Lessons
Gyro error from a sun azimuth
Compare the gyro's azimuth of the sun with the true azimuth to get the error, then take it off every gyro bearing if the gyro reads high — and add it if it reads low.
The rule
- The azimuth checks the instrument — it is not a bearing to plot.
- Error = gyro azimuth − true azimuth; if the gyro's figure is bigger, it reads high.
- Reads high: subtract the error from every gyro bearing. Reads low: add it.
- Keep the tenth of a degree — never round the error away.
- Check it again every watch and after a large alteration of course.
Worked example
| True azimuth of the sun, from the tables | 118.4 ° |
| Azimuth of the sun read off the gyro repeater | 119.8 ° |
| Bearing of the light read off the gyro repeater | 246.3 ° |
- Gyro error = 119.8 − 118.4 = 1.4° high (the gyro says more than the truth)
- True bearing of the light = 246.3 − 1.4 = 244.9°
Common mistake
Adding a high gyro error to the bearing instead of taking it off.
A gyro that reads high already makes every bearing too big; adding the error pushes it further the same way, so the bearing ends up out by twice the error and the position line sits twice as far off.