Reverse movement of planets (grahas), or planets becoming
retrograde, in Hindu astrology is referred to as Vakri Bhavana. The planets of
the solar system always move forward on their path of revolution around the sun.
But sometimes, seen from the earth, they appear as if for some time they are
moving backward. This relative backward motion of the planets is called
vakrigati (retrograde motion) and the phenomenon is called vakri bhavana.
Out of traditional nine planets of Hindu astrology, sun and
moon are never retrograde; Rahu and Ketu, which are shadow grahas represented
by the points of intersection of the planes of lunar and solar paths, are
always retrograde; and other grahas which are planets become retrograde for
some time during each revolution.
According to Varahamihira, the planets become more powerful
when they are retrograde (Laghu Jatakam 2/8). That means auspicious planets
become more auspicious and inauspicious become more inauspicious in vakri
state, majority of astrologers agree on this.
Retrograde or reverse motion of a planet happens when the component
of velocity of the planet in a particular direction is less than the velocity
component of the earth in that direction. This phenomenon is something like two
trains moving in the same direction on parallel rail lines, but with different
speeds. Seen from the faster train, the slower one appears to be moving backward.
The planets of our solar system revolve around the sun in the
elliptical paths of different radii, and the linear velocity decreases with the
increase of orbit-radius. These differences are the cause of relative motion
and retrograde effect when seen from the Earth. If observed from the Sun, all
planets will be seen in direct motion only.