Applying
An aspect is applying when the two planets are still closing on the exact angle, so the contact hasn't happened yet. Applying aspects are the ones a horary judgment rests on — they're the chart's future tense.
The fuller picture
Two planets applying to an aspect are on their way to it. The orb — the gap still left between them and the exact angle — is the distance they have to travel, and once they reach it the aspect has perfected and the planets begin to separate. Because an applying aspect describes an event that hasn't happened, it's the tense a horary question is usually asking about, and the practice this glossary follows goes further than most: applying aspects are what it reads, and separating ones are treated as history rather than evidence.
Applying is not the same as favourable, and that's the mistake worth heading off. An applying opposition is an approaching quarrel; an applying square is friction on its way. What "applying" tells you is that the thing is still to come — what arrives is whatever the aspect is. Our own interface makes that distinction visible on purpose, colouring an applying aspect by its nature rather than painting everything green because it's in the future.
Here's the part that a definition blurs. "Still closing" is a statement about relative motion, not about which planet is in front. Both bodies are moving, and the point at which the aspect would be exact moves with the slower one, so a faster planet can be behind and still be losing ground. There's a real divergence here between the practice we follow and our engine. The source measures the arc from one significator to the other planet's present degree, treating the target as if it were standing still, and that arc is also the number it turns into a timing. Our engine solves both motions properly, which is astronomically right and occasionally gives the opposite answer: on one worked chart the source reads an applying trine and the engine reports the same trine separating, because the target's aspect point is running away faster than the applicant closes. It's a narrow case — it needs two slow movers, or one near a station — and when it happens the two readings disagree about the tense of the event, not just the timing.
An applying aspect isn't a promise either, because two things can break it before it lands. If the applying planet turns retrograde before reaching exactness, it withdraws and the aspect never completes — refranation, and the test is a genuine station, not a planet that's merely crawling. And a third planet can get there first, which is prohibition. Retrograde motion doesn't always spoil an application, though: where two significators complete their aspect while one of them is going backwards, the aspect still perfects, and it perfects suddenly.
One practical thing rides directly on the applying arc: timing. The degrees left to travel give the number of units, and the sign the significator sits in and the type of house it occupies give the unit — days, weeks, months or years. So the same 6° of orb can be six days or six months depending on where the planet is standing. Worth knowing that the source counts completed degrees, so 6°59′ reads as six units rather than seven, and our own calculation rounds to the nearest degree instead. On that example the two differ by a whole unit.
In your chart
Open any aspect in the instrument and the motion row says applying or separating in words, toned by what's actually coming — an approaching trine reads as good news, an approaching square as a warning — and where the aspect applies you also get a perfects-in estimate computed from the two bodies' own daily motions in the chart. Applying aspects are drawn as solid lines on the wheel and separating ones dashed, and in record mode a focused applying line animates the draw from the faster planet's end. One caveat we'd rather state than hide: our applying test is a forward sample rather than a solve, so an extremely tight aspect involving the Moon can be reported the wrong way round — there's a case on record at eight arcminutes.
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