Practical Navigation Course
Unit 4 · Celestial Navigation

Chapter 17: Azimuths and Amplitudes

Comparing a computed true bearing of a celestial body with the observed compass or gyro bearing gives compass error at sea, far from any landmark.

Azimuth observations

Observe the body's bearing with an azimuth circle on the gyro repeater, note the exact time, and compute the true azimuth from LHA, declination and latitude — by Pub 229, an azimuth table, or the ABC/sight-reduction formula. Gyro error equals true azimuth minus gyro bearing; if the gyro reads higher than truth, the error is high and is subtracted from gyro bearings to get true. The sun, moon, planets and Polaris all serve; Polaris requires only the Polaris table for a latitude and azimuth check.

Amplitudes

An amplitude is observed when the body's centre is on the celestial horizon — for the sun, when its lower limb is about two-thirds of a diameter above the visible horizon (or centre about 0.7 × SD high). The amplitude angle A is measured north or south of east at rising, or of west at setting, and converts directly to a true bearing. It is the simplest compass check available because sin A = sin dec / cos L.

  • Named E or W for rising/setting, then N or S with the declination
  • Amplitude 'OE': observed sun bearing at theoretical rising
  • Convert amplitude to azimuth, then compare with the compass bearing

Formula cards

Amplitude

sin A = sin(dec) / cos(latitude)

A is measured from the prime vertical (east or west).

Time azimuth

tan Z = sin(LHA) / (cos L tan dec − sin L cos LHA)

Equivalent to a Pub 229 azimuth extraction.

Key takeaways

  • • Gyro error = true azimuth − gyro bearing; gyro high means subtract.
  • • Amplitude is taken at theoretical rising/setting: sin A = sin dec / cos lat.
  • • Check gyro error at least once per watch or daily, and log it.

Knowledge check

Immediate feedback with rationales. Answer each question to see the explanation.

Chapter 17 quiz

0 of 3 answered

  1. 1.The true azimuth of the sun is 093.0° and the gyro bearing is 095.5°. Gyro error is:

  2. 2.Amplitude of the sun is observed when:

  3. 3.With declination 12°N and latitude 40°N, the amplitude is approximately: