Compass & how to check it Flashcards
Do we need a compass?
YES. All ships, irrespective of size.
Six parts of a compass
Compass
Flinders bar
Soft iron Spheres (x2)
Fore-and-aft magnets
Heeling magnets
Athwartship Magnets
departure checks of a compass (M°)
list 6
(spares also checked)
- freedom of movement of the gimbal
- card floating freely and level
- liquid free of bubbles and clear
- compass card displayed and readable
- no liquid leaks around seals
- no tools left in area
What is Variation
the angular difference between True and Magnetic North. Measured East or West of True North.
What is Deviation
The angular difference between Magnetic North and Compass North. Measured East or West of Magnetic North
What is on a deviation card?
(list 6)
- Date + location
- name of compass adjuster
- CoC and signature of compass adjuster
- name and signature of Master
- Residual Deviation
- Location of all correctors
Other:
7. Residuals coefficients
8. Heeling Magnet
9. Flinders Bar
How does a gyro compass work
Finds True North as determined by the rotation of the Earth.
Gyroscope is a wheel spinning at a high rate of speed and mounted to have the freedom to turn its axis of spin in any direction.
Two important properties:
1. Gyroscopic Inertia (rigidity in space)
2. Precession
What is an Azimuth mirror?
An azimuth mirror is used in conjunction with a compass.
Enables operator to measure bearings of celestial and terrestrial objects.
How often should we check our compass
-at least once every watch
-after a large heading alteration.
How do we check a compass
- Transit
- Amplitude of the Sun
- True Azimuth of a heavenly body
Explain Compass Error check via transit
when 2 well charted objects are in transit, a compass bearing is taken.
true bearing is taken from the chart
comparison of compass bearing and true bearing = Compass Error
Variation found from chart, leaves us with the actual deviation
deviation compared with compass card to see if compass card is accurate.
Compass Check via Amplitude of the Sun
@ sunrise & sunset only!
semi diameter above horizon (due to refraction)
take a bearing of the sun
find the declination of the sun for that day (Nautical Almanac)
note latitude
enter Nories, use correct table = true bearing of sun
compare azimuth of sun against your gyro or mag compass bearing
Compass Check via Azimuth of a heavenly body
@ any time of day / night
take bearing of the sun. Note the time (UT)
Find GHA and declination of the sun for that time. (nautical almanac)
Use GHA and longitude to work out LHA.
Go to Vol 1 / 2 of the Sight Reduction tables with : LHA, LAT and DEC
Extract the Azimuth (True Bearing) of the Sun
Compare the Azimuth of the sun against your gyro or mag compass bearing.
