1The apparent temporary reverse motion of a planet (from east to west), resulting from the relative orbital progress of the earth and the planet.
- ‘Mars goes through apparent retrogradation every 25.7 months.’
- ‘The actual propositions quoted by Ptolemy as proved by Apollonius among others show mathematically at what points, under each of the two hypotheses, the apparent forward motion changes into apparent retrogradation and vice versa, or the planet appears to be stationary.’
- 1.1The orbiting or rotation of a planet or planetary satellite in a reverse direction from that normal in the solar system.
- ‘From the sum of all these small motions will result a daily retrogradation of the points of intersection of the equator and the ecliptic that is to say, of the equinoxes; and though it is extremely small, yet, as it is daily repeated in the same direction, the effect at the end of a year will be very sensible.’
- ‘Moreover, the distance that the planet travels between one retrogradation and the next is quite variable.’
Mid 16th century: from late Latin retrogradatio(n)- (see retro-, gradation).