What the four layers answer
Natal synastry compares two fixed birth charts. Partner progressions ask a different question: where does each person’s symbolic day-for-a-year chart stand on one chosen life date, and how do those changing positions relate to the other chart? Keeping four layers prevents one close aspect from losing its source. A progressed Moon contact between the two moving charts is not the same calculation as one person’s progressed Moon contacting the other person’s natal Sun.
The first layer compares both progressed charts. The second fixes Person B’s natal positions while advancing Person A. The third reverses that direction. The fourth is the natal baseline. The calculator reports the same five major angular relationships in every layer and orders them by measured orb. Counts are navigation aids rather than compatibility scores.
Worked reading: one contact in its correct layer
Suppose Person A’s progressed Venus is 14° Taurus and Person B’s natal Moon is 14° Scorpio. Their separation is 180°, so the contact belongs in Progressed A × Natal B. If Person B’s progressed Moon has moved to 18° Scorpio, the same Venus–Moon pair is four degrees apart in the progressed-to-progressed layer and may fall outside a one-degree orb.
The two rows answer separate geometric questions. The calculator therefore does not promote a natal contact into every progressed table or describe a changing position as a permanent relationship trait.
How the symbolic dates are calculated
For each person, elapsed time from the recorded birth moment to the comparison date is divided by 365.24219 days. That age in tropical years advances the ephemeris by the same number of days after birth. A person aged 36.25 tropical years maps to a planetary sky roughly 36.25 days after birth. Because the partners usually have different birth dates and ages, their symbolic ephemeris dates are different even though the life comparison date is shared.
The displayed positions are apparent geocentric tropical longitudes calculated for those symbolic moments. The astronomy supplies coordinates; using one symbolic day for one lived year is an astrological convention. A narrow orb shows closer geometry but does not establish an event, intention, relationship outcome, or causal influence.
Birth-time and interpretation limits
A recorded clock time determines the exact birth moment and therefore the fractional symbolic day. The Moon can move far enough during that fraction to change a close result. Historical UTC offsets also matter: today’s city offset may be wrong for a past birth date. This version intentionally omits Ascendant, Midheaven, progressed houses, and house overlays because astrologers use several progression methods for angles. Use the natal synastry and progressed-angle tools separately when those distinctions are needed.
Start with the natal baseline, identify the moving layer that contains a contact, and read the exact degrees before considering symbolism. A chart comparison cannot determine consent, compatibility, honesty, or what either person will do. Read the full progressed-synastry guide for a layer-by-layer workflow.
Calculation details and limits
- Astronomy Engine JavaScript documentation ↗ describes the planetary position library used by the local chart engine.
- Secondary progressions guide explains the site’s tropical-year and symbolic-day calculation.
- Natal synastry calculator supplies the unchanged two-chart baseline used here.