topic 4 Parts of Marine Sextant.pptx

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1. NAV 4 (TOPIC 4) Parts of the Marine sextant Learning Objectives: Determine the different parts of marine sextant.

2. The sextant is an instrument used to measure angles. is to measure the angle between horizon and a celestial body such as the Sun, the Moon, or a star, used in celestial navigation to determine latitude and longitude 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 What is Sextant?

3. PARTS OF SEXTANT

4. PARTS OF SEXTANT The sextant is made up of the following parts: 1. The frame. 2. The handle. 3. The telescope or monocle. 4 . The arc. 5 . The index arm. 6. The clamp. 7 . The micrometer drum. 8 . The micrometervernier . 9 . Electric light. 10. The index mirror. 11. The index mirror clips. 12. The index mirror (first) adjustment screw. 13. The index mirror shades. 14. The horizon mirror. 15. The horizon mirror clips. 16. The horizon mirror (second) adjustment screw. 17. The horizon mirror (third) adjustment screw. 18. The horizon mirror shades .

5. PARTS OF SEXTANT 1. The frame. This is either of rigid metal construction or moulded plastic. It incorporates three legs upon which the sextant rests when in the horizontal position. The upper side of the frame is referred to as the Plane of the instrument. The curved, lower part of the frame is referred to as the lower limb of the instrument. 2. The handle. The handle is fitted with a button switch and has a compartment for batteries to power the electric light on the index arm. 3. The telescope or prismatic monocle. This is used to enlarge the observed object and to make accurate observations easier. The telescope usually has a magnification of 4 X and a field of view of 5°. The monocle on the other hand has a magnification of 6 X and a field of view of 8½°. Higher magnification is desirable when observing vertical sextant angles of distant shore objects to obtain a clearer view of the sun’s lower limb when taking sights. A wide field of view is important when trying to locate stars which are not easily found because of the apparent size.

6. PARTS OF SEXTANT 4 . The arc. If the sextant is of the metal variety, the arc will be constructed of a thin strip of metal which has a low co-efficient of expansion. It is “let in” flush with the “lower limb” of the sextant and is graduated from 0° to 120° “on” the arc and from 0° to -5° “off” the arc. 5 . The index arm. The index arm is mounted on a circular base plate and is free to rotate on a central axis underneath the index mirror. The arrow of the index arm is known as the index mark. 6 . The clamp. By exerting finger pressure, the clamp disengages or unclamps the index arm from the gearing cut into the lower limb of the sextant. The index arm can then be moved to the required angle. On releasing pressure, the index arm is automatically clamped in position. 7 . The micrometer drum. While whole degrees are read directly from the arc, minutes of arc are read from the micrometer drum. Turning the micrometer drum screws the worm along the rack and permits fine adjustment of an observed angle or altitude.

7. PARTS OF SEXTANT 8 . The micrometervernier . The micrometervernier has five or six graduations which correspond to 0.2’ or 10” of arc respectively. Minutes of arc are read off the micrometer drum opposite the vernier index mark. Seconds or decimals of a minute of arc are read where one of the vernier graduations lines up perfectly with one of the minute graduations on the micrometer drum. 9 . The electric light. The light bulb is contained in a special holder so arranged that the light illuminates the arc, micrometer drum and vernier . It is operated by pressing the button(s) in the handle and is useful for taking twilight sights. 10. Index mirror. The index mirror is rectangular in shape and set in a frame attached to the moveable index arm. When properly adjusted it should be perpendicular to the plane of the instrument. Its purpose is to reflect light to the horizon mirror. 11. The index mirror clips. These are spring clips that hold the mirror to its frame. 12. Index mirror adjustment screw (first adjustment). This is an adjustment screw situated on the centre line of the mirror. By adjusting the screw the mirror is pushed against the spring clips and moved to the perpendicular position. 13. The index mirror shades. These shades are made of high quality optical glass and are neutral or coloured light filters. They are used to reduce the intensity of the sun’s reflected image and to protect the eyes. There are normally four shades. 14.The horizon mirror. This mirror receives the image from the index mirror and reflects it back to the observer through the telescope. It can be either a rectangular or circular mirror held in an appropriate frame. As the horizon mirror occupies only half the space enclosed by its frame, the observer is able to see both the reflected image and the horizon at the same time. Like the index mirror it needs to be perpendicular to the plane of the instrument and parallel to the index mirror when the index arm is set to zero. 15. The horizon mirror clips. These are spring clips holding the horizon mirror in its frame.

