The Sextant Flashcards
What is a Sextant?
An angle measuring device
What are the Fixed Errors of a Sextant?
Graduation Error
Collimation Error
Prismatic (shade) Error
Centring Error
Instrument Error
Adjustable Error: Perpendicularity Error
A perpendicular error is when the Index Mirror is not perpendicular to the plane of the
instrument.
To check, turn the sextant round, hold it 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
Adjustable Error: Side Error
A 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 sextant error exists. To correct this situation, Adjust the screw furthest from the frame at the back of the Horizon mirror until no step is seen.
Adjustable Error: Index Error
An 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.
Adjusting Index Error:
a) turn the screw at the top of the mirror and then re-check for side error and
again for index error.
or
b) turn the micrometer and take the sextant reading and apply it to the Sextant
Altitude. (If it off, it is on: if it is on, it is off.)
Fixed Error: Graduation Error
A graduation error occurs when the graduations on the arc, micrometer and/or the
screw thread are not cut accurately.
Fixed Error: Centering Error
A centering error will occur when the pivot of the Index Bar is not at the center of the
circle of which the arc forms a part.
Fixed Error: Collimation Error
Collimation errors happen when the telescope is not parallel with the plane of the
instrument.
Fixed Error: Prismatic Error (Shade Error)
An optical error is when the shades are not optically flat
Where do we find the Tables for Range through VSA?
Norries
Where do we find declination and other celestial navigational tables?
Nautical Almanac
Once we have adjusted our sextant for Errors, which error will we check daily on passage?
Index Error, Via the micrometer.
Ensure it matches the expected error
