The angular bearing of a celestial body measured clockwise from true north, used to check compass error.
In practice
On watch, the second or third officer typically takes an azimuth of the sun at sunrise or sunset when the sun is low enough to appear through the azimuth mirror without excessive refraction. The observation takes only a few minutes and gives an independent check on gyro performance that does not rely on any electronic system. Regular azimuth observations, entered in the bridge log with the resulting compass error, provide a documented record of compass reliability and are inspected during Port State Control examinations. On vessels not equipped with a gyrocompass, an accurate azimuth observation is the only means of determining total compass error on passage.
Regulatory detail & full definition
An azimuth is the horizontal angle of a celestial body measured clockwise from true north, used principally to check the error of the gyrocompass or magnetic compass by comparing the observed azimuth of the sun, moon, or a star with its calculated true azimuth at that moment and position. Bowditch (American Practical Navigator) details the azimuth calculation using sight reduction tables such as HO 229 or AP3270, or by modern calculator, using the vessel's latitude, the body's declination, and its local hour angle. The difference between observed and calculated azimuth reveals total compass error (variation plus deviation combined, for a magnetic compass).
On watch, the second or third officer typically takes an azimuth of the sun at sunrise or sunset when the sun is low enough to appear through the azimuth mirror without excessive refraction. The observation takes only a few minutes and gives an independent check on gyro performance that does not rely on any electronic system. Regular azimuth observations, entered in the bridge log with the resulting compass error, provide a documented record of compass reliability and are inspected during Port State Control examinations. On vessels not equipped with a gyrocompass, an accurate azimuth observation is the only means of determining total compass error on passage.