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Sextant and it's Errors

Make sure you use the shades on the sextant when looking at the SUN

When taking a bearing in an Oral Exam, make sure you push the shades away so you can see the object (but not the SUN)

Remember the initials (PSI) the same initials that's on Sub-aqua diving bottles

(P) Perpendicular Error - (1st Error)

(S) Side Error - (2nd Error)

(I) Index Error - (3rd error)

 

Now remember the letters that go with it

(PI) Perpendicular error - is when the (Index) glass is not perpendicular to the plane of the instrument

(SH) Side Error - is when the (Horizon) mirror is not perpendicular to the plane of the instrument

(I - H - I) Index Error - is when the (Horizon) mirror and the (Index) glass are not parallel to each other

 

(Q) What does (Perpendicular) mean?

(a) 090 degrees

If asked to find if there is an error in the sextant then do the following

(1st Error)

Set the Index bar between 30 and 40 degrees, hold the sextant horizontally with the arc furthest away from you, look into the index glass with the sextant tilted to a small angle, check if the true and reflected arcs are in line

If they are not in line then adjust the screw on the index glass (The index glass has only one screw)

 

(2nd Error)

Set the index bar at ZERO, hold the sextant nearly horizontal, look into the eye piece to see if the true and reflected arcs are in line

If they are not in line then adjust the screw nearest the plane of the instrument on the horizon mirror (There are two screws on the horizon mirror, use the one nearest the base of the sextant)

 

(3rd Error)

Set the index bar to ZERO, hold the sextant vertically, look into the eye piece to see if the true and reflected arcs are in line

If they are not in line then adjust it with the screw furthest away from the base of the sextant on the horizon mirror or adjust the vernier wheel

If it's ON the ARC then subtract it from your final bearing

If it's OFF the ARC then add it to your final bearing

most captains will expect you to read the vernier wheel to see what the index error is

 

To find the distance your off a land object (Usually a lighthouse) you would find the height of it on the chart, find the angle from the base to the top of the lighthouse allowing for any errors, and go into "NORRIES TABLES" and find out the distance in the chapter "Distance by vertical angles"

 

A sextant is used to find vertical and horizontal angles, it was first used to find the ships position by aligning up the sun with the horizon and by knowing the time of day you could find the Latitude on a chart

To find the longitude they needed an accurate time-keeping clock, the chronometer was the answer for this as it kept real accurate time, now they can do it using nautical tables and trigonometric sight reduction tables to find the longitude by using the Sun, moon or any one of 57 visible stars

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