How StarMapper calculates your lines
Astrocartography maps from different sites can disagree, and it is rarely obvious why. This page explains exactly how StarMapper computes your map — the ephemeris, the projection method, the time zone handling — and where its honest limits are.
Planet positions: Swiss Ephemeris
StarMapper computes planetary positions in your browser with Swiss Ephemeris (v2.10, compiled to WebAssembly) — the standard high-precision ephemeris developed by Astrodienst from NASA JPL data, and the engine behind most professional astrology software. If the module cannot load in your browser, we fall back to the VSOP87-based astronomy-engine library; the two agree to within arcminutes, which is a mile or two on the map. Rising and setting lines are additionally cross-checked against an independent Swiss Ephemeris–based calculation service.
Line geometry: the in-mundo (Jim Lewis) method
Lines are computed in mundo: from each body's true position in the sky (right ascension and declination, including its ecliptic latitude). Your MC line is the meridian where the planet culminates; the ASC line traces every point on Earth where it sat exactly on the eastern horizon at your birth moment. This is the method Jim Lewis used for the original Astro*Carto*Graphy maps, and the method Astro.com uses today.
Why other calculators show different lines
There are two legitimate conventions. The in-mundo method (used by StarMapper and Astro.com) uses the body's real position in the sky. The zodiacal method (the default on some other calculators) first projects the body onto the ecliptic, discarding its celestial latitude. For bodies that sit far from the ecliptic — Pluto, Chiron, sometimes the Moon and Venus — the two methods can place rising and setting lines hundreds of miles apart. That single difference explains most “every site gives me a different map” confusion. Remaining differences between serious tools come down to ephemeris choice and sidereal-time models: a few miles at most.
Birth time, time zones, and DST
Your birth city is geocoded with OpenStreetMap, and its time zone is resolved from the IANA time-zone database with full historical daylight-saving rules — so an October 1993 California birth is treated as PDT, not PST. Getting this wrong shifts every line on the map, which is why we resolve it from the historical record rather than asking you to guess.
Relocated charts: Placidus houses
Clicking a city on your map (or searching one) shows your relocated chart there: the same birth moment recast for that location. Planets keep their natal zodiac degrees — only the angles and house cusps move. We compute the relocated Ascendant, Midheaven, and house cusps with Swiss Ephemeris' house routine using Placidus houses, the mainstream default and what Astro.com uses, so you can compare charts one-to-one. Inside the polar circles, where Placidus is mathematically undefined, we fall back to Porphyry — the same behavior as Swiss Ephemeris itself — and label the chart accordingly.
How close to a line counts
A line is a zone, not a wall. Its influence is generally strongest within roughly 150 miles (250 km), still noticeable out to about 300 miles (500 km), and fades into background beyond that. You do not need to live directly on a line for it to matter — and a city between two lines feels both.
Accuracy limits, honestly
Angular lines move with your birth time — roughly 15 miles per minute of clock error, so five minutes of uncertainty shifts rising and setting lines by 60–85 miles. If your birth time is approximate, read lines as broader corridors, and treat MC/IC lines (which shift the same way) with the same caution. If you only know the day, the Sun through Saturn lines are still broadly usable; Moon lines are not.
What the free map includes
Every map draws 48 planetary lines — Sun through Pluto plus Chiron and the North Node, each at the Ascendant, Midheaven, Descendant, and IC — with a written interpretation for every line, plus a relocated chart (new Ascendant, Midheaven, and Placidus house placements) for any city you click or search. The North Node is computed as the true (osculating) lunar node — the same convention Astro.com uses — rather than the mean node; the two can differ by up to about a degree and three quarters. Selecting a line also shows its influence zone and marks where it crosses other lines — the simplest on-map form of parans. Classic rise/culminate parans and local space lines are on our roadmap and not yet included; if your practice depends on them today, a tool like Astro.com or Astro-Seek covers them.
Questions about a specific chart, or found a discrepancy with another calculator? We take accuracy reports seriously — tell us about it.