Sextant

  • Error of Perpendicularity: This is caused when the index glass is not perpendicular to the plane of the instrument. To check for this, clamp the index bar about the middle of the arc, and holding the sextant horizontally, with the arc away from you, look obliquely into the index mirror till the arc of the sextant and its reflection on the index mirror are seem simultaneous. If in alignment, the error does not exist. If not, turn the adjustment screw at the back of the index glass, until they are aligned
  • Side Error: This is caused by the horizon glass not being perpendicular to the plane of the instrument. Clamp the index bar at 0 degree 0.0’. Hold the sextant vertically and look at the heavenly body. Turn the micrometer one way and then the other, while looking at the body. The reflected image of the body will move above and below the direct image and should pass exactly over it. If the reflected image passes to the left or right of the direct image, side error exists. This error can be removed by turning the second adjustment screw (the top screw behind the horizon glass) until the true and reflected horizons appear in the same line.
  • Index Error: This is caused if the index mirror and the horizon glass are not exactly parallel to each other when the index is set at 0 degree 0.0’. Basically, this is the difference between the optical zero of the sextant and its graduated zero, termed OFF the arc if the optical zero lies to the right of the graduated zero and termed ON the arc if the optical zero lies to the left of the graduated zero. There are three methods of obtaining the index error of a sextant:

    A) By observing the horizon: Clamp the index at 0 deg 0.0’ and, holding the sextant vertical, look at the horizon. The reflected image and the direct image should appear in a perfect line. If not, turn the micrometre until they coincide exactly. The reading of the micrometer, ON or OFF the arc gives the IE

    B) By observing the star or planet: Clamp the index at 0 deg 0.0’ and holding the sextant vertical, look at the star/planet. The reflected and direct image must coincide. If not turn the micrometer till they do. The reading of the micrometer, ON or OFF the arc gives the IE

    C) By observing the Sun: Set the index at about 32’ ON the arc. Hold sextant vertical and look at the sun, using shades. The reflected image of the sun would appear below the direct image. Turn the micrometer until their closer limbs just touch. Note reading ON the arc.
    Set the index at about 32’ OFF the arc and look at the Sun. The reflected image of the sun would appear above the direct image. Turn the micrometer until their closer limbs just touch. Note reading OFF the arc.
    The name of IE is the name of the reading having a higher numerical value.
  • Error of Collimation: This is due to the axis of the telescope not being parallel to the plane of the instrument. The telescope is attached to the sextant in such a manner that it cannot tilt. These modern sextants are therefore not provided with any collimating screws

Non-Adjustable Errors Of Sextant

  • Graduation Error: Due to the inaccurate graduation of the main scale on the arc or of the micrometre/vernier
  • Centring Error: Caused if the pivot of the index bar is not situated at the geometric centre of the arc. This can be caused due to a manufacturing defect or due to careless handling.
  • Shade Error: The shades should be so mounted that their glass surfaces are normal to the rays of light passing through them. If not, the distortion would result. The greater number of shades used, the greater the chances of distortion.
  • Optical Errors: Caused by prismatic errors of the mirrors or aberrations in the telescope lens
  • Wear on the rack and worm: This causes a backlash, leading to inconsistent errors. Wearing down of the worm can be due to lack of lubrication, the presence of dust particles, careless handling


This is the angle at the observer between the plane of the observer’s sensible horizon and the direction to his visible horizon. Dip occurs because the observer is not at sea level. The value of dip increases as the height of eye of observer increases. The values of dip are given in the cover page of the nautical almanac and in nautical tables (Nories), as a function of the height of eye

Pointers on the use of sextant

  1. Always check the errors before use
  2. Focus the telescope while looking at the horizon and make a mark on the circumference of the stem
  3. During use, hold the sextant steady. For this, stand with feet slightly apart for balance with hands holding the sextant steady
  4. 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
  5. Stand as close as far as practicable to the centerline of the ship
  6. USe appropriate dark shades while observing the sun
  7. If backlash error exists remember to rotate the micrometre in one direction only
  8. Altitudes of stars and planets should be taken during twilight
  9. 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
  10. 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
  11. 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
    1852 X Tan VSA

Care and maintenance of a sextant

  1. Do not put too much stress on the index bar when grasping a sextant
  2. Never touch the arc. It will smear it. These aren’t oleophobic per se
  3. Ensure that worm and rack are clean
  4. Coat worm and rack with vaseline when not using it for too long
  5. Mirrors, lenses and shades should be wiped clean with a soft cloth
  6. After each use, gently wipe the index mirror, horizon glass
  7. Put it in the box when not using it
  8. Do not bump the sextant anywhere
  9. Avoid exposure to sunlight
  10. 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.