Celestial

Question 1

What are the fixed errors of a sextant?

Answer 1

1. Graduation Error
   Is when the graduations on the arc, micrometer and / or the screw threat are not cut accurately
2. Collimation Error
   Is when the telescope is not parallel with the plane of the instrument
3. Centering Error
   Is when the pivot of the Index Bar is not at the centre of the circle of which the arc forms a part
4. Shade Error
   Is when the shades are not optically flat

Question 2

What are the adjustable errors on a sextant?

Answer 2

P.S.I

1. Perpindicularity Error
   Is when the Index mirror is not perpendicular to the plane of the instrument. To check turn the sextant round, hold is horizontal and look into the index mirror. If the true and reflected arcs do not line up then the error exists. To adjust for this error turn the screw at the top of the mirror
2. Side Error
   Is when the horizon mirror is not perpendicular with the plane of the instrument. To check for this set the arc to zero, hold the sextant on the side and look through the telescope at the distant horizon. If the true and reflected horizons do not line up then the side error exists. To adjust for this error turn the screw at the tops of the mirror.
3. Index Error
   Is when the index and horizon mirrors are not parallel with each other when the arc is set to zero. To check for this, set the arc to zero, hold the sextant vertical and look at the distant horizon. If the true and reflected horizons do not line up, the index error exists. To adjust for this error you can either:
   a) turn the screw at the top of the mirror and then re-check for side error and for indext error
   or
   b) turn the micrometer until the horizons are level. Make a note of the micrometer and use this figure in your calculations. On / Off the arc.

Question 3

Name all the parts of the Sextant

Answer 3

Question 4

Compass Checks

Answer 4

1. Transit - comparing the true transit of two charted objects against the observed compass bearing of the same objects. The difference between the two bearings is the compass error.
2. Amplitude of the sun at sunrise or sunset.
   -Take a bearing of the sun rising/setting, a semi-diameter above the horizon (due to the refraction in the atmosphere)
   -Find the Dec of the sun for that day (Nautical Almanac)
   -Note our latitude
   -Enter Norries, use the correct table for true bearing of sun (True Azimuth at rising or setting)
   -Compare against the sun’s compass (gyro/magentic) bearing
3. Azimuth of a heavenly body
   -Take a bearing of the sun, note the time
   - Find the GHA and the Dec of the sun for that time
   - Work through chosen longitude E or W and GHA to calculate LHA
   - Go to the Sight Reduction Tables with the LHA, Lat and declination
   - Extract the Azimuth (True Bearing) of the sun
   - Compare Azimuth of the sun against your gyro/compass bearing

Question 5

Celestial Position Fixing

Answer 5

MerPass - Meridian Passage at Noon (Given in Nautical Almanac)

Measure the angle of the sun at noon, note the time.

Combine Dec with sextant reading and this will give latitude.

By working out out how far E or W of GMT - you can work out Long.

SunRunSun - two lines of position. Position line advanced along a ship’s track and crossed with a later observations