Sextant

Question 1

What are the fixed errors of a sextant?

Answer 1

• Graduation error
• Collimation error
• Centring error
• Shade error

Question 2

What are the adjustable errors of a sextant?

Answer 2

• Perpendicularly error
• Side error
• Index error

Question 3

What is graduation error?

Answer 3

Fixed error - When graduations on the arc, micrometer and/or the screw thread are not cut accurately.

Question 4

What is collimation error?

Answer 4

Fixed error - When the telescope is not parallel with the plane of the instrument. Depending on the sextant, can be adjustable or fixed error.

Question 5

What is centring error?

Answer 5

Fixed error - When the pivot of the index bar is not at the centre of the circle of which the arcs forms a part.

Question 6

What is shade error?

Answer 6

Fixed error - When the shades are not optically flat.

Question 7

What is perpendicularly error?

Answer 7

Adjustable error - When the Index mirror is not perpendicular with 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.

Question 8

What is side error?

Answer 8

Adjustable error - When the horizon mirror is not perpendicular with the plane of the instrument. To check 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 adjust for this error, turn the screw at the top of the mirror.

Question 9

What is index error?

Answer 9

Adjustable error - 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 recheck for side error and again for index error
b) turn the micrometer and take the sextant reading and apply it to the sextant altitude

Question 10

What is a sextant?

Answer 10

Measure the angle between an astronomical object and the horizon for the purposes of celestial navigation.