SEXTANT SEXTANT an instrument for measuring angles on

SEXTANT • SEXTANT, an instrument for measuring angles on the celestial sphere. • The sextant basically consists of a telescope, a half silvered horizontal mirror which the telescope "looks" through and a moving arm on which the index mirror is fixed. • By manipulating this arm a star or other celestial body can be made to appear on the horizon. The shades are to use when the object being looked at is bright - such as the sun. 9/9/2020 Capt/ Elkady www. groups. yahoo. com/group/captelkady

SEXTANT Sextant Index Arm 9/9/2020 Capt/ Elkady Release

Parts of Sextant • The index arm is a movable bar of the same material as the frame. It pivots about the center of curvature of the limb. • The release is a actuated clamp that keeps the tangent screw engaged with the limb’s teeth. 9/9/2020 Capt/ Elkady www. groups. yahoo. com/group/captelkady

Parts of Sextant • Micrometer drum graduated in minutes of altitude. One complete turn of the drum moves the index arm one degree along the arc. • Next to the micrometer drum and fixed on the index arm is a vernier that reads in fractions of a minute. 9/9/2020 Capt/ Elkady www. groups. yahoo. com/group/captelkady

Parts of Sextant • The vernier shown is graduated into ten parts, permitting readings to 1/10 of a minute of arc (0. 1'). Some sextants have verniers graduated into only five parts, permitting readings to 0. 2'. 9/9/2020 Capt/ Elkady www. groups. yahoo. com/group/captelkady

Parts of Sextant • The index mirror is a piece of silvered plate glass mounted on the index arm, perpendicular to the plane of the instrument, with the center of the reflecting surface directly over the pivot of the index arm. 9/9/2020 Capt/ Elkady www. groups. yahoo. com/group/captelkady

Parts of Sextant • The horizon glass is a piece of 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 surfaces are parallel when the micrometer drum is set at 0°, if the instrument is in perfect adjustment. 9/9/2020 Capt/ Elkady www. groups. yahoo. com/group/captelkady

Parts of Sextant • Shade glasses of varying darkness are mounted on the sextant’s frame in front of the index mirror and horizon glass. They can be moved into the line of sight as needed to reduce the intensity of light reaching the eye. 9/9/2020 Capt/ Elkady www. groups. yahoo. com/group/captelkady

Parts of Sextant • The telescope screws into an adjustable collar in line with the horizon glass and parallel to the plane of the instrument. 9/9/2020 Capt/ Elkady www. groups. yahoo. com/group/captelkady

Theory of the Sextant • The pervious figure shows the elements of the sextant • the index arm may be adjusted coarsely, with the loaded quick release levers on the bottom, or by turning the micrometer drum. 9/9/2020 Capt/ Elkady www. groups. yahoo. com/group/captelkady

Theory 9/9/2020 Capt/ Elkady

Theory • The sextant relies on the optical principle that if a ray of light is reflected from two mirrors in succession the angle between the first and last direction of the ray is twice the angle between the mirrors. 9/9/2020 Capt/ Elkady www. groups. yahoo. com/group/captelkady

Theory • 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. 9/9/2020 Capt/ Elkady www. groups. yahoo. com/group/captelkady

Theory • Bring the body down to the horizon by moving the arm along the arc and then clamp the arm. Using the micrometer knob make small adjustments while gently swaying the instrument slightly from side to side until the heavenly body just brushes the horizon. 9/9/2020 Capt/ Elkady www. groups. yahoo. com/group/captelkady

Horizon Sun Ob ser ved Alti tu de MEASUREMENT OF THE SUN ALTITUDE BY SEXTANT 9/9/2020 Capt/ Elkady www. groups. yahoo. com/group/captelkady

Swinging the Sextant Right Altitude Using the micrometer knob make small adjustments while gently swaying the instrument slightly from side to side until the heavenly body just brushes the horizon. 9/9/2020 Capt/ Elkady When this is achieved instantly make a note of

Swinging the Sextant Wrong Altitude 9/9/2020 Capt/ Elkady www. groups. yahoo. com/group/captelkady

Reading to Sextant 9/9/2020 Elkady Capt/ www. groups. yahoo. com/group/captelkady

How To Read Vernier To read the Vernier scale for fractions of an arc minute, find the one short line on the stationary scale that is directly opposite any line on the rotating drum. 9/9/2020 Capt/ Elkady www. groups. yahoo. com/group/captelkady

