3. (Science: astronomy) The arc of the horizon between the true east or west point and the center of the sun, or a star, at its rising or setting. At the rising, the amplitude is eastern or ortive: at the setting, it is western, occiduous, or occasive. It is also northern or southern, when north or south of the equator. The arc of the horizon between the true east or west point and the foot of the vertical circle passing through any star or object.
6. (Science: mathematics) An angle upon which the value of some function depends; a term used more especially in connection with elliptic functions. Magnetic amplitude, the angular distance of a heavenly body, when on the horizon, from the magnetic east or west point as indicated by the compass. The difference between the magnetic and the true or astronomical amplitude (see 3 above) is the variation of the compass.