The word aspect comes from the latin word aspiciō which means “catch sight of”, so it is basically the way two planets “see each other”. Technically, it is the angular distance between them; the type of aspect depends on the angle they form. For example, Venus at 12 Virgo and the Moon at 12 Taurus make a 120 degrees angle; a 120 degrees aspect is called Trine. We are not going to consider every possible angle formed by each pair of planets, but only those related to sacred geometry.
Major Aspects
Major aspects are also called ptolemaic aspects, because they were those described by Claudius Ptolemy in his Tetrabiblos—one of the most important book of western astrology. There are others, which are known as minor aspects, but we are not going to discuss them here. Each aspect describes a peculiar type of relationship between the planets involved.
 Conjunction0° |
Conjunction is not strictly speaking an aspect, because in this case planetary forces tend to merge in a single astral energy. Whether the conjunction is going to be harmonious or unstable depends by the planets involved. You can’t mix water and oil. |
 Opposition180° |
In an opposition, planets are facing each other, which is not necessarily something bad. In this case the final result also depends by which planets we are dealing with. It could turn out to be a strenuous tug-of-war as well as a collaboration between complementary forces. |
 Trine120° |
Usually it occurs between planets in signs of the same element. Their relationship is harmonious and stable because they are on the same wavelength. The nature of the element indicates where the collaboration takes place. |
 Square90° |
In a relationship, a square is a symbol of stiffness and tension: each part wants to bring water to its mill. This aspect takes place between signs whose nature (masculine/feminine) is in contrast. |
 Sextile60° |
Sextile is an harmonious aspect based on mutual support, although it is less stable and strong than a trine. This could also be a pro since it prevents harmony from becoming a golden prison or a forced collaboration. It usually occurs between planets of the same nature (masculine/feminine). |
In astrology aspects are defined in different ways according to their characteristics. The following is a list of technical terms used to describe them:
| Dexter and Sinister |
An aspect is always formed by the fastest planet. When the planet receiving it occupies a following sign, the aspect is sinister; when it is in a precedent one, dexter. For example: a planet in Taurus making a trine to a planet in Virgo is making a sinister aspect; a planet in Aries making a sextile to a planet in Aquarius is making a dexter aspect. Usually, dexter aspects are stronger and more effective than sinister aspects. |
| Applying and Separating |
Basically, an aspect is applying when it is going to become more and more perfect. It is separating when perfection has already occurred, and it is getting more and more loose. Applying aspects concern the future; separating, the past. This distinction is of particular importance in forecasting techniques, such as horary astrology. |
| Associate and Dissociate |
An aspect is associate when the planets involved are transiting in two signs that form the same aspect; it is dissociate otherwise. For example: an opposition between Venus at 14 Cancer and Jupiter at 14 Capricorn is an associate aspect (signs in opposition); Mercury at 29 Gemini and Mars at 1 Capricorn form a dissociate aspect. Associate aspects are far more effective than dissociate ones, to the point that in ancient times the latter were basically ignored. |
| Partile and Platick |
An aspect is partile when the planets involved are on the same degree of the respective signs; it’s called platick, otherwise. For example, Mercury at 4 Libra and Saturn at 4 Sagittarius form a partile sextile. In my opinion this distinction has no practical use. |
You wrote: “ For example, Mercury at 4 Libra and Saturn at 4 Scorpio form a partile sextile.” Libra and Scorpio would make not be 60° apart, thus the aspect would not be a sextile.
Thanks for reporting the error, I fixed it with 4 Sag