A marine sextant is nothing more than a device designed to measure the angle between two objects with a great deal of precision.
In celestial navigation, these two objects are
a celestial body (star, sun, moon, or planet)
the visible horizon
In coastal navigation, measure the vertical angle to find the range from an object of known height.
Also used for measuring the angular distance between two terrestrial object.
OPTICAL PRINCIPLES OF MARINE SEXTANT
When ray of light reflected from a plane surface, angle of incidence equals angle of reflection
If ray of light undergoes 2 reflections in same plane, angle between first and last direction is equal to twice angle that two reflecting surfaces make to each other
• As a consequence, graduations on a sextant limb are such that 1° of actual arc on limb reads on scale of sextant limb as 2°
ARC or The LIMB
In most marine sextants, brass or aluminum comprise the frame. A frames come in various designs. Teeth mark the outer edge of the limb. Each tooth marks one degree of altitude. The altitude graduations along the limb mark the arc. Some sextants have an arc marked in a strip of brass, silver or platinum in laid in the limb.
Index Arm
The index arm is a moveable bar of the Same material as the frame. It pivots about the centre of curvature of the limb.
Tangent Screw
Tangent screw mounted perpendicularly on the end of the index arm. Where it engages the teeth of the limb. Because the observer can move the index arm through the length of the arc by rotating the tangent screw, this is sometimes called an “endless tangent screw”.
INDEX ARM RELEASE
This is spring-actuated clamp that keeps the tangent screw engaged with the limb’s teeth. The observer can disengage the tangent screw and move the index arm along the limb for rough adjustment.
Micrometer Drum
The ends of tangent screw mounts a micrometer drum, graduated in minutes of altitude. One complete turn of the drum moves the index arm one degree along the arc.
vernier
Next to the micrometer drum and fixed on the index arm is a vernier, that reads in fraction of a minute. The vernier shown in a graduated into ten parts, permitting reading to 1/10 of a minute arc ( 0.1’ ).
Index Mirror
Index mirror is a piece of silvered plate glass mounted on the index arm, perpendicular to the plane of the instrument, with the centre of the reflecting surface directly over the pivot of the index arm.
Horizon Glass / Mirror
The horizon glass is a piece of a optical glass silvered on its half nearer the frame. It is mounted on the frame, perpendicular to the plane of the sextant. The index mirror and horizon glass are mounted so that their surface are parallel when the micrometer drum is set at 0°.
Shade Glasses
Shade glasses of varying darkness are mounted on the sextant’s frame in front of index mirror and horizon glass. They can be moved into the line of sight as needed to reduce the intensity of the light reaching to the eye.
Telescope
The Telescope, screws into an adjustable collar in line with the horizon glass and parallel to the plane of the instrument.
Handle
Sextants are designed to be held in the right hand. Some have a small light on the index arm to assist in readings altitudes. The batteries for this light are fitted inside a recess in the handle.
Sextant- Sun Sights
Move the index arm with the drum slowly until the sun appears to be resting exactly on the horizon, tangent to the lower limb. Then make the observation.
When observing the moon, follow the same procedure as for the sun.
Sextant Star and Planet Sights
Set the index arm and micrometer drum on 0° and direct the line of sight at the body to be observed. Then, while keeping the reflected image of the body in the mirrored half of the horizon glass, swing the index arm out and rotate the frame of the sextant down. Keep the reflected image of the body in the mirror until the horizon appears in the clear part of the horizon glass. Then make the observation.
Reading the Sextant
Reading a micrometer drum sextant is done in three steps.
The degrees are read by noting the position of the arrow on the index arm in relation to the arc.
The minutes are read by noting the position of the zero on the vernier with relation to the graduations on the micrometer drum.
the fraction of a minute is read by noting which mark on the vernier most nearly coincides with one of the graduations on the micrometer drum.
Errors of the Sextant
Non Adjustable Sextant Error
1. Prismatic Error– occurs when the faces of the shade glasses and mirrors are not parallel.
2. Graduation Error– occurs in the arc, micrometer drum, and vernier of a sextant which is improperly cut or incorrectly calibrated.
3. Centering Error– results if the index arm does not pivot at the exact centre of the arc’s curvature.
Adjustable Sextant Error
1.Error of Perpendicular - is existing, when index mirror is not perpendicular to the frame of the sextant .
Place the index arm at about 35° on the arc and hold the sextant on its side with the index mirror up and towards the eye.
Observe the direct and reflected views of the sextant arc, if the two views are not joined in a straight line, the index mirror is not perpendicular.
Make the adjustment using screws behind the index mirror. (1st adjustment screw)
2. Side Error - is existing, when horizon glass is not perpendicular to the frame of the sextant.
Set the index arm at zero and direct the line of sight at a star. Then rotate the tangent screw back and fwd, so that the reflected images passes alternately above and below the direct view. If the reflected image passes directly over the un-reflected image, no side error exist. If it passes to one side, side error exists.
• An alternative method is to observe a vertical line, such as one edge of the mast of another vessel. If the direct and reflected portions do not form a continuous line, the horizon glass not ppr to the frame of the sextant.
A third methods involves holding the sextant vertically as in observing the altitude of a celestial body. Bring the reflected image of the horizon into coincide with the direct view until it appears as a continuous line across the horizon glass.
Make the appropriate adjustment using second adjustment screw; behind the horizon glass which is furthest away from the frame
3. Error of Collimation– is existing, if the line of sight through the telescope is not parallel to the plane of the instrument.
To check the parallelism of the telescope, insert it in its collar and observe two stars 90° or more apart. Bring the reflected image of one into coincidence with the direct view of the other near either the right or left edge of the field of view. Then tilt the sextant so that the stars appear near the opposite edge. If they remain in coincidence, the telescope is parallel to the frame; if they separate it is not.
Adjust the collar to correct for non-paralle
4. Index Error –
Is the error remaining after removed perpendicularity error, side error, and collimation error. The index mirror and horizon glass not being parallel when the index arm is set exactly at zero is the major cause of the index error.
Set the instrument at zero and direct the line of sight at the horizon. Adjust the sextant reading as necessary to cause both images of the horizon to come into line. The sextant’s reading when the horizons comes into line is the index error
index error by the sun
Clamp the index arm at about 32’ off the arc and observe the true and reflected images to the position of limb upon limb. Repeat the observation with index arm set at about 32’ on the arc, and note the two readings of both observations.
The numerical value of the index error is the difference between the two readings divided by two, and would be called 'on the arc' if the 'on the arc' reading were the greater of the two, and 'off the arc' if the 'off the arc' reading were the greater
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