8. PARTS OF SEXTANT 16. The horizon mirror adjustment screw (second adjustment). This adjustment screw is situated on the centre line of the horizon mirror. It may be at the top or bottom of the mirror, depending on its design. It is used to adjust the horizon mirror to the perpendicular position. 17. The horizon mirror adjustment screw (third adjustment). The adjustment screw is located on the edge of the mirror and is used to adjust the horizon mirror parallel to the index mirror when the index arm is set to zero. 18. The horizon mirror shades. These are used to reduce the intensity of a brightly lit horizon and are similar to the index mirror shades. Two or three horizontal shades are normally fitted.

9. 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 Sextant). The sector-shaped part is called the frame.

10. Side Error • 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. ERRORS OF SEXTANT

11. Index Error • This error can be found using the horizon. The sextant’s altitude is set to zero and then the two images of the horizon are aligned. The Index Error can then be read off. • If the sextant altitude reads high, the correction is subtractive and termed “On the Arc.” “Off the Arc” is the opposite. • After Index Error has been applied, the Sextant Altitude it is referred to as the Observed Altitude.

12. How to find the index error of the sextant by 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 micrometer until they coincide exactly. The reading of the micrometer, ON or OFF the arc gives the IE

13. How to find the index error of the sextant by 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.

14. The purpose of altitude correction • The corrections made to the Hs (sextant altitude) are necessary because the mathematical premise of celestial navigation has the observer taking a sight to the center of the celestial object from the center of the earth. Since we cannot physically replicate that assumption we have to factor in some additional corrections.

15. The Purpose of Altitude Correction • The corrections made to the Hs (sextant altitude) are necessary because the mathematical premise of celestial navigation has the observer taking a sight to the center of the celestial object from the center of the earth. Since we cannot physically replicate that assumption we have to factor in some additional corrections.

16. Dip • The True Horizon is at 90° to the Earth’s gravitational field. It coincides with the apparent horizon at sea level. However the Apparent Horizon starts to dip below the horizontal plane as the height of (the observer’s) eye increases. • The error is due to the height of the eye above the Sea. The formulae are; • Dip = 0.97 x Square Root ( Ht of Eye in feet) • Dip = 1.76 x Square Root ( Ht of Eye in meters) • Dip is subtracted from the Observed Altitude to give Apparent Altitude.

17. Refraction • It is due to the passage of the optical beam through the layers of air with different density that are above the earth's surface. • Refraction should be subtracted from the apparent altitude to get the True Altitude

18. Semi-Diameter • This correction applies to the sun and moon. The moon tables are at the rear of the Nautical Almanac and are a subject of another newsletter. Because of the elliptical shape of the Earth’s orbit, the semi diameter of the sun is subject to change.

19. Parallax • A correction due to parallax, particularly that sextant altitude correction due to the difference between the apparent direction from a point on the surface of the earth to celestial body and the apparent direction from the center of the earth to the same body.

20. Altitude by the Sextant • Sextant Altitude • Index Error To get the Observe Altitude • Dip the Height of Eye To get the Apparent Altitude • Refraction (always subtract) • SD: LL add; UL subtract • To get the True Altitude

21. Summary To use the sextant the telescope must be focused on the horizon. The celestial body to be shot, found and the sextant aimed at it. Bring the body down to the horizon by moving the arm along the arc and then clamp the arm. 07/05/2019 REVIEWED, VALIDATED AND APPROVED

22. References STCW Code Table A-II/1 Function 1: Navigation at the Operational Level CMO No. 67 series of 2017. Revised PSG for BS Marine Transportation http://geocenter.survey.ntua.gr/main/labs/carto/academic/persons/bnakos_site_nafp/documentation/american_practical_navigator.pdf Bowditch's American Practical Navigator, Chapter IV. By Nathaniel Bowditch, 1802. Admiralty Nautical Almanac, UK Hydrographic Office. NP-314 07/05/2019 REVIEWED, VALIDATED AND APPROVED