A Treatise on Astrology, Liber 536 by Aleister Crowley, 1917

The Aspects of the Planets

If an observer standing upon the earth fixes the position of any two planets in the sky, he will find that he must shift the position of his telescope through a certain angle. At certain angles, as has been previously explained, the influence of the two planets react upon each other, and those angles are called the aspects. These aspects are as follows:

1. Conjunction. Planets are said to be in conjunction when the angle between them is zero. That is to say that the telescope does not have to be shifted at all in the plane of the Zodiac.

2. Parallel. Planets a.re said to be parallel when the angle between them is zero. That is, the telescope docs not have to be shifted at all in the plane at right angles to that of the Zodiac.

3. Semi-sextile, The semi-sextile aspect is 30 degrees.

4. Semi-square. The angle is of 45 degrees.

5. Sextile. The angle is of 60 degrees.

6. Square or quartile. The angle is of 90 degrees.

7. Trine. The angle is of 120 degrees.

8. Sesqui-quadrate. The angle is of 135 degrees.

9. Opposition. The angle is of 180 degrees.

These are also certain lesser aspects: 72 degrees, 144 degrees, and 150 degrees. These are. of very little if any importance and can, generally speaking, be neglected . . The semi-sextile and semi-square aspects are also weak, especially the semi-square. The same is true of the sesqui-quadrate.

In the old-fashioned traditional astrology, there was a convention to consider the trine, sextile, and semi-sextile aspects as good ; the quartile and opposition as bad and the conjunction as doubtful. Modern research, however, has led us to modify this very crude conception. In this book will be found numerous examples of exceptions to this rule, which is merely a rough generalisation.

The parallel aspect is of great, but little understood, importance. Its principal function appears to be to confirm and strengthen other aspects. It is most effective when the parallel is near zero; that is to say when the forces of the two planets in parallel lie as nearly as possible in the great plane which contains the total force of the solar system.