Midterm Flashcards
– 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)
Sextant
Latin word Sextant comes from
Sextans
Latin word “sextans” meaning in English
The Sixth Part
Reason why Sextant is its name
It is named like this because its arc approximates one-sixth of a circle
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
Sextant Altitude
– 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
Object’s Altitude
an essential part of celestial navigation used as a term for making the measurement of an object’s altitude
Sighting/Shooting the object/Taking a sight
normally made of brass but some “lightweight” models are of aluminum alloy on which other parts are mounted
Frame
– mounted with its axis parallel to the plane of the frame
– adjustable for the characteristics of the individual observer’s eye
Telescope
mounted on the upper end of the index arm directly over its pivot point
Index Mirror
– 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
Index Shade Glasses
– 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
Horizon Glass
similar to the index shades, but of lesser density, and serve to reduce the glare of reflected sunlight on the horizon
Horizon Shade Glasses
indicates the number of degrees of an angle
Graduated Arc
the lower part of the frame and carries the arc which is graduated in degrees
Limb
– 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
Micrometer Drum
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
Tangent Screw
graduated into ten parts, permitting readings to 1/10 of a minute of arc (0.1’)
Vernier
– 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
Clamping Lever or Release Levers
– 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
Index Arm
mounted on the frame at a location and angle for good balance and easy grip with the right hand
Handle
produced by the index glass not being perpendicular to the plane of the instrument
Perpendicularity Error
If Perpendicularity Error is present what should be done
turn the first adjustment screw at the back of the index glass, until they appear in alignment
caused by the horizon glass not being perpendicular to the plane of the instrument
Side Error
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?
turning the second adjustment screw until the true and reflected horizons appear on the same line
– 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
Collimation Error
– 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
Index Error
Non-Adjustable Errors
Prismatic Error
Graduation Error
Centering Error
occurs when the faces of the shade glasses and mirrors are not parallel
Prismatic Error
error due to lack of parallelism in the shade glasses
Shade Error
error due to lack of parallelism in the shade glasses
Shade Error
occurs in the arc, micrometer drum, and Vernier of a sextant which is improperly cut or incorrectly calibrated
Graduation Error
results if the index arm does not pivot at the exact center of the arc’s curvature
Centering Error
must be corrected for errors of the instrument and observer, and other corrections depending on the celestial body being observed
Sextant Altitude (Hs)
– 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
Observed Altitude (Ho)
a plane which passes through the observer’s eye and is at right angles to the vertical of the observer
Sensible Horizon
plane is perpendicular to the true vertical and is tangent to earth’s surface
Geoidal Horizon
plane from the eye of the observer and tangents to earth’s surface
Geometrical Horizon
– 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
Visible Horizon
– 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
Celestial Horizon
coordinates sued to define a position on the celestial sphere with reference to an observer on the earth
Altitude & Azimuth
– an instrument error of the sextant
– occurs when the index and horizon mirrors gets lightly out of adjustment
Index Error
– 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
Dip/Height of Dip
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
Semi-Diameter Correction
– 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
Refraction
– 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
Parallax Correction
Where the Parallax Correction is published in the Nautical Almanac
Horizontal Parallax
– 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
Polaris
– a measure of the distance from the assumed position to the line of position
Intercept
Three Parts consisting the 2102-D Star Finder
- A rigid white plastic disk with stars printed on it, called the “star base” or “white disk”
- a set of 9 transparent templates with blue printing, which we call the “blue template “
- a special template with red printing
brightest stars in all the sky
Magnitude One
stars as bright as the Bgi Dipper stars
Magnitude Two
stars not quite as bright as Big Dipper stars, but brighter than the stars on the handle of the Little Dipper
Magnitude Three
How many Magnitude One Stars are there and are navigational
20 and all are navigational stars
How many Magnitude Two Stars are there and how many are navigational?
75 and 30 are navigational
How many Magnitude Three Stars are there and how many are navigational?
200 and only 7 are navigational
– 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
Nautical Almanac
the essential requirement for anyone learning and practicing astronavigation published by John William Norie
Norie’s Nautical Table
– 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
L.A.N./Noon Sight
