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NAV 4 > Midterm > Flashcards

Midterm Flashcards

1
Q

– an instrument used to measure the angle between any two visible objects
– its primary use is to determine the angle between a celestial object and the horizon
– common uses include sighting the sun at solar noon and sighting Polaris at night, to find one’s latitude (in northern latitudes)

A

Sextant

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2
Q

Latin word Sextant comes from

A

Sextans

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3
Q

Latin word “sextans” meaning in English

A

The Sixth Part

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4
Q

Reason why Sextant is its name

A

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

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5
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 if any

A

Sextant Altitude

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6
Q

– angle between a celestial object and the horizon
– 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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7
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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8
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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9
Q

– mounted with its axis parallel to the plane of the frame
– adjustable for the characteristics of the individual observer’s eye

A

Telescope

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10
Q

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

A

Index Mirror

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11
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
– employed when making observation of the sun

A

Index Shade Glasses

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12
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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13
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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14
Q

indicates the number of degrees of an angle

A

Graduated Arc

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15
Q

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

A

Limb

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16
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
– has 60 graduations, each representing 1’ of arc
– mounted on a shaft, having a pinion gear at the other end called tangent screw

A

Micrometer Drum

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17
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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18
Q

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

A

Vernier

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19
Q

– spring-loaded clamps that hold the tangent screw against the teeth of the limb
– when squeezed together, they release the tangent screw and allow the index arm to be move easily along the arc

A

Clamping Lever or Release Levers

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20
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

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21
Q

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

A

Handle

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22
Q

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

A

Perpendicularity Error

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23
Q

If Perpendicularity Error is present what should be done

A

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

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24
Q

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

A

Side Error

25
Q

If the true horizon and its reflection in the mirror half of the horizon glass does not appear in alignment and side error is present, what should be done?

A

turning the second adjustment screw until the true and reflected horizons appear on the same line

26
Q

– due to the axis of the telescope not being parallel to the plane of the instrument
– altitude measured will be greater than their actual values

A

Collimation Error

27
Q

– the error remaining after the navigator has removed perpendicularity error, side error, and collimation error
– caused by the index glass and the horizon glass not being parallel to each other, when the index bar is at zero

A

Index Error

28
Q

Non-Adjustable Errors

A

Prismatic Error
Graduation Error
Centering Error

29
Q

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

A

Prismatic Error

30
Q

error due to lack of parallelism in the shade glasses

A

Shade Error

31
Q

error due to lack of parallelism in the shade glasses

A

Shade Error

32
Q

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

A

Graduation Error

33
Q

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

A

Centering Error

34
Q

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

A

Sextant Altitude (Hs)

35
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
– the altitude used in all celestial navigation calculations

A

Observed Altitude (Ho)

36
Q

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

A

Sensible Horizon

37
Q

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

A

Geoidal Horizon

38
Q

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

A

Geometrical Horizon

39
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

40
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

41
Q

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

A

Altitude & Azimuth

42
Q

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

A

Index Error

43
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

44
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

45
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

46
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
– always added to the altitude reading since the reading obtained at the surface (apparent altitude) will always be lesser than the true altitude obtained from the center of the earth
– this error is published in the Almanac under the name “Horizontal Parallax” or HP

A

Parallax Correction

47
Q

Where the Parallax Correction is published in the Nautical Almanac

A

Horizontal Parallax

48
Q

– located in the northern sky
– part of the star constellation “Little Dipper” or properly known as Ursa Minor
– not usually seen until the sky has become quite dark
– no star is located exactly at either pole, but it is less than one degree from the north celestial pole
– alternately transits the upper and lower branches of each celestial meridian in completing its diurnal circle

A

Polaris

49
Q

– a measure of the distance from the assumed position to the line of position

A

Intercept

50
Q

Three Parts consisting the 2102-D Star Finder

A
  1. A rigid white plastic disk with stars printed on it, called the “star base” or “white disk”
  2. a set of 9 transparent templates with blue printing, which we call the “blue template “
  3. a special template with red printing
51
Q

brightest stars in all the sky

A

Magnitude One

52
Q

stars as bright as the Bgi Dipper stars

A

Magnitude Two

53
Q

stars not quite as bright as Big Dipper stars, but brighter than the stars on the handle of the Little Dipper

A

Magnitude Three

54
Q

How many Magnitude One Stars are there and are navigational

A

20 and all are navigational stars

55
Q

How many Magnitude Two Stars are there and how many are navigational?

A

75 and 30 are navigational

56
Q

How many Magnitude Three Stars are there and how many are navigational?

A

200 and only 7 are navigational

57
Q

– a publication describing the positions of a selection of celestial bodies for the purpose of enabling navigators to use celestial navigation to determine the position of their ship while at sea
– contains astronomical data for the entire year which assists the navigator in making astronomical calculations on ship

A

Nautical Almanac

58
Q

the essential requirement for anyone learning and practicing astronavigation published by John William Norie

A

Norie’s Nautical Table

59
Q

– a convenient and simple element in navigation dating back several centuries
– the time at which the sun is directly on your meridian of Longitude
– the highest point in the sky the sun will reach on a particular day at your location
– results in a line of position which coincides with the ship’s latitude

A

L.A.N./Noon Sight

NAV 4 (6 decks)

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