Self Assessment

Self Assessment No. 3 – § 09 – 13


51. Why is a sextant called an “instrument of double reflection?”

52. Explain the principle of the sextant.

53. Where is the sextant certificate (if any) located, what types of Sextant Certificates are there and what is the value of each?

54. What are the following readings on the micrometer sextant? (The upper half of each of the figures below represents a magnified view of the main scale on the arc of a vernier sextant, while the lower half is the vernier scale. In each case state the reading of the sextant).


a.  40º 20’ 30”  ON the arc

b.  00º 20’  OFF the arc

c.  62º 40’ 20” ON the arc

d.  01º 12’ 6” OFF the arc or 01º 12’ 24”







e. 60º (10’ + 4’) = 60º 14’ ON the arc

f. 40º 10’ 20”   ON the arc

g. 32º 20’ (+4’ 20”) = 32º 24’ 24”ON the arc

h. 01º 20’ (+1º 20”) = 01º 28’ 40”OFF the arc





55. Explain the three adjustable errors to which a sextant may be subject, and how each of these errors may be corrected.

56. If the Index Error is reasonably small, why is it preferable not to adjust the sextant for it at all, but to allow for it when measuring altitudes?

57. Describe the three methods of determining Index Error

58. Explain how to take a Sextant Altitude of the Sun

59. Explain how to take a Sextant Altitude of a Star or Planet

60. Draw a diagram indicating the three horizons, and define each of the four altitudes, explain the relationship of each altitude to each horizon.

61. Briefly explain the purpose of each individual correction necessary to convert a sextant altitude to a true altitude.

62. Using the Altitude Correction Tables on Page 11 of the ANC pamphlet convert each of the following sextant altitudes to true altitudes, showing all the relevant factors involved – I.E., Obs Alt, Dip,  Apparent Altitude, main and additional corrections:-

Body Observed Sextant Altitude Index Error Height of Eye Date

a. Sun L.L.  35º 15.5’ +03.1  15 FT  05TH Jan

b. Sun U.L. 44º 16.8’ -01.5  10 FT  30TH Sep

c. ALDEBARAN 58º 11.2’ -02.0’  12 FT  01ST Jul

d. VENUS ♀ 46º 12.3’ +01.7’  18 FT  15TH Mar

e. MARS ♂ 29º 14.9’ -00.7’  09 FT  06TH May





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