Midterm

Question 1

– 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)

Answer 1

Sextant

Question 2

Latin word Sextant comes from

Answer 2

Sextans

Question 3

Latin word “sextans” meaning in English

Answer 3

The Sixth Part

Question 4

Reason why Sextant is its name

Answer 4

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

Question 5

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

Answer 5

Sextant Altitude

Question 6

– 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

Answer 6

Object’s Altitude

Question 7

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

Answer 7

Sighting/Shooting the object/Taking a sight

Question 8

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

Answer 8

Frame

Question 9

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

Answer 9

Telescope

Question 10

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

Answer 10

Index Mirror

Question 11

– 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

Answer 11

Index Shade Glasses

Question 12

– 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

Answer 12

Horizon Glass

Question 13

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

Answer 13

Horizon Shade Glasses

Question 14

indicates the number of degrees of an angle

Answer 14

Graduated Arc

Question 15

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

Answer 15

Limb

Question 16

– 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

Answer 16

Micrometer Drum

Question 17

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

Answer 17

Tangent Screw

Question 18

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

Answer 18

Vernier

Question 19

– 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

Answer 19

Clamping Lever or Release Levers

Question 20

– 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

Answer 20

Index Arm

Question 21

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

Answer 21

Handle

Question 22

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

Answer 22

Perpendicularity Error

Question 23

If Perpendicularity Error is present what should be done

Answer 23

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

Question 24

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

Answer 24

Side Error

Question 25

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?

Answer 25

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

Question 26

– 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

Answer 26

Collimation Error

Question 27

– 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

Answer 27

Index Error

Question 28

Non-Adjustable Errors

Answer 28

Prismatic Error Graduation Error Centering Error

Question 29

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

Answer 29

Prismatic Error

Question 30

error due to lack of parallelism in the shade glasses

Answer 30

Shade Error

Question 31

error due to lack of parallelism in the shade glasses

Answer 31

Shade Error

Question 32

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

Answer 32

Graduation Error

Question 33

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

Answer 33

Centering Error

Question 34

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

Answer 34

Sextant Altitude (Hs)

Question 35

– 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

Answer 35

Observed Altitude (Ho)

Question 36

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

Answer 36

Sensible Horizon

Question 37

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

Answer 37

Geoidal Horizon

Question 38

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

Answer 38

Geometrical Horizon

Question 39

– 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

Answer 39

Visible Horizon

Question 40

– 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

Answer 40

Celestial Horizon

Question 41

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

Answer 41

Altitude & Azimuth

Question 42

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

Answer 42

Index Error

Question 43

– 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

Answer 43

Dip/Height of Dip

Question 44

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

Answer 44

Semi-Diameter Correction

Question 45

– 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

Answer 45

Refraction

Question 46

– 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

Answer 46

Parallax Correction

Question 47

Where the Parallax Correction is published in the Nautical Almanac

Answer 47

Horizontal Parallax

Question 48

– 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

Answer 48

Polaris

Question 49

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

Answer 49

Intercept

Question 50

Three Parts consisting the 2102-D Star Finder

Answer 50

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

Question 51

brightest stars in all the sky

Answer 51

Magnitude One

Question 52

stars as bright as the Bgi Dipper stars

Answer 52

Magnitude Two

Question 53

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

Answer 53

Magnitude Three

Question 54

How many Magnitude One Stars are there and are navigational

Answer 54

20 and all are navigational stars

Question 55

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

Answer 55

75 and 30 are navigational

Question 56

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

Answer 56

200 and only 7 are navigational

Question 57

– 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

Answer 57

Nautical Almanac

Question 58

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

Answer 58

Norie’s Nautical Table

Question 59

– 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

Answer 59

L.A.N./Noon Sight