Sextant and Compass Flashcards
4 - Fixed errors
**Graduation error
Graduation on arc mcormeter and or screw are not accurate
Collimation
Telescope isn’t lined up
Centering error**
Optical error mirror and shade**
Adjustable error on the sextant
Perpendicularity Error:
This occurs when the index mirror is not perpendicular to the plane of the sextant.
To correct it, adjust the screw at the rear of the index mirror until the true and reflected arcs align3.
**Side Error: **
This happens when the horizon mirror is not perpendicular to the plane of the sextant.
To correct it, adjust the center screw at the back of the horizon mirror until the true and reflected horizons align3.
Index Error:
This occurs when the index mirror and horizon mirror are not parallel to each other when the index arm is set at zero.
To correct it, adjust the screw nearest to the plane of the instrument until the true and reflected images
Parts of a sextant
Parts of a compass
Perminant Magnetism
Vertical permanent heeling magnet
Fore-and-aft permanent magnets
Athwartship permanent magnets
Induced
Vertical soft iron flinders bar
Soft iron balls, kelvin balls
Way to check a compass
Transit
Amplitude of the sun
Azimuth of a heavily body
How often should you check the compass?
Once a watch or every major change of course
How do you do the calculation for azimuth of a heavenly body
- Take a bearing of the sun
- Find the GHA and DEC for that time
- Find the local LHA = GHA + LHA (Almanac)
- GO to the SRT with LHA, LAT, DEC
- Find the true bearing of the sun
Compare calculation to ships compass that is the error.
GHA = Grenich hour angle
DEC = Declination
LHA = Local hour angel
SRT = Sight reduction table
Amplitude of the sun calculation
- Take a bearing and the time when the sun is half the diameter on the horizion .
- Note the Lat, and time (Alminanc) will give decilantion.
- Dec + Lat in the (SRT) will give True bearing
Compass error is the difference.
SRT sight reduction table
How do you check your echo sounder
**Lead and line **
Throw it over the side roughly where the transducer is and compare it to your reading making sure to check the setting and see if it is set to depth below keel, transducer or from waterline.
Dipping distance
- Take a bearing of a object with a known hight as it goes over the horizion
- In Nories nautical table calculate the high.
- Add then your height of eye calculation
- That will be the range then plot on the chart for a fix.
Horizontal sextant angle
Measure the angel between three points on land ABC
you need the angel between A&B as well as B&C
using a station pointer you then mark it out and your position on a chart.
Horizontal sextant angle no station pointer
Measure the angel between three points on land ABC
you need the angel between A&B as well as B&C
Say A to B was 50degrees and b to C was 40 degrees
90 - 50 = 40 degrees if you measure off 40 degrees from both and B then where the two points mees that would be the center of the circle
Do the same for B and C
90 - 40 = 50
where the points of b and c meet is the center of that circle.
**Where the two circles cross that is you position. **
Vertical sextant angle
VSA
- Take a bearing and angel to top of a known object
- Note time
- Go to Norris tables and look height of object table (range by vertical sextant angle)
- Mark on the chart and that will be closest point to danger.
- Adding height of eye and tide will take us further from the object and tide.
- If asked to include it then i would calculate my poisiton below MHWS and then add it to the hieght of the object.
- index error on the sextant need to be included
- Tables are calculated with a hight of eye of 9m
Running fix
- Take a bearing of an object
- the do an EP adding for leeway and tide aft a hour
- take another bearing of the same object
- move the bearing line through out EP position and where the two bearing lines cross that is our position.
- You can then work backward to find you starting position