How To Read Vernier Count the number of spaces this short line is away from the long line at the top of the stationary scale. Each space corresponds to 2/10 of an arc minute, Add the full degrees, arc minutes, and fractions of an arc minute to get the full angle reading. 9/9/2020 Capt/ Elkady

How To Read Vernier 9/9/2020 Capt/ Elkady

How To Read Vernier 9/9/2020 Capt/ Elkady

How To Read Vernier 9/9/2020 Capt/ Elkady www. groups. yahoo. com/group/captelkady

Sextant Errors Non adjustable Errors • 1. Prismatic Error - shades and mirrors not parallel. • 2. Graduation Errors - limb construction faulty. • 3. Centering Error - index arm pivot wrong with respect to center of limb curve. 9/9/2020 Capt/ Elkady www. groups. yahoo. com/group/captelkady

Sextant Errors • Adjustable Errors • 1. Perpendicularity Error - index mirror not perpendicular to frame. • 2. Side Error - horizontal glass not perpendicular to frame. • 3. Collimation Error - telescope not parallel to frame (adjustable on some). • 4. Index Error - index mirror and horizon glass not parallel. 9/9/2020 Capt/ Elkady www. groups. yahoo. com/group/captelkady

1 -Perpendicular error • Adjust first for perpendicularity of the index mirror to the frame of the sextant. To test for perpendicularity, place the index arm at about 35° on the arc and hold the sextant on its side with the index mirror up and toward the eye. Observe the direct and reflected views of the sextant arc, as illustrated in Figure 1612 a. If the two views are not joined in a straight line, the index mirror is not perpendicular. 9/9/2020 Capt/ Elkady www. groups. yahoo. com/group/captelkady

1 -Perpendicular error • If the reflected image is above the direct view, the mirror is inclined forward. If the reflected image is below the direct view, the mirror is inclined backward. Make the adjustment using two screws behind the index mirror. 9/9/2020 Capt/ Elkady www. groups. yahoo. com/group/captelkady

1 -Perpendicular error Testing the perpendicularity of the index mirror. Here the mirror is not perpendicular. 9/9/2020 Capt/ Elkady

2 - Side Error • An error resulting from the horizon glass not being perpendicular is called side error. • To test for side error, set the index arm at zero and direct the line of sight at a star. • Then rotate the tangent screw back and forth so that the reflected image passes alternately above and below the direct view. If, in changing from one position to the other, the reflected image passes directly over the unreflected image, no side error exists If it passes to one side, side error exists. 9/9/2020 Capt/ Elkady www. groups. yahoo. com/group/captelkady

Side Error Testing the perpendicularity of the horizon glass. On the left, side error does not exist. At the right, side error does exist. 9/9/2020 Capt/ Elkady www. groups. yahoo. com/group/captelkady

3 - Index Error • Index error is the error remaining after the navigator has 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 index error. • To test for parallelism of the mirrors, 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 horizon comes into line is the index error. 9/9/2020 Capt/ Elkady www. groups. yahoo. com/group/captelkady

Index Error by Sun 9/9/2020 Capt/ Elkady www. groups. yahoo. com/group/captelkady

Index Error • I. E =(60 -Off arc) – On arc 2 I. E =(60 -28. 2) – 32. 8) 2 I. E= 0. 5 on arc 9/9/2020 Capt/ Elkady www. groups. yahoo. com/group/captelkady

Index error By horizon set the instrument line of sight at 9/9/2020 at zero and direct the Capt/ Elkady the horizon. Here the index error does not exist

Index error By horizon set the instrument line of sight at 9/9/2020 at zero and direct the Capt/ Elkady the horizon. Here the index error does exist

Questions • Complete 1 - The part of a sextant mounted directly over the pivot of the index arm is _______. 2 - Because of the reflecting properties of a sextant, if the sextant altitude reads 60° on the limb, the actual arc of the limb from 0° to the 60° reading is _____. 9/9/2020 Capt/ Elkady www. groups. yahoo. com/group/captelkady

Questions 3 - Index error of a sextant is primarily caused by the horizon glass not being parallel to the index mirror 4 - A marine sextant has the index arm set at zero and the reflected image of the horizon forms a continuous line with the actual image. Is this sextant has index error or not? 9/9/2020 Capt/ Elkady www. groups. yahoo. com/group/captelkady

Questions Complete 5 - In order to remove side error from a sextant, you should adjust the horizon glass to make it _______to the sextant frame. 6 - What causes the error of collimation with regards to the four adjustments to a sextant? 9/9/2020 Capt/ Elkady www. groups. yahoo. com/group/captelkady