WEEK VII—NAVIGATION
Tuesday Lecture
Compass Error by an Azimuth
The easiest and most accurate way to find the error of your compass is, first, to find the bearing of the sun by your pelorus. If you set your pelorus, so that it will exactly coincide with the course you are steaming as shown by the compass in your chart house and then get a bearing of the sun by noting where the shadow from the pelorus vane cuts the circumference, this bearing will be the bearing of the sun by compass. At the same time, get your true bearing of the sun from the Azimuth Tables. The difference between the two will be the compass error, marked East or West according to the following rule which put in your Note-Book:
1. Express your Compass Bearing and your True Bearing by NEW compass reading.
2. If TZ is to the right of CZ, C.E. is East. Formula: True - Right - East.
3. If TZ is to the left of CZ, C.E. is West. Formula: True - Left - West.
You must now remember that what you have is a Compass Error, consisting of both Variation and Deviation. To find the Deviation, the Variation and C.E. being given, is merely to apply the rules already given you under Dead Reckoning. For instance, if you had a C.E. of 10° W and a Variation of 4° E, the Deviation would be 14° W.
Put this example in your Note-Book:
LAT 20h 59m 57s Lat. 4° 55' N Dec. 10° 39' 30" N
Ship heading N 11° W. CB of (.) S 88° E. Variation 10° W. What was the ship's true course and Deviation of Compass on direction ship was heading?
Compass Variation and Deviation
CZ 92° (New compass reading)
TZ 80° (New compass reading)
——
CE 12°
CE = 12° W
Variation 10° W
——
Deviation 2° W
True course being sailed N 23° W or 337°.
Course being sailed
Let us now work out some of the following examples:
1. L.A.T. 22h 14m 18s
Lat. 30° 29' S
Dec. 17° 28' 44" N
Ship heading S 84° W
Compass Bearing 44°
Variation 10° W.
Required T.C. and Deviation on ship's loading.
2. August 29th, 1919. CT 2h 29m 18s A.M. Longitude 120° 19' 46" E. Latitude 44° 14' N. Ship heading 98°.
Compass Bearing S 42° E.
Variation 4° E.
Required T.C. and Deviation on ship's heading.
3. June 17th, 1919. CT 4h 18m 44s A.M. Longitude 60° 14' 59" E. Latitude 38° 48' 00" S. Ship heading SW x S.
Compass Bearing 40°
Variation 12° W.
Required T.C. and Deviation on ship's heading.
Etc.