Sextant Flashcards
an instrument used to measure the angle between any two visible objects
Sextant
Primary Use of Sextant
to determine the angle between a celestial object and the horizon
Common Uses of Sextant
include sighting the sun at solar noon and sighting Polaris at night, to find
one’s latitude (in northern latitudes)
Sextant comes from what Latin word
Sextans
The Latin “Sextans” means what
the sixth part
Why was sextant named sextant?
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
Sextant Altitude
angle between a celestial object and the horizon
Object’s Altitude
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
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
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
Micrometer Drum
How many graduations does a micrometer drum have and what do they represent?
60, each representing 1’ of arc
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
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
How many seconds equates to an error of a nautical mile in the position when using a sextant?
4
caused by the horizon glass not being perpendicular to the plane of the instrument
Side Error
How to check for perpendicularity error?
turn the first adjustment screw at the back of the index glass, until they appear in alignment
due to the axis of the telescope not being parallel to the plane of the instrument
Collimation Error
How to remove side error
It can be removed by turning the second adjustment screw until the true and reflected horizons appear on the same line
the error remaining after the navigator has removed perpendicularity error, side error, and collimation error
Index 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
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
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
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
– 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
plane from the eye of the observer and tangents to earth’s surface
Geometrical 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
Measured with a sextant
Altitude
Measured with a compass
Azimuth
– an instrument error of the sextant
– occurs when the index and horizon mirrors gets lightly out of adjustment
Index Error
Sextant reading is zero, what is the error?
No error
Sextant reading is not zero, what is the error?
the amount of error is the index correction
index mark is to the left of the zero (on the arc) of the limb, what is the reading?
the reading is too large
index mark is to the right of the zero (off the arc)
the reading is too low
– 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
Parallax Correction
