Sextant

Question 1

What is the principle that the sextant works on?

Answer 1

The sextant works on the principle that the angle between the mirrors is half the angle between the first ray of light entering the sextant, and the last ray of light leaving it.

Question 2

How is the sextant used to take a reading from the sun?

Answer 2

Set the sextant to zero, and point it at the sun.
By moving the index arm, point the sextant back down to the horizon until both the sun and the horizon can be seen in the horizon mirror.
Then adjust with the vernier micrometer until the lower limb of the sun is just touching the horizon.
Record the precise time and take a reading from the sextant.

Question 3

How is the sextant used to take a reading from a star?

Answer 3

Find the approximate angle of the star in a data table, and set the sextant to that angle.
Point the sextant at the horizon, the star should appear in the horizon mirror.
Adjust until the star is on the horizon, then take a reading.

Question 4

What is off the arc', and how is reading a value off the arc’ different?

Answer 4

`off the arc’ is the scale that is below zero on the sextant.
Off the arc readings are only really used for checking errors.
When reading off the arc it is important to bear in mind that the micrometer has to be subtracted to find the minutes and decimals.

Question 5

How does one ensure the time of the reading is accurate to the second?

Answer 5

When you take the reading start counting the seconds until you can check the time. Then subtract these seconds from the time to get the time you took the reading.

Question 6

How do you read the vernier?

Answer 6

Whichever line on the vernier lines up with a line of the micrometer is the reading that should be taken. If none line up, take the value between the two lines that are displaced the same amount.

Question 7

What are the three adjustable errors and what causes them?

Answer 7

1. Perpendicularity. - The index mirror must be perpendicular to the plane of the instrument.
2. Side error - The horizon mirror must be perpendicular to the plane of the instrument.
3. Parallelism - The two mirrors must be parallel when the sextant is set at zero.

Question 8

How do you check for an error of perpendicularity?

Answer 8

Set the index arm roughly in the middle of the arc, hold the sextant horizontally and view the arc directly. The arc should line up with the reflected arc in the index mirror.

Question 9

How do you fix an error of perpendicularity?

Answer 9

Using the first adjustment screw on the back of the index mirror. Make sure to try tightening the screw before you loosen it!

Question 10

How do you check for side error?

Answer 10

Set the index arm at zero and view the horizon with the sextant tilted at about 45 degrees. The true and reflected horizons should be continuous.
or
Set the index arm at zero, and vertically view a distant source of light (star). There should not be a horizontally displaced double image. If there is, error is present.

Question 11

How do you fix side error?

Answer 11

Using the second adjustment screw on the back of the horizon mirror furthest from the plane of the instrument.

Question 12

How do you check for an error of parallelism?

Answer 12

Set the index arm to zero and vertically view the horizon. The true and reflected horizons should be continuous.
or
Set the index arm at zero and view a distant source of light (star). There should be no vertically displaced double image.

Question 13

How do you fix an error of parallelism?

Answer 13

Using the third adjustment screw on the back of the horizon mirror closest to the the plane of the instrument.

Question 14

What can happen when an error of parallelism is fixed?

Answer 14

Fixing an error of parallelism can cause side error. So, when parallelism has been fixed, side error must be checked again, until both errors are not present.

Question 15

How do you check for the removal of adjustable errors?

Answer 15

Set the sextant at zero and view the sun. Move the index arm until you see two images of the sun that just touch. Take this reading. Do this on the arc and off the arc. add these two readings and divide by four to get the semi diameter of the sun. If the diameter is the same as the diameter in the nautical almanac, there is no adjustable error.

Question 16

What is sometimes wrongly called index error?

Answer 16

Errors of parallelism.

Question 17

What is index error?

Answer 17

Index error is any error that is left when all the adjustable errors have been removed.

Question 18

List the six non-adjustable errors and describe them.

Answer 18

Collimation - The telescope must be parallel to the plane of the instrument.
Shade error - Imperfections or poor manufacture of shades. Faces should be flat and parallel.
Prism error- Imperfections in manufacture of mirrors and telescope lenses
Centring error - the pivot of the index arm must be central to the arc
Vernier error - poor manufacture of the vernier can lead to inaccurate readings.
Graduation/scale error - poor manufacture or wear of worm and rack can produce errors.

Question 19

How can you determine the index error?

Answer 19

By looking at the calibration/inspection certificate.
or
By setting the sextant at zero and viewing the sun. Adjust the index arm until you can see two images of the sun, just touching. Take this reading and take another reading off the arc. Whichever reading is bigger, is which way the error is. Divide the difference by two to find the error.