A graha is vakri when its apparent motion through the zodiac reverses. The Sun and Moon never retrograde, Rahu and Ketu are always retrograde (they are the nodes and move backwards by nature), and the other five do so for part of every year. In Jyotish retrogradation is a source of strength, not of weakness: it is the basis of cheshta bala, the strength of motion, one of the six components of shadbala.