The amplitude of a body is the angle at the observer’s zenith or the arc of his rational horizon contained between the observer’s prime vertical, and the vertical circle through the body, at theoretical rising or setting.
Looking at the figure NESW is the celestial sphere as depicted in equidistant projection on plane of observer’s rational horizon. P – is the elevated pole. X1 and X2 – Celestial body rising and setting respectively. Centre of each celestial body is on the rational horizon. Z – is the Zenith of the observer QZ- is the latitude of the observer.
Important points to consider when taking amplitude Amplitude is always taken when the heavenly body is either rising or setting on the horizon. Normally amplitude is taken of sun and moon and not of stars and planets as they are not visible during rising or setting. Amplitude is always taken when the true altitude of the sun or moon is 0. For the sun, this is when the lower limb of the sun is about 1/2 its diameter above the horizon – this is to compensate for maximum refraction which occurs when the body is on the horizon
Procedure for observing amplitude –
Take the gyro bearing of the sun or moon when the body is rising/ setting. For Sun, take the observation when its lower limb is about 1/2 its diameter above the horizon. Note the GMT and the DR of the vessel at the time of taking the observation. Obtain the body’s declination for that time. Calculate the amplitude using the formula :
Sin Amplitude = Sin Declination x Sec Latitude Amplitude can also be found by referring to Norie’s or Burton’s nautical tables. Note when rising, the amplitude is named E…… N or S (according to the name of the declination) and when setting it is named W…… N or S (according to the name of the declination). It is read as, for e.g. 10 degrees north of east.
