Generally, all planets move from one Rasi to the other in clock-wise direction. Rahu and Ketu move in anti clock-wise direction. Sometimes, we see that a planet, instead of moving in clock-wise direction in the Zodiac seems to be moving in anti clock-wise direction. Whenever a planet is moving in anti clock-wise direction in the Zodiac, it is said to be in Retrogression. Sun and Moon will never retrograde. Rahu and Ketu are always in retrogression.
A retrograding planet will always aspect the rear sign with half aspect. A retrograding planet will give beneficial results, if a friendly planet is placed in the 12th Sign from retro planet. It will give bad results if an enemy planet is placed in the 12th Sign from retro planet. So, let us analyze few example charts.
