Required Information –
- GMT
- Longitude and Latitude
- Gyro heading and Compass heading
- Gyro bearing of SUN (As most probably the azimuth circle will give you Gyro bearing and not Magnetic bearing).
Calculations –
Calculate for Nautical Almanac the GHA of the SUN
GHA (Day and Hour ) : __________
INCREMENT (Minutes) : ___________
Add both you will get GHA of SUN.
Then GHA (SUN) + Longitude (+ if E / – if west) = LHA(SUN)
Leave this information aside we will need it later, Next, we will find DECLINATION of the Sun using Almanac (Days page).
DECLINATION (Day and Hour)
+/- D-Correction ()
With this you will get your DECLINATION.
Now we need to find LHA
If LHA is more than 180 (P=360-LHA ) & If LHA is less than 180 (P = LHA).
A = TAN (LATITUDE) / TAN (P)
When hour angle is between 270 to 090, A is named opposite to latitude.
when hour angle is between 090 and 270 Degree, A is named same as latitude.
B= TAN (DECLINATION) / SIN (P)
B is always named same as declination.
C = A +/- B
If A and B both have same sign then we add them, If they have diffrent sign we substract them and put the sign of the bigger side.
Tan Azimuth = 1/[(C * Cos (Lat)]
Azimuth takes combined name of C and Hour angle – If LHA is between 0 and 180, It is named “West”, If LHA is between 180 and 360, It is named as EAST.
NOW Compare TRUE AZIMUTH with the GYRO AZIMUTH of SUN, the diffrence is your compass error (GYRO).
HOW TO FIND COMPASS ERROR (MAGNETIC).
Initially check your VESSEL heading in gyro and magnetic for example if gyro heading is 050 and magnetic heading is 054 then you know the difference is 4 , Now when you observe the bearing of the sun using azimuth circle you get Gyro azimuth lets say it is 080 then what will be your compass bearing? It will be 084, Similary to calculate compass error (magnetic) we use Observed Azimuth magnetic and the True Azimuth.