Determine compass error using the azimuth of a star.

Sample calculation (Azimuth-Stars):

Let’s LEARN WITH AN EXAMPLE –

On 05 May 2006 at 11h00m00s UTC, a vessel in position 04°30’N 010°00’W observed Canopus – bearing 145° by compass. Find the compass error. If variation was 4.0° East, calculate the deviation.

GHA Aries  (05d 11h): 028° 10.7°
Increment (00m 00s): 000° 00.0
GHA Aries: 028° 10.7
Longitude (W): – 010° 00.0′ (minus- since longitude is westerly)
LHA Aries: 018° 10.7

SHA Canopus: 263° 59.0
LHA Canopus: 282° 09.7′

Declination:
S 052° 42.1
P= 360 – 282° 09.7° = 77° 50.3″ (IF LHA>180, P=360-LHA)
A = Tan Latitude/ Tan P
A = Tan 04° 30′ / Tan 77° 50.3
A = 0.016960803 S (A is named opposite to latitude, except when hour angle is between 090° and 270°)

B = Tan Declination / Sin P
B= Tan 052° 42.1′ / Sin 77° 50.3
B
= 1.342905601 S (B is always named same as declination)

C = A+B = 1.359866404 S (C correction, A+/- B: If A and B have same name- add, If different name subtract)

Tan Azimuth = 1/(C X Cos Latitude)
Tan Azimuth = 0.737640253
Azimuth = S 36.4° E (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= 143.6°
Compass Azimuth= 145.0°
Compass Error = 1.4° West
Variation = 4.0° East
Deviation = 5.4° West

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