NATARAJACompass error — azimuth and amplitude
Learning outcomes & permitted supports
- Find gyro error from a celestial azimuth: error = True − Compass, named by 'compass least, error East'.
- Split magnetic error into variation (chart) and deviation (ship): deviation = error − variation.
- Take an amplitude correctly: sin A = sin dec / cos lat, centre on the TRUE horizon, named from east/west toward the declination.
- Choose the moment: bodies LOW for azimuth accuracy; know the whole celestial day as one routine.
Supports in this lab: engine true azimuths (validated 609/609 incl. 12 azimuth cross-checks ≤0.01°), compass rose with injectable gyro error/variation/deviation, amplitude horizon strip with visible-vs-true comparison, navigator's-day capstone strip.
Glossary: compass error · variation · deviation · amplitude · twilight
Azimuth check — the sky's bearing vs the compass card
Sun: alt 29.4° · TRUE azimuth Zn 85.0° (engine)
| Gyro bearing observed | 83.5° |
| Gyro error (True − Gyro) | 1.5°E |
| Magnetic bearing observed | 83.0° |
| Total magnetic error | 2.0°E |
| Variation (chart) | 1.0°W |
| Deviation (error − variation) | 3.0°E |
Compass least, error East; compass best, error West. Here the gyro reads 83.5° against a true 85.0° — least → East. The deviation goes in the table against the ship's head; it changes when the head changes.
Amplitude — the rising Sun against due east
2026-08-17 · dec 013° 26.2′N · sin A = sin dec / cos lat = 0.232 / 0.945 → A = 14.2° · rising bearing 75.8°
Take the bearing when the Sun's CENTRE sits on the TRUE horizon — its lower limb about a semidiameter and a half above the visible one (refraction has already lifted the image). The small circle marks the bearing if you jump early at the visible horizon — 0.3° off at this latitude. And watch the inset: the same declination costs more amplitude the higher your latitude.
The error ladder (T · V · M · D · C)
| T — True | 63.5° |
| V — Variation | 1.0°W |
| M — Magnetic | 64.5° |
| D — Deviation | 3.0°E |
| C — Compass | 61.5° |
| Error (V + D) | 2.0°E |
Down the ladder (True → Compass) SUBTRACT easterly errors; up the ladder add them. "True Virgins Make Dull Company — Add Whisky" carries the memory; the sky's azimuth check supplies the one honest number the whole ladder hangs from.
The navigator's day — the whole course in one strip
One day, one position — every tick computed by the engine. This is the whole course in a single strip: the labs you have walked are the tools of a single watch-keeping day. Lab OP-13 walks the noon-latitude and longitude sums step by step.