CELESTIAL NAVIGATION Flashcards
What is Declination?
The angle of the body measured from the celestial equator north or south, it is similar to latitude.
What is GHA?
Greenwich Hour Angle - The angle from the prime meridian to the body measured westwards (CW).
What is Altitude?
The angle vertically measured from the observer’s horizon to the body. A sextant is used to take this precise angle.
What is an Azimuth?
The bearing of the body. The angle is normally calculated in a quadrangle form and then converted into a 3 figure notation.
What is a Zenith?
The point on the observer’s meridian directly above the observers head, this is at 90deg to the observer’s horizon.
What is TZD?
True Zenith Distance -
90deg - altitude.
angle from the body to the observer’s zenith.
What is SHA?
Sidereel Hour Angle - the angle from the first point of Aires to the star measured westwards/clockwise.
What is LHA?
Local Hour Angle - The angle from the observer’s meridian (their longitude) to the body measured westwards/CW.
What is the First Point of Aires?
The point where the suns ecliptic crosses the celestial equator during the vernal equinox.
SEXTANT - First correctable error:
- Define;
- Check;
- Correct:
DEFINE - Error of perpendicularity -
The index mirror is not perpendicular to the plane of the instrument.
CHECK -
Set the index arm between 30-40 on the arc;
Hold it horizontally outwards
Check that the true and reflected image of the arc is consistent.
CORRECT-
Use the adjustment screw behind the index mirror until the reflected image is the same as the true image.
SEXTANT - Second correctable error:
- Define;
- Check;
- Correct:
DEFINE - Side error
The horizon mirror is not perpendicular with the plane of the instrument.
CHECK -
Set to 0 on the arc and hold the sextant vertically.
select a star and observe if the two stars are side by side, if yes then NO error.
or
Set to 0 and hold the sextant just out of the horizontal position and if the two horizon images are misaligned then an error exists.
CORRECT-
Use the adjustment screw on the horizon mirror furthest from the plane of the instrument until the two images are side by side.
SEXTANT - Third correctable error:
- Define;
- Check;
- Correct:
DEFINE - Index error
When the horizon and index mirrors are not parallel to each other when the arm is at 0deg.
CHECK -
Set at 0deg and hold vertically. If the horizon is misaligned then index error exists.
CORRECT-
Adjust the screw on the back of the horizon mirror closest to the plane of the instrument.
SEXTANT - Name the six non-correctable errors:
Centring, Prismatic, Shade, Graduation, Collimation, Worm and Rack.
What are the two ways for determining compass error?
Azimuth and Amplitude
Explain the process of taking an azimuth?
-Can be taken at any time of day and can be used to take the bearing of a cel. body.
A= (TanLat)/(TanLHA)
named opposite to lat except when LHA is between 90 and 270
B=(TanDec)/(SinLHA)
always named the same as the dec
C=A+/-B
+ if named same - if named differently
TanAz=1/CcosLat
Named N or S based on C
E if LHA 180-360
W if LHA 000-180
