Method and accuracy

In one paragraph

The map takes your birth moment in Universal Time, computes where the Sun, Moon, planets, the true North Node and Chiron stood in the sky, and finds every place on Earth where each one was rising, setting, overhead or underfoot. Everything runs in your browser. We tested the planet positions against NASA JPL Horizons, NASA's own reference for planetary positions, and tested every kind of line point against Horizons and against itself. The largest position error we found for any planet is about a thousandth of a degree, and a degree is 111 km at the equator, so the lines sit well under a kilometer from where NASA's data puts them.

Is astrocartography accurate?

It depends which part you mean. The calculation of the lines can be checked, and we tested ours. The meaning given to the lines is a symbolic tradition, so it cannot be tested in the same way.

The calculation. A line marks where a planet was rising, setting, overhead or underfoot at a given moment. That is ordinary astronomy, and it can be measured against independent data. We compared our planet positions and line points with NASA JPL Horizons; the full results are in the tables below. In practice, the accuracy of your map depends far more on your birth time than on the math: the Earth turns 1 degree every 4 minutes, so 4 minutes of error moves every line by about 111 km at the equator.

The meaning. Whether living near a Venus line brings more warmth into your life is not something astronomy can confirm. There is no scientific evidence that planetary lines shape events. Astrologers present the readings as a tradition and a vocabulary for thinking about places, and that is how this site presents them too: "astrologers associate", never "this will happen". See what astrocartography is for more on this.

1. From your birth details to a moment in time

Your local birth time is converted to Universal Time with the full IANA time zone database, including daylight saving and historical rule changes. Browsers do not carry all historical rules before 1970, so for older births we add a correction table built from the IANA data. If the clocks were turned back and your birth time happened twice, or turned forward and it never happened, the map tells you which reading it used.

This step matters more than any other: the Earth turns one degree every four minutes, so a birth time that is four minutes off moves every line by about 111 km at the equator. If you are not sure of your time, choose how sure you are and the map shades the zone where each line could fall.

2. Where the planets were

Positions come from published astronomical theory: VSOP87 for the Sun and planets (with light time, aberration and nutation), the full lunar theory in Jean Meeus's Astronomical Algorithms for the Moon, Meeus for Pluto, and a table from NASA JPL Horizons for Chiron (valid 1900 to 2100). The North Node is the true (osculating) node. We compared 500 random moments between 1900 and 2100 for every body with NASA JPL Horizons (ephemeris DE441):

Body	Largest error found (degrees)	Typical error (median)
Sun	0.00045	0.00005
Moon	0.00499	0.00067
Mercury	0.00062	0.00007
Venus	0.00086	0.00009
Mars	0.00099	0.00013
Jupiter	0.00054	0.00013
Saturn	0.00056	0.00014
Uranus	0.00082	0.00013
Neptune	0.00106	0.00033
Pluto	0.00092	0.00010
Chiron	0.00031	0.00005

The true node agrees with Horizons to within 0.021 degree, and the sidereal time (the Earth's rotation angle) to within 0.62 seconds of time.

3. How the lines are drawn

• MC line: the longitude where the planet's hour angle was zero, that is, where it was on the meridian above you. It runs straight from pole to pole.
• IC line: 180 degrees away, where the planet was on the meridian below.
• AC and DC lines: for each latitude, the longitude where the planet's altitude was exactly zero in the east (rising) or the west (setting). They curve, and they stop at latitude 90 minus the planet's declination, beyond which the planet did not rise or set that day.

Conventions: lines are geocentric and geometric. They use the center of the planet at the true horizon, without atmospheric refraction or the Moon's parallax, which is the usual convention in astrocartography. By default each line uses the planet's real position in the sky, including its small latitude north or south of the zodiac (called in mundo). You can switch to "on the ecliptic", which uses the zodiac degree only; then a planet on its AC line sits exactly on the relocated Ascendant. The two methods can differ by several degrees of longitude for Pluto (up to 6.9 degrees in our tests), a few degrees for Venus, Mars and the Moon, and not at all for the Sun.

4. The tests

What was tested	Result
Right ascension of each body vs NASA JPL Horizons (330 tests)	largest error 0.0035 degree
Declination vs NASA JPL Horizons	largest error 0.0018 degree
Points on MC and IC lines: is the planet exactly on the meridian there? (checked with Horizons, 176 points)	largest error 0.0013 degree
Points on AC and DC lines: is the planet exactly on the horizon there? (Horizons)	largest error 0.0061 degree
Every point of 800 line sets and 7.5 million curve points: self-consistency	errors below 0.0000000000002 degree
Distance to the nearest line vs a brute-force search (60 random places)	0.000 km difference
Parans: both planets exactly on their angle at each paran latitude (23,760 parans, 60 charts)	errors below 0.0000000004 degree, none missed
Local space lines: start at the birth place, leave in the planet's compass direction, stay on one great circle	errors below 0.000000000006 degree

Two honest limits. First, no published astrocartography map lists its line longitudes, so we could not compare against another program's map; the check rests on the astronomical definitions and on NASA's independent data. Second, the accuracy of your map is limited by your birth time far more than by the calculation.

5. Parans, local space and the relocated chart

Parans are found by scanning every latitude for the moment when two planets reach their angles together, then refining each crossing; each result is checked by putting both planets back on their angles at that latitude. Local space lines leave your birth place in each planet's compass direction (azimuth) at your birth and follow a great circle around the world. The relocated chart keeps your birth moment and recalculates the houses for the new place, in Placidus, Whole Sign or Equal houses; above about 66 degrees of latitude, where Placidus cannot be calculated, Whole Sign houses are used and the map says so.

6. Distances

Distances to lines are great-circle distances on a sphere with the mean Earth radius of 6,371 km, measured to the nearest point of each line. The flat map stretches the far north and south; the globe view shows distances as they really are.

What the calculation cannot do

It cannot tell you what will happen in a place. Astrocartography is a symbolic tradition, not a science, and a careful calculation does not make it a prediction. Use it as one way to reflect on places, next to the practical facts of work, cost, family and law.