Compass error by azimuth and amplitude

What you do on the bridge

1. Gyro error by azimuth
2. The standard compass, and the comparison
3. Amplitude at sunset

The oral questions, and what a good answer covers

The key points are in our own words. In the course, these questions come once you have done the job.

1. When do you check the compass error, and how?Often asked

   A good answer covers:

   • By azimuth of a celestial body, by amplitude at sunrise or sunset, or by a charted transit
   • Gyro error first, then the standard compass by comparison with the corrected gyro
   • Every result in the compass error book, and any change from the usual error looked into

   Show me on the bridgeThe sun is low to the west. Stand at the repeater and look at it through the azimuth ring.

2. What is an amplitude, and when exactly do you take the bearing?Often asked

   A good answer covers:

   • The true bearing of the sun measured from east at rising or west at setting
   • Found from the declination and the latitude, so no time or longitude is needed
   • Taken when the sun's centre is on the celestial horizon
   • Less reliable in high latitudes, where the sun rises at a shallow angle
3. Compass error, variation and deviation: how do they fit together?Often asked

   A good answer covers:

   • Compass error is the difference between true and the compass reading
   • Variation is the earth's part, read from the chart for the place and year
   • Deviation is the ship's own magnetism, and changes with heading
   • Deviation is compass error less variation; compare it with the deviation card for the heading
   • Going from compass to true, add easterly error and take off westerly
4. The gyro reads high. What does that mean for the bearings you take?

   A good answer covers:

   • A gyro that reads high has a westerly error, so subtract it to get true
   • Every bearing off the repeater carries the same error until it is applied
   • Apply it to bearings for fixes, and to the course set on the autopilot
   • A sudden change in gyro error is a sign of a fault; compare with the magnetic compass

How the lesson runs

The tutor's narration from the lesson, step by step. On the bridge, each control lights up as it is named.

1. Brief

   We're in open water east of the Ambrose Channel, about forty minutes before sunset. Every bearing we take off the gyro carries its error, so we need to know what that error is.

   We'll take an azimuth of the sun to find the gyro error, use the corrected gyro to check the standard compass, log the comparison, and finish with an amplitude as the sun sets.

2. Gyro error by azimuth

   Go to a wing, stand at the repeater, aim at the sun and press T. Then open the compass panel at the helm projector, pick that sun bearing in the error book, and choose Azimuth.

   Work the true bearing of the sun from the almanac extract and the formula there. It needs to be within half a degree, and the gyro error within one degree. Remember, a gyro that reads high has a westerly error.

3. The standard compass, and the comparison

   Now the magnetic compass. You read it through the projector at the helm, so there is no need to climb to the monkey island. Apply the gyro error to the gyro heading to get true, and compare it with the compass heading.

   In Compass comparison, enter the gyro and compass readings, the chart variation, the deviation off the card for this heading, and the gyro error. Fill in the compass error and the observed deviation as well, and check the deviation against the card within a degree.

4. Amplitude at sunset

   As the sun sets, take one more bearing, this time when its centre is on the horizon, which is when the lower limb looks about half a diameter above the sea. Too early or too late and refraction spoils it.

   Enter it in the book as an Amplitude. Work it from the declination and the latitude, counted from west at sunset. The error should agree with your azimuth within a degree.