Why did copernicus have to keep small epicycles in his models? actually, copernicus did not use epicycles. unlike the ptolemaic system, all the planets were treated the same. his model did not predict planetary positions as well as the ptolemaic system. it was the only way to explain retrograde motion. which planet has the longest duration of retrograde motion as viewed from earth? mars jupiter saturn uranus neptune which planet has the shortest duration of retrograde motion as viewed from earth? mars jupiter saturn uranus neptune

Neptune has the longest duration of retrograde motion as viewed from Earth.

Mars has the shortest duration of retrograde motion as viewed from Earth.

Explanation:

The planets move along the space from west to east, although this movement is interrupted for brief intervals by a retrograde movement from east to west.

Retrogradation occupies a small part of the planet's motion that normally develops directly. The durations of retrogradation for the planets are: Mercury 23 days, Venus 42 days, Mars 73 days, Jupiter 123 days, Saturn 138 days, Uranus 151 days and Neptune 158 days.