The Sextant

Question 1

What is a sextant?

Answer 1

The sextant can be used to measure the angle between the horizon and a visible object or when used horizontally the angle between two objects

Question 2

What can the sextant be used for?

Answer 2

1) Measure the altitude of a celestial body
2) Horizontal sextant angle
3) Vertical sextant angle

Question 3

What are the fixable errors?

Answer 3

• Perpendicular error = The index mirror not perpendicular to the plane of the instrument. To correct you must look through the index mirror and the reflected image and make sure they are aligned and if not adjust the index mirror screw.
• Side error = The horizon mirror not perpendicular to the plane of the instrument. To correct you adjust so that the reflected and direct images of the horizon appear as a continuous line, if they not in a line adjust the two screws behind the horizon mirror.
• Index error = The index mirror and horizon mirror not parallel to each other. To correct you need to set the instrument to zero and adjust so that both images of the horizon come into a line, the reading is the error. A positive IE should be subtracted and a negative IE should be added.

Question 4

What are the non-correctable errors?

Answer 4

1 - Prismatic = two faces of the mirror not parallel to each other
2 - Worm and rack = caused by wear on the gearing rack
3 - Graduation = may be found on the arc, vernier or micrometer scale
4 - Shade error = faces of the shades not parallel to plane of the sextant
5 - Collimation = axis of the telescope not parallel to the plane of the sextant
6 - Centering = pivot of the index bar is not in the exact centre of the curvature of the arc