Compass Error Calculation by Azimuth of Planets

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Let’s learn how to calculate compass error by the Azimuth of any planet. We will understand this with the help of an easy example.

Sample calculation (Azimuth- Planets):


On 06 May 2006 at 22h20m 10s UTC, a vessel in position 48°00’N 050°00’E observed Mars bearing 327° by compass. Find the compass error. If variation was 4.0° East, calculate the deviation.


GHA Mars (06d 22h): = 089° 55.7′
Increment (20m 10s): = 005° 02.5
v (0.9)             =     00.3
GHA Mars: = 094°58.5′


Longitude (E):  (+) 050° 00.0′ (plus-since longitude is easterly)
LHA Mars:                 144° 58.5


Declination (06d 22h): = N 024° 18.6
d (0.2): = 00.1 (D correction)
Declination Mars: = N 024° 18.5

Now we will FIND – P and use ABC method for further calculations.
P= 1440 58.5 (If LHA 180° P=LHA)
A = Tan Latitude/ Tan P
A = Tan 48° 00′ / Tan 144° 58.5
A = 1.584646985 N (A is named opposite to latitude, except when hour angle is between 090° and 270°).
B = Tan Declination/Sin P
B= Tan 024° 18.5′ / Sin 144° 58.5′
B = 0.787011353 N (B is always named same as declination)
C = A+B = 2.371658338 N (C correction, A+/- B: If A and B have same name-add, If different name- subtract).


Tan Azimuth = 1/(CX Cos Latitude)
Tan Azimuth = 0.6301399
Azimuth = N 32.2° W (Azimuth takes the combined name of C correction and Hour Angle – If LHA is between 0°and 180°, it is named “west”, if LHA is between 180° and 360, it is named “east).


True Azimuth= 327.8°
Compass Azimuth= 327.0°
Compass Error = 0.8° East
Variation = 4.0° East (you can find this in ECDIS when finding compass error onboard)
Deviation = 3.2° West

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