The aspects

An aspect is an angle between two planets. Classical astrology knows five, and for a definite reason: only these five arise from dividing the circle by its simplest ratios.

The five

Conjunction (0°) — the same place. Neither good nor bad: it joins, and what comes of it depends on who is joining.
Sextile (60°, the circle divided by six) — easy, but without momentum. An opportunity that has to be taken.
Square (90°, divided by four) — tension. Two signs of different elements that will not look at one another.
Trine (120°, divided by three) — flow. Same element: what meets here passes without effort — including the unwelcome.
Opposition (180°, divided by two) — facing. Visibility and conflict at once.

The minor aspects — semi-sextile, quincunx, quintile — are later additions and do not appear in older practice.

Orbs belong to planets, not to aspects

Here the tradition departs most clearly from current usage. Modern: one orb per aspect, eight degrees for a trine, say. Classical: each planet has its own orb, and two planets are in aspect when the distance closes to the sum of their halves.

The Sun carries the largest orb, Mercury and Mars smaller ones. The consequence: an aspect of the Sun acts over a greater distance than the same aspect between two small planets. Anyone coming from modern practice gains some aspects and loses others. The five angles themselves are read differently there too, and a present-day guide to the aspects sets out how.

Application and separation

The point at which most misreadings begin.

An applying aspect is not yet complete: the faster planet is running towards the slower. It describes something that is coming.

A separating aspect is past. It describes something that has happened.

The same angle therefore means two different things according to which way it runs. In horary this decides the answer; in a natal chart it decides whether a disposition is still ahead.

Sign boundaries

An aspect that would come about arithmetically but would only complete after a change of sign counts, in the tradition, as not completed. The planet leaves its sign, and with it the conditions under which the aspect would hold.

Modern software draws such aspects. Anyone reading classically checks them.

And reception

Finally, what modern accounts almost always omit and the tradition weighs heavily: reception. If a planet stands in a sign the other rules, that other receives it — and a difficult aspect with reception often ends well, while a harmonious one without reception can come to nothing.