1. Junior Navigation Chapter 2
The Sextant
Homework
Q & A
Junior Navigation
Chapter 2

2. ■ Identify the parts of a sextant and understand how a sextant works. 6
Objectives:
■ Identify the parts of a sextant and understand how a sextant works.
■ Read the angle measured by a sextant.
■ Determine index error in a sextant.
■ Determine index corrections and apply them to sextant altitudes.
■ Describe how to handle, maintain, and stow a sextant.
■ Prepare for taking your practice sights.
■ Demonstrate and describe techniques for taking sights on the sun.
■ Describe safety precautions and methods for taking sights from a boat at sea.
■ Record the time of a sight.
■ Identify the ideal and practical accuracy limits of celestial navigation, and factors affecting accuracy.
■ Identify sights that may be erroneous in a run of sights.
■ Describe basic sight requirements for this course.

3. Follow the Student Manual for guidance6
Practical Exercises:
Problems 1 through 5
Follow the Student Manual for guidance

4. direct reflected REF: ¶ 96
6. If the sextant is set to 0°00.0', and you aim the sextant telescope at the horizon, you see both ______ and ________ images of the horizon through the telescope.
direct
reflected
REF: ¶ 9

5. 7. What is the sextant reading in Figure 2-11? a. 50°00.2' b. 50°00.8'
c. 50°07.0'
d. 50°09.7'
Solution:
The mark on the arm is close to 50°. The zero mark on the vernier is slightly below (higher reading) the 0 mark on the drum, so the minutes will be between 00' and 01' and the solution will be between 50° and 51°. The tenths of minutes is the value of the vernier line where its extension is continuous onto the drum, which is 8, so the sextant reading is 50°00.8', which is answer b.
Figure 2-11
REF: ¶

6. 8. What is the sextant reading in Figure 2-12? a. 25°50.2' b. 25°53.4'
c. 25°57.4'
d. 26°02.4'
Solution:
The mark on the arm is between 25° and 26°. The 0 mark on the vernier is just above 53' and the vernier is a continuous line at 4', so the reading is 25°53.4', which is answer b.
Figure 2-12
REF: ¶

7. b. Which view indicates side error?9
9. In the telescope views diagrammed in Figures 2-13a and Figures 2-13b, the sextant is set to 0°00.0' and aimed at the sun. a. Which view indicates index error?
b. Which view indicates side error?
Figure 2-13a indicates that index error is present.
Figure 2-13b indicates that side error is present.
REF: ¶
Figure 2-13

8. b. What is the index correction (IC)?10
10. When the two images of the sun are brought into coincidence, the reading on the limb is - 1° (off the arc) and the reading on the micrometer drum is 57.8'. a. What is the index error (IE)?
b. What is the index correction (IC)?
IE = -60' +57.8' = -2.2'
IC = +2.2'
Solution:
Sextant reading is -1 ° +57.8'. The index mark on the arm will be slightly below 0°, which is "Off the arc."
REF: ¶

9. a. What is the index error (IE)?11
11. When the two images of the sun are brought into coincidence, the sextant reading is 0° +1.7' (on the arc).
a. What is the index error (IE)?
b. What is the index correction (IC)?
IE = +1.7'
IC = -1.7'
REF: ¶

10. a. What is the index error (IE)?12
12. When the horizon is brought into continuity, the sextant reading is 0° +3.3' (on the arc).
a. What is the index error (IE)?
b. What is the index correction (IC)?
IE = +3.3'
IC = -3.3'
REF: ¶

11. b. What is the index correction (IC)?13
13. When the horizon is brought into continuity, the sextant reading is - 1° (off the arc) and the reading on the micrometer drum is + 1.5'. a. What is the index error (IE)?
b. What is the index correction (IC)?
IE = -60' + 1.5' = -58.5'
IC = -IE = -(-58.5') = +58.5'
REF: ¶

12. 14
14. For the following sight data, compute the corrected sextant altitude. a. hs 28°15.8' IC +7.3' b. hs 47°27.3' IC -2.5' c. hs 46°27.2' IE 3.2' on the arc
d. hs 18°59.8' IE 2.2' off the arc
Solution:
corrected
hs IC sextant altitude
a °15.8' +7.3' °23.1'
b °27.3'  ' 47°24.8'
c °27.2'  '  46°24.0'
d °59.8'  +2.2'  19°02.0'
REF: ¶

13. 15. To care for your sextant properly you should:
a. scatter a half inch of silica gel in the bottom of the sextant box.
b. always put on gloves before handling the mirrors.
c. keep the brass arc bright with emery cloth
d. use fresh water on a swab to remove salt from the mirrors.
REF: ¶ 34

14. 16. When should you use shade glasses to observe the sun?
You should always use shade glasses to observe the sun.
REF: ¶

15. 17. Why would a navigator take an upper-limb observation of the sun?
When the lower limb is obscured by cloud cover.
REF: ¶ 40 - Part 4

16. 18. The purpose of "swinging the arc" when using a sextant is to:
observe as wide an arc of the horizon as possible.
b. make certain that the sextant is vertical.
c. obtain the largest possible altitude reading.
d. improve the chances of seeing the celestial body.
REF: ¶ 40 - Part 5

17. a. turn the boat's bow into the waves. 19
19. The most important rule to observe when taking sights from a small vessel at sea is always:
a. turn the boat's bow into the waves.
b. sit in the bottom of the cockpit so you won't fall overboard.
c. wear a safety harness.
d. remove the telescope from the sextant.
REF: ¶ 46

18. To enable the observer to see over wave crests.20
20. For greatest accuracy in a sight, where would a large height of eye be preferable -on a hazy horizon or on a rough sea?
On rough seas
Why?
To enable the observer to see over wave crests.
REF: ¶ 47

19. six-foot to eight-foot beam seas a haze on the horizon 21
21. Which of the following would have the least effect on the accuracy of a celestial LOP?
six-foot to eight-foot beam seas
a haze on the horizon
observing the upper limb of the sun rather than the lower limb
d. missing the correct time by 5 seconds
REF: ¶ 57, 58

20. a. The third sight in this run is inconsistent and probably bad 22
22. In the following runs of sights, which sights are more likely to be bad?  
a. Time hs b. Time hs c. Time hs
°12.4' °27.8' °22.8'
°22.7' °12.2' °06.6'
°01.3' °07.8' °49.2'
°44.2' °03.4' °44.6'
°54.3' °58.5' °16.6'
a. The third sight in this run is inconsistent and probably bad
b. The first sight in this run is inconsistent and probably bad
c. The fourth sight in this run is inconsistent and probably bad
REF: ¶ 60, 61, & Figure 2-9 in SM

21. 23
23. In question 22, which body is east of you and which is west of you?
In run 22a, the sights are increasing in altitude, so the body is east of you.
In runs 22b and 22c, the sights are decreasing in altitude, so the bodies are west of you.
REF: ¶ 60, & Figure 2-9 in SM

22. End of Homework Q & A Junior Navigation Chapter 2
The Sextant
End of
Homework
Q & A
Junior Navigation
Chapter 2