Astrocartography
How we calculate it
Every number this site uses, written down. If you disagree with one, you can at least see what it is.
Where the positions come from
The Swiss Ephemeris, which is the same source professional astrology software uses and is derived from NASA JPL's planetary data. We compute geocentric positions for the ten traditional bodies, Sun through Pluto.
Your birth place is geocoded to coordinates and used to work out the timezone, which converts your local birth time to universal time. After that the birth place plays no part in the calculation. The lines depend only on the moment.
How the lines are derived
Each planet's position is converted from ecliptic coordinates to equatorial ones, using the obliquity of the ecliptic for your date rather than a fixed value. That gives a right ascension and a declination, which is what places a body against the rotating Earth.
- MC: CulminatingThe planet was highest in the sky here, associated with visibility, career and public life.
- IC: Anti-culminatingThe planet was directly beneath here, associated with home, roots and private life.
- AC: RisingThe planet was rising on the horizon here, associated with how you come across and start things.
- DC: SettingThe planet was setting here, associated with partnership and how you meet other people.
The rising and setting curves are drawn at one-degree steps of latitude, and stop where a planet becomes circumpolar. That is, where from that latitude it never rises or never sets at all. Those gaps are real, not missing data.
The distance bands
How close a place has to be to a line before it means anything. These follow the conventional astrocartography orb of roughly one degree of longitude for a direct hit, widening to the 200 to 300 mile range most published guides give as the outer limit.
| Distance | Reads as | Meaning |
|---|---|---|
| Up to 120 km | Strong | On the line, in effect. |
| Up to 240 km | Nearby | Close enough to read as present. |
| Up to 400 km | Broad | A background influence at most. |
| Up to 650 km | Wide | The far edge of anything worth saying. |
| Beyond 650 km | Out of range | Too far to mean anything. |
Distances are great-circle, measured to the nearest point on the line. For the vertical MC and IC lines that is computed exactly; for the curved rising and setting lines it is measured to the drawn points, which is accurate to a few kilometres. Far inside the bands themselves.
What your birth time does: the biggest source of error
The Earth turns 15 degrees of longitude every hour, and so does your whole map. Not one line at a time. The entire pattern rotates together, rigidly.
- 1 minute of error → about 28 km
- 5 minutes → about 140 km
- 15 minutes → about 415 km
- 30 minutes → about 830 km
- 1 hour → about 1,660 km
Those are equator figures and shrink towards the poles, but the scale is the point: a birth time rounded to the half hour can move a line from Paris to Frankfurt. That is why the tool asks how precise your time is, and shades a band showing the answer.
If you do not know your birth time at all, the shape of your map is still exactly right. Every latitude where a line bends or stops is correct. What is unknown is only how far east or west the whole thing sits. The slider on the tool lets you see that directly.
What this is not
The astronomy above is exact and checkable. What a planetary line is said to mean about a place is symbolic interpretation, and a different kind of claim entirely. We keep those apart deliberately, and nothing here predicts anything.
Parans
A paran is a latitude where two planets were angular at the same moment, where two of your lines cross. We find them by walking latitude and watching for the point at which two lines swap sides, then interpolating between the two steps either side of it.
Pairs of the same planet are excluded: your Venus MC line crossing your Venus rising line is the geometry of one planet, not two planets meeting, and is not read as a paran.
The orb is one degree of latitude, about 110 km, which is the received figure. Longitude plays no part at all: a paran holds right around the world at its latitude, which is the part most people find surprising.