1historical In Ptolemy's third lunar model: the angle between the true apogee of an epicycle (as used in the second lunar model) and a point on the epicycle designated the mean apogee (introduced by Ptolemy in order to correct a discrepancy observed at the octants).
2rare The angle between the ecliptic and the great circle joining the centres of the moon and of the earth's shadow in a lunar eclipse, which indicates the position of the part of the moon that is first eclipsed.
Mid 19th century; earliest use found in William Whewell (1794–1866), college head and writer on the history and philosophy of science. From Hellenistic Greek πρόσνευσις inclination towards, in Ptolemy in senses 1 (Almagest 5. 5) and 2 (Almagest 6. 10) from προσνεύειν to incline towards (from ancient Greek προς to + νεύειν to nod) + ancient Greek -σις.