Books
- Celestial Navigation for Yachtsmen by Mary Blewitt
- Celestial Navigation in the GPS Age by John Karl
- Emergency Navigation by David Burch
- How to use Plastic Sextants by David Burch
- Replace light cord provided on sextant with soft webbing as used for sail ties. p17
- Either discard or trim the plastic cup on the eye piece as it prevents full range of adjustment of the telescope. p17
- Crucial adjustments/settings: Index Error (mirrors not parallel to each other), Side Error (mirrors not perpendicular to the frame). p25f
- Side Error Adjustment Procedure using two casino dice. p26ff
- Bodies appear as duplicate images. Vertical separation is due to index error. Horizontal separation is due to side error. p30
- Determine Index Error and Correction:
- Index Error is most crucial for accurate results. p36
- Horizon Index Correction Method (page 51): Set the arm and drum at 0.0*. Observe the horizon. Turn the dial clockwise till the horizon splits. Turn the dial counter-clockwise till there is a single horizon. Read the arc and note as "A(way) touch". Continue turning counter-clockwise till the horizon splits again. Read the arc and note as "A(way) leave". Repeat five or six times. Reverse direction and repeat five or six times, recording all data. p51ff
- Solar Index Correction Method (page 64ff, page 146ff): Toward / Clockwise / Angle Decreasing
- Set the arm and drum at 0.0*. Observe the sun.
- Turn counter-clockwise till the reflected sun is a noticeable distance below the direct sun.
- Reverse direction and turn clockwise till the reflected sun just contacts the lower limb of the direct sun. Read the arc, subtract the value from 60', and note as "ON", (the arc of the sextant).
- Continue turning clockwise till the reflected sun is just touching the upper limb of the direct sun. Read the arc, and note the result as "OFF" (the arc).
- Subtract "OFF" from "ON" and divide by two to find the Index Correction.
- As a check, add the "OFF" and "ON" values and divide by four to obtain the measured semi-diameter of the sun. Compare the measured semi-diameter with the value in the Nautical Almanac. Repeat several times.
- Solar Index Correction Method (page 73, page 146ff): Away / Counter-Clockwise / Angle Increasing
- Set the arm and drum at 0.0*. Observe the sun.
- Turn clockwise till the reflected sun is a noticeable distance above the direct sun.
- Reverse direction and turn counter-clockwise till the reflected sun just contacts the upper limb of the direct sun. Read the arc and note as "OFF" (the arc of the sextant).
- Continue turning counter-clockwise till the reflected sun is just touching the lower limb of the direct sun. Read the arc, subtract the value from 60', and note the result as "ON" (the arc).
- Subtract "OFF" from "ON" and divide by two to find the Index Correction.
- As a check, add the "OFF" and "ON" values and divide by four to obtain the measured semi-diameter of the sun. Compare the measured semi-diameter with the value in the Nautical Almanac. Repeat several times.
- Index Correction measurement is an integral part of the sight session and must be done, and values recorded, at the start and end of each session. p78
- Sight Procedure: p92ff
- Before going on deck, record compass heading/(GPS Course Over Ground) and knotmeter reading/(GPS Speed Over Ground).
- Verify Index Correction using either the horizon or the semi-diameter of the sun.
- When shooting the sun, select a combination of index mirror shades that provides a clear, distinct circular sun without any perception of 'brightness'.
- View the horizon, directly below the sun, through the sextant telescope and rotate the index arm till the sun appears in view through the sextant telescope.
- Rotate the sextant about the sighting line axis while slowing bringing the sun down (away/counter-clockwise) to the horizon.
- Stop adjusting when the sun just skims the horizon at the lowest point in the sextant's rotation.
- Record the time. Read the sextant setting. Re-check the sextant setting. Record the height and the name of the celestial object viewed.
- Sight each body selected for the round of sights.
- After completing the round of sights, record compass heading/(GPS Course Over Ground) and knotmeter reading/(GPS Speed Over Ground).
- Choice of Celestial Bodies p101ff
- Star and planet sights are best done by precomputing the height and bearings. Use StarFinder 2102-D.
- Select three stars that are about 120* apart, bright, and about the same height. Not below 15* and not above 75*.
- Set and Wait Method p104
- Only works with bodies that are rising or setting, that is bodies that are some distance from the observer's meridian.
- For rising bodies, turn the drum toward/clockwise to place the body above the horizon, then reverse direction (away/counter-clockwise) to sink the body about 4' of arc below the horizon. Rotate the sextant till the body just touches the horizon. Record the time and height.
- For setting bodies, turn the drum toward/clockwise to place the body above the horizon, then reverse direction (away/counter-clockwise) to place the body about 4' above the horizon. Rotate the sextant till the body just touches the horizon. Record the time and height.
- Averaging Celestial Sights p113ff
- When using sight reduction software, determine the intercept distance and bearing for each sight. Average the intercepts. (first approximation) p115f
- Fit Slope Method: When using sight reduction software, calculate the rate of change of the height of the body (Hc) at a time before the round of sights and at a time after the round of sights taking into account the change in DR position between the two times selected. Plot the height vs time for these two and shift the resulting line vertically for a best fit with the sextant heights (Hs) of the actual sights. (second approximation) p118f
- Steps for Taking Sights: p122f
- 1. Measure the Index Error in the Away (increasing sextant angle) direction..
- 2. Use the "Set and Wait" procedure outlined above. p104
- 3. Take at least four sights in the Away direction.
- 4. Analyze the data using the "Fit Slope Method" outlined above. p118f
- Accuracies of 5 to 6nm are possible with a plastic sextant and the above methods vs 1 to 2nm with metal sextant.
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- When taking a sight: all final adjustments should be done in the "away" (counter-clockwise, increasing arc) direction.
- Primer of navigation by George Mixter.
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