11

The SextantThe Sextant

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Junior NavigationJunior NavigationChapter 2Chapter 2

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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.

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Practical Exercises:

Problems 1 through 5

Follow the Student Manual for guidance

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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.

directreflected

REF: ¶ 9

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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'

REF: ¶ 16 - 18

Figure 2-11

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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'

REF: ¶ 16 - 18

Figure 2-12

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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: ¶ 25 - 27 Figure 2-13

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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)?

REF: ¶ 25 - 30

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."

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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: ¶ 25 - 31

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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)?

REF: ¶ 25 - 31

IE = +3.3'

IC = -3.3'

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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: ¶ 25 - 30

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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

altitudea. 28°15.8' +7.3'

28°23.1'b. 47°27.3'  -2.5' 47°24.8'c. 46°27.2'  -3.2'  46°24.0'd. 18°59.8'  +2.2'  19°02.0'

REF: ¶ 30 - 32

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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

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16. When should you use shade glasses to observe the sun?

You should always use shade glasses to observe the sun.

REF: ¶ 38 - 40

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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

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18. The purpose of "swinging the arc" when using a sextant is to:

a. 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

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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

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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

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21. Which of the following would have the least effect on the accuracy of a celestial LOP?

a. six-foot to eight-foot beam seas

b. a haze on the horizon

c. observing the upper limb of the sun rather than the lower limb

d. missing the correct time by 5 seconds

REF: ¶ 57, 58

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22. In the following runs of sights, which sights are more likely to be bad?  

a. Time hs b. Time hs c. Time

hs07-14-32 35°12.4' 18-58-22 43°27.8' 18-09-58

35°22.8'07-15-41 35°22.7' 19-00-16 43°12.2' 18-11-06

35°06.6'07-16-59 35°01.3' 19-01-40 43°07.8' 18-12-09

34°49.2'07-18-12 35°44.2' 19-03-02 43°03.4' 18-13-16

34°44.6'07-19-20 35°54.3' 19-04-22 42°58.5' 18-14-26

34°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

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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

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The SextantThe Sextant

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