Marine Sextant: Principles and Usage

• Marine Sextant: Part A

Marine Sextant Overview

• A marine sextant is a precision optical instrument used to measure altitudes and angles.
• Sextants were essential for ship navigation.
• The name "sextant" is derived from the length of its graduated arc, approximately one-sixth of a circle.
• The operation of a marine sextant is based on the optical law that when a ray of light is twice reflected by two mirrors in the same plane, the angle between the first and last rays is twice the angle between the two mirrors.
• Sextants can measure angles up to twice their arc length, with a 60° arc enabling measurements of up to 120°.
• Modern sextants typically have graduations from 0° to 125° "on" the arc and a 5° "arc of Excess" on the opposite side of zero, readings on this part are considered "off" the arc.

Principle of the Sextant

• When light reflects off a plane mirror, the incident ray's angle equals the reflected ray's angle, with both rays and the normal lying in the same plane.
• When light undergoes 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.

Parts of Sextant

• A sextant is shaped like a sector (60 degrees or 1/6th of a circle, hence "Sextant" from the Latin "Sextans").
• The sector-shaped part is called the frame.
• Major components of a sextant include:
  • The frame.
  • The handle.
  • A telescope or monocle.
  • A rising piece.
  • The arc.
  • The index arm.
  • A clamp.
  • A worm and rack.
  • A micrometer drum.
  • A micrometer vernier.
  • An electric light.
  • An index mirror.
  • Index mirror clips.
  • An index mirror (first) adjustment screw.
  • Index mirror shades.
  • A horizon mirror.
  • Horizon mirror clips.
  • A horizon mirror (second) adjustment screw.
  • A horizon mirror (third) adjustment screw.
  • Horizon mirror shades.

Detailed Description of Parts

• Frame: Made of rigid metal or molded plastic, incorporating three legs for horizontal positioning and includes the upper "Plane of the instrument" and lower "lower limb."
• Handle: Fitted with a button switch and battery compartment for the electric light on the index arm.
• Telescope/Prismatic Monocle: Enlarges the observed object, with telescopes typically having 4X magnification and a 5° field of view; monocles offer 6X magnification and an 82° field of view.
• Rising Piece: Connects the telescope or monocle to the frame via an adjustable slide with a milled head screw, altering the area of the horizon mirror viewed.
• Arc: Metal variety using a thin strip with low expansion, flush with the lower limb, graduated from 0° to 120° "on" the arc and 0° to -5° "off" the arc.
• Index Arm: Mounted on a circular base plate, rotates on a central axis under the index mirror; the index mark is an arrow on the index arm.
• Clamp: Disengages/unclamps the index arm from the gearing via finger pressure, allowing movement to the required angle. On release, the arm automatically clamps.
• Worm and Rack: Tangent screw engages with the gearing on the lower limb; a quick release clamp disengages the worm for index arm movement.
• Micrometer Drum: Whole degrees are read from the arc, minutes from the micrometer drum; turning the drum adjusts the observed angle.
• Micrometer Vernier: Has 5 or 6 graduations corresponding to 0.2' or 10", read opposite the vernier index mark for minutes and seconds/decimals.
• Electric Light: Illuminates the arc, micrometer drum, and vernier; used for twilight sights.
• Index Mirror: Rectangular, in a frame on the index arm; perpendicular to the instrument plane to reflect light to the horizon mirror.
• Index Mirror Clips: Spring clips holding the index mirror to its frame.
• Index Mirror Adjustment Screw (First): On the center line, adjusts mirror perpendicularity using the spring clips.
• Index Mirror Shades: High-quality optical glass, neutral or colored filters, to reduce sun intensity and protect the eyes (typically four shades).
• Horizon Mirror: Receives the image from the index mirror and reflects it to the observer; either rectangular or circular. It needs to be perpendicular to the instrument plane and parallel to the index mirror when the index arm is set to zero.
• Horizon Mirror Clips: Spring clips holding the horizon mirror in its frame.
• Horizon Mirror Adjustment Screw (Second): On the center line, used to adjust horizon mirror perpendicularity.
• Horizon Mirror Adjustment Screw (Third): On the mirror's edge, adjusts the horizon mirror parallel to the index mirror when the index arm is at zero.
• Horizon Mirror Shades: Reduce the intensity of a brightly lit horizon.

How a Sextant Works

• A sextant uses two mirrors, one half-silvered to allow viewing of the horizon.
• The navigator uses the half-silvered mirror to view the horizon.
• The other mirror on a movable arm is positioned so the reflected object aligns with the horizon.
• Readings from the scale measure the angular distance between them.
• A sextant measures angles accurately to the nearest ten seconds (degrees, minutes, and seconds).

How to Use a Sextant

• Always check and correct any errors before use.
• Focus the telescope on the horizon and mark the stem's circumference.
• Stand with feet apart for balance when holding the sextant.
• Swing the sextant while observing altitude with a celestial body.
• Measure altitude at the lowest point on the arc, adjusting for ship's centerline.
• Use shades to protect eyes when observing the sun.
• Correct persistent backlash error by rotating the micrometer in one direction only.
• Preferable to take altitudes of stars and planets during twilight.
• Avoid using the sextant at night due to the false impression of a good horizon from strong moonlight.
• Determining longitude requires a chronometer; the Earth moves 15 degrees per hour.

Determining Location

• GPS has largely replaced sextants due to the complexity of using a sextant.
• Finding latitude requires measuring the angle between the horizon and the sun at its highest point (noon).
• Tables provide the line of latitude where the sun should be on a given day.
• As an example, on December 21, if the sun is directly overhead the Tropic of Capricorn, your latitude is 23.5 degrees S.