CELESTIAL NAVIGATION Flashcards
(17 cards)
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
Explain the process of taking an amplitude?
Can only be taken at sunrise/set. Semi diameter of the sun should be above the horizon visually.
Calculate the true bearing of the sun and then compare to the gyro/compass bearing to obtain an error. Can be used with variation to calculate deviation of the ship’s head.
SinAmp= SinDec/CosLat
Named E if rising
W if setting
N or S on dec
Explain finding latitude from true altitude:
Sextant alt. to true alt:
-Sextant Altitude:
-Index Error (micrometer - On arc minus, off arc plus)
=Observed alt.
-Dip (from dip tables in almanac)
=Apparent alt.
- Main correction (for refraction taken from almanac)
- True altitude (if TZD is required then 90-TA)
-A0,A1,A2 all added to TA
- -1deg
=Latitude North (Can only be north for Northern Hemisphere)
