The speed of a planet in orbit around the Sun varies depending upon its distance from the Sun. This is based on Kepler’s second law of planetary motion by which the planet is fastest at perihelion, when it is nearest to the Sun and slowest at aphelion, when it is farthest. Nevertheless, this movement is direct in reality. But as viewed from the Earth, in the geocentric system which we follow, the planets sometimes appear to move in direct or backward motion in their orbit.
We have to consider this phenomenon for two groups of planets, the inner planets Mercury and Venus and the outer planets Mars, Jupiter and Saturn. The motion of Rahu and Ketu is always retrograde, as they are not physical entities but only mathematically calculated sensitive points and hence this phenomenon applies to them very briefly when they become direct. Rahu and Ketu also have a varying orbital motion like that of planets because of their elliptical orbits.
Mercury and Venus are inner planets and whenever they are ahead of the Sun, they become retrograde. Mars, Jupiter and Saturn being outer planets become retrograde when they are in 5th house, 6th house, 7th house or 8th house from the Sun or in other words whenever they are generally 120 degree to 240 degree away from the Sun.
The inner planets, on the other hand, have a much smaller sidereal period and hence the phenomena are slightly more complex. It would suffice to know that Mercury goes away from the Sun by a maximum of 27 degrees and Venus by a maximum of 47 degrees. Due to their smaller sidereal period these planets race past the Sun and on getting ahead they become retrograde and this cycle goes on.
Mercury becomes retrograde when it is between 15 degree to 27 degree ahead of the Sun and becomes direct when it is roughly identical degrees behind the Sun. Similarly, Venus becomes retrograde when it is between 27 degree to 47 degree ahead of the Sun and becomes direct when it is roughly identical degrees behind the Sun. Generally, Mercury becomes retrograde when it is in 2nd house from the Sun and becomes direct when it is in 12th house from the Sun. Similarly, Venus in 2nd house and 3rd house from the Sun becomes retrograde and in 11th house and 12th house from the Sun becomes direct.
The Sun and Moon are never retrograde. Mercury is retrograde three times in a year. Saturn and Jupiter every year for some months. Rahu and Ketu are always retrograde i.e. they always move in backward direction (left-hand side of the zodiac). It is said that malefic planets in retrogression will always give bad and unfavorable results. Benefic planets when retrograde generally provides good and auspicious results.
A planet in retrogression also gives the effect of its position in the previous house i.e., the house from which it recedes. Some astrologers opine that it gives the results of the house into which it recedes back and not of the previous house. Their periods of stay while in retrogression are not always the same. The Sun and Moon never become retrograde. During retrogression planets become extremely strong, beneficially they usually produce notable results like elevation in rank, and position, financial success. Name and fame etc.
Mantreshwar in his Phaladeepika has at several places extolled the virtues of a retrograde planet and has equated a retrograde Planet with an exalted planet in strength and effects even though he be in an enemy’s sign or in debilitating sign. Saravali states that benefics are greatly powerful and when retrograde and cause Raj Yoga but malefics when retrograde cause fruitless journeys, wanderings and misery. Mahadeva in his Jataka-Tattava has also stated that benefics when retrograde give wealth and splendor while malefics in retrogression cause many troubles and misery.
As per Mantreshwar, a retrograde planet, whether a natural benefic or malefic, if occupying a Quadrant or Trine House becomes super strong and super powerful. He then helps the native in every possible way for his self-improvement and also for taking him to the top at least for a while.