Retrograde is picked up from Latin word "retrogradus" meaning backward-step. Astronomically it is an optical illusion, caused by the acceleration of the earth with respect to other heavenly bodies.
According to NASA, the apparent retro motion of planets can be attributed to difference in orbital speeds of planets. For example Mars takes just about twice as long to make one trip around Sun as the Earth, and hence when Earth passes along the slower Mars, this illusion occurs.
This phenomenon can be further explained in a more generalised manner by taking an example of moving cars. Let’s imagine ourselves travelling in a car which is moving at a faster speed than another car, moving in the same direction but slightly ahead of us. Now as we approach the slower car it appears to be stationary for a moment and on coming parallel it seems to move slightly backwards. However as we keep moving on with our speed, the slower car seems to resume its movement in the forward direction. In reality the slower car is moving all the time; however the impression created is that the slower car had stopped, moved slightly backward and then resumed its journey. Thus, a perception is created that the slower car is moving in some sort of a loop. Hence the use of word "apparent" while referring to the retrograde motion of the planets; because as observed from the Earth the planet in the retrograde motion “appears” to do a “loop travel”.
Now, the generally in the astrological interpretation of retro planets it is thought that such planets become "unconventional" i.e. that they tend to behave in manner that is different from their stipulated character.
Planets in a chart are like different energy sources that manifest according to their PACs (Placement Aspect Conjunction). In case of a retro planet, the inherent energy still follows its marked course but it just “appears” to pause, take an inward turn & then get expressed. It is like taking a decision and before putting it into action or announcing it, retrospecting it over & over in one's own mind. This, from an outsider’s perspective emerges as if the person is slow, hesitant or indecisive. The reality however is that energies become more subjective, inner & personal.
Taking the example of the Mercury, the planet that controls intelligence, senses, communication. With Mercury retro the intelligence turns inwards, creating a tendency to repeatedly go back into past experiences and extract knowledge. The thought process is more in sub-conscious mind than in conscious side. The inner knowledge is much more than what is communicated at conscious level. Since the real intelligent self is less obvious to the outer world, the person does not seem to be sharp and rational.
Mars, the planet of action, energy and logic when goes retro would signify that the energy is held back, as instead of usual explosion, Mars(R) uses up its energy to revisit events and build up the action plan internally, that is brought out only if triggered by a reason/idea/determination. Though this process gives the natives a hidden potential for accomplishments but outwardly they appears to be meek, slow & sometimes stubborn.
When the beauty, luxury, balance and happiness of Venus turns retro, the creativity & expression are manifested internally. With the inner self glowing with the Venusian light, the search for a suitable outlet leads to difficulties or delays in matters of relationships and related satisfaction. Though the inner self of such natives becomes aesthetically and artistically refined, outwardly they appear to be struggling with the nitty-gritties of relationships.
With the Guru (Jupiter) becoming retro; ethics, beliefs, expansions are directed inwards. Seeking explanations, finding truth using the teacher that's within oneself becomes the goal. Unique insights, inner richness & optimism help the natives to realise their life path but outwardly they seem to be “not-in-line” with the social norms, snobbish or high-handed.
When the karmadhipati Saturn turns retro, characteristics like restrictions, discipline, organisation, endurance all get attached to inner self. "Look before you leap", can be out of fear, practicality, responsibility but it is a part of the inner expression. Redoing in order to achieve the right thing makes the natives appear self-conscious, insecure & ignorant.
Coming to the karmic Rahu & Ketu. Deemed to be always retrograde, they represent the baggages carried over from past lives - the things that were attained & that are yet to be achieved. Rahu takes us back & forth to the desired carried over from bygones, in the process magnifying them. Clouding the soul with it; Rahu probes the mind to work & rework to achieve those leftovers this time. Outwardly giving a materialistic, malefic & individualistic expression. Ketu, on the other hand sticks to the point of "no lust" to achieve things of past and tends to mask the soul to keep away from them. Outwardly appears to be detaching, non-motivating & obstructive.
The above presents a general synopsis of the characteristics of the planets in retrograde motion. The same however gets modified and individualised depending upon the lordships and other attributes attached to the planets.
MSc Operations Research, from Delhi University