Planets form angles to one another as they move through the heavens. If two planets are 90 degrees apart, they form a square. If they're 180 degrees apart, they are in opposition. Planets in aspect have twenty-four hour communication going on. The particular angle that separates any two planets describes the nature of their conversation. Astrologers use six angles most often, each of which produces a different type of relationship or "conversation" between the planets they join. Let's go over the meaning of each of the aspects.
Conjunction (0—8 Degrees)
When someone tells you that two things are operating "in conjunction," it means they're operating together. This holds true with planets, as well. Two (or more) planets conjoined are a team. Some planets pair up more easily than others—Venus and the Moon, for example, since both are feminine and receptive, or the Sun and Mars, since both are pretty feisty by nature. Planets in conjunction are usually sharing a house in your chart.
Sextile (60 Degrees)
The sextile links planets in compatible elements/ That is, planets in sextile are either in fire and air signs or in earth and water signs. Since these pairs of elements get along well, the sextile encourages an active exchange between the two planets involved, so these two pats of you will be eager to work together.
Square (90 Degrees)
The square puts planets at cross-purposes. There is friction between them and they never allow each other to stop moving. You can see squares operating in someone who is fidgety or constantly restless. Although they're uncomfortable and even aggravating at times, your squares point to places where tremendous growth is possible.
Trine (120 Degrees)
Trines are usually formed between planets of the same element, so they understand each other. They show an ease of communication not found in any of the other aspects, and they are traditionally thought of as "favorable." Of course, there is a downside to trines. Planets in these relationship are so comfortable that they can often get lazy and spoiled. (Sometimes they go so comfy they're boring.) Planets in trine show urges or needs that automatically support each other. The catch is that you've got to get them moving.
Quincunx (150 Degrees)
This aspect joins two signs that don't share a quality, element, or gender, and it is very difficult for them to communicate with each other. It's as if two strangers who don't speak each other's language are trying to tell each other a story in a noisy room. It's frustrating. For that reason, this aspect has always been considered to require an adjustment in the way the two planets are used. Planets in quincunx can often feel pushed, forced, or obligated to perform They seem to correspond to health issues.
Opposition (180 Degrees)
When two forces are in opposition, they're working against each other. They have the same mission and goal, but their techniques are very different. It's as if the two are standing across from one another with their arms folded, involved in a debate, neither willing to concede an inch. They can only break out of their standoff by firms becoming aware of one another and then compromising. This aspect is the least difficult of the traditionally known "hard" aspects because planets "at odds" with one another can come to some sort of a compromise.
From Llewellyn's 2011 Astrological Calendar. Click here for a full list of current-year almanacs and calendars.