Applying and separating
An aspect is applying when the faster planet is still moving toward exactness and separating once it has passed, and traditional astrology treats the applying half as the one that carries weight.
The reasoning is temporal rather than mystical. An applying aspect is a meeting that has not happened yet, so it describes what is still coming; a separating one describes what has already been said. Lilly and the horary tradition are strict about it, and natal practice inherits the emphasis.
Direction depends on motion, not on which planet is which. When either body is retrograde the sense reverses, and two planets can apply, separate and apply again around a station.
Frequently asked
How do I tell whether an aspect is applying?
Compare the two bodies' daily motion. If the faster one is closing the gap in degrees, the aspect is applying.
Do separating aspects matter at all?
Yes, but as background. They describe conditions already established rather than something still arriving.