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NAV 4 > Sextant > Flashcards

Sextant Flashcards

1
Q

an instrument used to measure the angle between any two visible objects

A

Sextant

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2
Q

Primary Use of Sextant

A

to determine the angle between a celestial object and the horizon

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3
Q

Common Uses of Sextant

A

include sighting the sun at solar noon and sighting Polaris at night, to find
one’s latitude (in northern latitudes)

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4
Q

Sextant comes from what Latin word

A

Sextans

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5
Q

The Latin “Sextans” means what

A

the sixth part

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6
Q

Why was sextant named sextant?

A

It is named like this because its arc approximates one-sixth of a circle

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7
Q

the angular height of an object, above the visible horizon or as read from the arc of a sextant, before correction for index error

A

Sextant Altitude

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8
Q

angle between a celestial object and the horizon

A

Object’s Altitude

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9
Q

the angular height of an object above the visible horizon as read from the arch of the sextant and correction for index error

A

Object’s Altitude

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10
Q

an essential part of celestial navigation used as a term for making the measurement of an object’s altitude

A

Sighting/Shooting the object/Taking a sight

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11
Q

normally made of brass but some “lightweight” models are of aluminum alloy on which other parts are mounted

A

Frame

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12
Q

mounted with its axis parallel to the plane of the frame

A

Telescope

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13
Q

mounted on the upper end of the index arm directly over its pivot point

A

Index Mirror

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14
Q

are of optically ground glass mounted perpendicular to the arc, and are pivoted so that they can be swung into or out of line of sight between the index and the horizon
mirrors

A

Index Shade Glasses

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15
Q

– mounted on the frame
– also perpendicular to the plane of the instrument when the index arm is set to exactly at 0°
– parallel to the index mirror

A

Horizon Glass

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16
Q

similar to the index shades, but of lesser density, and serve to reduce the glare of reflected sunlight on the horizon

A

Horizon Shade Glasses

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17
Q

indicates the number of degrees of an angle

A

Graduated Arc

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18
Q

the lower part of the frame and carries the arc which is graduated in degrees

A

Limb

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19
Q

– used to make fine adjustments when measuring angles and indicates minutes of a degree of angle
– attached to the lower end of the index arm

A

Micrometer Drum

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20
Q

How many graduations does a micrometer drum have and what do they represent?

A

60, each representing 1’ of arc

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21
Q

engages the worm teeth cut into the limb, and one full turn of the drum
moves the index drum by one-half degree on the arc

A

Tangent Screw

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22
Q

graduated into ten parts, permitting readings to 1/10 of a minute of arc (0.1’)

A

Vernier

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23
Q

spring-loaded clamps that hold the tangent screw against the teeth of the limb

A

Clamping Lever or Release Levers

24
Q

– pivots at one end to allow the attached index mirror to reflect an object onto the horizon glass and swings along the arc scale on the other end to indicate what the angle measures
– a type of ruler that determines direction or measures an angle

A

Index Arm

25
Q

mounted on the frame at a location and angle for good balance and easy grip with the right hand

A

Handle

26
Q

produced by the index glass not being perpendicular to the plane of the instrument

A

Perpendicularity Error

27
Q

How many seconds equates to an error of a nautical mile in the position when using a sextant?

A

4

28
Q

caused by the horizon glass not being perpendicular to the plane of the instrument

A

Side Error

29
Q

How to check for perpendicularity error?

A

turn the first adjustment screw at the back of the index glass, until they appear in alignment

30
Q

due to the axis of the telescope not being parallel to the plane of the instrument

A

Collimation Error

31
Q

How to remove side error

A

It can be removed by turning the second adjustment screw until the true and reflected horizons appear on the same line

32
Q

the error remaining after the navigator has removed perpendicularity error, side error, and collimation error

A

Index Error

33
Q

occurs when the faces of the shade glasses and mirrors are not parallel

A

Prismatic Error

34
Q

error due to lack of parallelism in the shade glasses

A

Shade Error

35
Q

occurs in the arc, micrometer drum, and Vernier of a sextant which is improperly cut or incorrectly calibrated

A

Graduation Error

36
Q

results if the index arm does not pivot at the exact center of the arc’s curvature

A

Centering Error

37
Q

must be corrected for errors of the instrument and observer, and other corrections depending on the celestial body being observed

A

Sextant Altitude

38
Q

the Hs value corrected to read as though the altitude had been measured with reference to the celestial horizon at the earth’s center, on a perpendicular plane passing through the observer’s zenith and the body

A

Observed Altitude (Ho

39
Q

a plane which passes through the observer’s eye and is at right angles to the vertical of the observer

A

Sensible Horizon

40
Q

plane is perpendicular to the true vertical and is tangent to earth’s surface

A

Geoidal Horizon

41
Q

– also called apparent horizon
– the circle which bounds the observer’s view of the earth’s surface in a clear atmosphere
– its range will depend upon the refractive index of the air and the observer’s height of eye

A

Visible Horizon

42
Q

plane from the eye of the observer and tangents to earth’s surface

A

Geometrical Horizon

43
Q

– the plane which passes through the center of the earth and is at right angles to the observer’s vertical and therefore parallel to the sensible horizon
– also called rational horizon

A

Celestial Horizon

44
Q

– coordinates sued to define a position on the celestial sphere with reference to an observer on the earth

A

Altitude & Azimuth

45
Q

Measured with a sextant

A

Altitude

46
Q

Measured with a compass

A

Azimuth

47
Q

– an instrument error of the sextant
– occurs when the index and horizon mirrors gets lightly out of adjustment

A

Index Error

48
Q

Sextant reading is zero, what is the error?

A

No error

49
Q

Sextant reading is not zero, what is the error?

A

the amount of error is the index correction

50
Q

index mark is to the left of the zero (on the arc) of the limb, what is the reading?

A

the reading is too large

51
Q

index mark is to the right of the zero (off the arc)

A

the reading is too low

52
Q

– an error due to the fact that the horizon is not level with the height of the eye above sea level
– varies in amount according to the height of eye and is always subtractive

A

Dip/Height of Dip

53
Q

The altitude of a body’s center above the rational horizon is the quantity required in navigational calculation. We cannot locate the center of the sun or moon (left image) , so the altitude of the edge nearest the horizon is measured and the semi-diameter as given in the Nautical Almanac is added to the altitude observed with the sextant.

A

Semi-Diameter Correction

54
Q

– refers to the change in direction of a wave due to a change in velocity in the wave as it passes between mediums of different refractive indexes
– this error is the same error as why the sun appears just on the horizon when in reality it has already dipped below the horizon
– the bending of light rays as it passes through the atmosphere
– makes the reading larger than what it actually is

A

Refraction

55
Q

– the change in direction of an object as seen from two different positions
– caused by the proximity of bodies of the solar system to the earth resulting in a difference in altitudes measured from the surface of the earth and from the center of the earth

A

Parallax Correction

NAV 4 (6 decks)

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