An opposition aspect in astrology, is where two planets appear opposite each other from the point of view of the earth. They are 180 degrees apart from each other. This is the 360 degree circle divided by two.
Number Two Symbolism
In number symbolism, the number two is a number of division and separation. The unity of the number one is broken. In the Bible after each day of creation it says 'and it was good' except for the second day where this phrase is missed out. Why was this? It is because with the division created by having two forces there can be disharmony, separation, conflict and opposing ideas. In the symbol of the Tao, we have both yin and yang, the two primary forces. If we look around us we see the world divided in two in all aspects of creation, man and woman, night and day, up and down, right and left. Everything we live by is a polarity.
Yet in some way, this division is also necessary and fundamental to creation. If everything remained a unity and a oneness there would be no separation, no freedom of choice between opposites, no right and wrong and nothing to be done or achieved. This nature of duality or the principle expressed by opposition is that there is dynamic betwen the two poles that animates creation.
We even see this is politics, we have the Republicans and the Democrats in the United States. Through the opposition and debate of these two sets of values we try to find what is the correct path. We need both of these entities to allow our political system to reach a correct balance. If both parties always agreed with another we would have no room for gaining a deeper understanding into the issues.
Opposition in Astrology
When the Sun and Moon are in opposition, it is the time of the full moon. The full light of the Sun is reflected back by the moon. In a similar way, when two planets are in opposition, they reflect back the qualities of each other. There is, however, a tension between the two because they do not agree, as they are coming from different sides. Somehow though they are vital to each other to reach an understanding and a balance.
In Astrology, the opposition of two planets in the sky also represents two fundamental forces in opposition in the psyche of a person, that are in need of reconciliation. Often a person associates with one side of the opposition and sees the other side as being out there in the world. They therefore find it in another person because they don't acknowledge it as being part of themselves.
Moon & Mars in Opposition
Let's take an example. A person is born under an opposition between the Moon amd Mars. This person has both these forces inside of them. They have a desire to powerfully express their emotions. Since the two planets are in opposition though, they may associate with the Moon and feel emotional but not powerful.
What happens is they are continually attracted to powerful people, since they resonate with the unexpressed Mars inside of themselves. They may have a succession of abusive or aggressive partners or just people who are strong and powerful that they admire. These people are showing them what they actually have inside of them. Once of course they acknowledge their own power, they will no longer need to seek it in the outside world. This is the opposition in action.
People with opposition aspects in their natal chart therefore, often learn through relationship with another and in this way learn to acknowledge more of what is inside of themselves.