Can you identify the following parts to this sextant?
i) Index Glass, ii) Horizon Glass, iii) Index bar, iv) Arc, v) Clamp, vi) Micrometer, vii) Telescope, viii) 1st Adjustment Screw, ix) 2nd Adjustment Screw, x) 3rd Adjustment Screw.
Did you identify the parts correctly?
Causes and correction of three adjustable errors are:
a) Side Error:
It is caused by the horizon glass not being perpendicular to the plane of the instrument.
To check the side error, by day, clamp the index bar at zero, hold the sextant horizontally and observe the horizon through the telescope. If the true horizon and its reflection in the mirror half of the horizon glass, appear in alignment, side error is not present. If they do not side error exists. It can be removed by turning the second adjustment screw until the true and reflected horizons appear in the same line.
To do this at night, clamp the index bar at zero and holding the sextant vertically, observe a star through the telescope. If the star and its reflection are not displaced horizontally, side error is absent. If they are displaced horizontally, the error exists and can be eliminated by adjusting the 2nd adjustment screw till there is no horizontal displacement between them. (2 marks for causes and 2 marks for correction)
b) Index Error:
It is caused by the index glass and the horizon glass not being parallel to each other, when the index bar is at zero.
To find the index error, by day, using the horizon, clamp the index bar at zero and holding the sextant vertically, view the horizon through the telescope. If the true horizon and its reflection appear in the same line, Index error is not present. If they appear displaced vertically, turn the micrometer drum till they are in the same line. The micrometer reading then is the index error, which is on the arc if the micrometer reading is more than zero and off the arc if it is less than zero.
To eliminate index error, clamp the index bar at zero and looking through the telescope, turn the third adjustment screw, till the true horizon and its reflection appear in alignment.
c) Error of perpendicularity:
It is produced by the index glass not being perpendicular the plane of sextant. To check for this error, clamp the index bar at about the middle of the arc, and hold the sextant horizontally with the arc away from the observer. Look obliquely into the index mirror till the arc of the sextant and its reflection in the index mirror, are seen simultaneously.
If they appear in alignment, error of perpendicularity is not present.
If not in alignment, turn the first adjustment screw at the back of the Index glass,
A useful video talks about the reading of a sextant. NB. This does not cove errors or reading off the arc.
There is also an animated model of a sextant here – Sextant