Navigation and Seamanship

2.2.4. Error of sextant

There are two types of errors.

1) Adjustable errors.

2) Non-adjustable errors.

Adjustable errors are of four types.

i) Error of perpendicularity :

If the index mirror is not perpendicular to the plane of sextant the error is found and it is called error of perpendicularity.

To find out this error, hold the sextant horizontally face upwards and arc away. Move the index bar along the arc looking through the index mirror, till the true and reflected are seen at the same time. If they are not in continuous line, there is error. This error can be adjusted by turning the small screws behind the index mirror.

ii) Side error :

If the horizon glass is not perpendicular to the plan of the sextant, this error is formed. To find out the error, hold the sextant horizontally clamping the index bar at 0^o. Look at the horizon through the telescope. If the true horizon and the reflected horizon are seen in a continuous line, there is a side error, if they are not a line, there is side error. This can also be adjusted by turning the upper screw behind the horizon glass.

iii) Index error : 

When the index arm is at zero mark, the index mirror and horizon glass should be parallel to each other. If they are not parallel, the error is found and it is called index error.

To find out this error, hold the sextant vertically keeping the index bar at zero mark and look at the horizon. If the true and the reflected horizon are in a continuous line, there is no error. If not, this can be adjusted by turning the tangent screw till they are in a line. The reading at that time is the index error. this can be adjusted by turning the lower screw behind the horizon glass. If the index error is on the arc, it should be subtracted and if it is off the arc, it should be added to all reading taken by that section.

iv) Collimation error :

When the telescope is screwed in place, its longitudinal axis should be parallel to the plane of the sextant. If not, the error is found called Collimation error.