Transit = where planets are NOW vs where they were at your birth.
Example: you were born with Sun at 10° Aries.
Today, transiting Jupiter is at 10° Aries - that's a Jupiter Return (happens every 12 years).
Major life expansion period.
Or transiting Saturn is opposite your natal Moon - challenging emotional period.
Apps use transits for: (1) Daily predictions - "Mars transiting your 10th house: career boost this week", (2) Alert systems - "Venus entering your 7th house tomorrow: great for romance", (3) Major transit tracking - Saturn Return, Jupiter Return, Uranus Opposition at age 42.
To implement: fetch user's birth chart (natal positions) once, store it.
Then call our API daily/hourly for current positions.
Compare current to natal: conjunction (0°), opposition (180°), trine (120°), square (90°), sextile (60°).
Generate predictions based on aspects.
Professional tier ($37/mo) includes transit calculation endpoint that does comparison automatically - you send natal data + current date, get back all active transits with interpretations.
Way easier than coding aspect detection yourself.
Response includes orb (how exact the aspect is), applying/separating (is aspect getting stronger or weaker), peak date (when aspect is exact).
Essential for timing-based predictions.
Automatic transit detection vs manual aspect calculations