Astrology and the History of Computing

In antiquity, astronomy and astrology were not separate disciplines — the same scholars who tracked the planets also read meaning into their positions. The most remarkable surviving object from that world is the Antikythera mechanism, a hand-cranked bronze device recovered from a Roman-era shipwreck off the Greek island of Antikythera.

Built around 100 BCE (estimates span roughly 205–60 BCE), it is widely described as the world's oldest known analog computer. Of its original gearwork, surviving fragments preserve around 30 bronze gears, with more reconstructed from later analysis. Twenty-first-century X-ray imaging revealed thousands of tiny Greek characters across its surfaces — effectively an instruction manual — describing the motions of the Sun, Moon, and the five planets known at the time, and tracking astronomical cycles such as the 19-year Metonic cycle and the eclipse-predicting Saros cycle.

It is more precise to call it an astronomical calculator than a purpose-built astrological one: it modelled the heavens and predicted eclipses and calendar events. But in a period when celestial calculation and celestial interpretation were one craft, it stands as the earliest evidence of people building machines to do astronomy's hardest arithmetic for them. The Greeks who could build it were the same culture that formalized the houses and aspects of the birth chart.

For most of astrology's history, casting a chart by hand took a skilled practitioner an hour or more: looking up planetary positions in printed ephemeris tables, interpolating between dates, and converting local mean time. It was exactly the kind of repetitive, error-prone arithmetic that early computers were good at.

In the late 1960s the French astrologer André Barbault lent his interpretive texts to Astroflash, an early computerized horoscope service running on an IBM mainframe. Astroflash terminals were installed in high-traffic public places — first in Paris, then, in 1969, a much-publicized installation at Grand Central Terminal in New York that printed a fourteen-page horoscope in about two minutes — and the service produced thousands of chart reports a month. It was one of the first times astrology met a computer at commercial scale, and it drew sharp criticism from other astrologers who felt the craft was being mechanized.

A few years later, in 1973, the mathematician Neil F. Michelsen — who worked at IBM and had learned to program chart calculations on an IBM 1130 — founded Astro Computing Services. ACS generated charts on demand and, in 1976, began publishing The American Ephemeris, a series of computer-generated planetary tables built on rigorous astronomical data. Michelsen's ephemerides became a standard reference — a reminder that astrological practice has always depended on good astronomy.

When affordable microcomputers arrived in 1977 — the Commodore PET and the Apple II among them — astrology software followed almost immediately. In Ann Arbor, Michigan, the self-taught programmer and musician Michael Erlewine bought a Commodore PET and founded Matrix Software that same year, widely credited as the first company to put astrological calculation onto personal computers.

Erlewine fit a working chart program — geocentric and heliocentric positions, multiple coordinate systems — into 8 kilobytes of memory, and in 1980 published the Manual of Computer Programming for Astrologers, a how-to that taught a generation of hobbyists to code their own chart engines. The themes are familiar from any history of early personal computing: hard math, tight memory, and enthusiasts sharing programs on cassette tape. Astrology was, in its small way, one of the first things people wanted their new home computers to do.

That first generation of microcomputer astrology software has a living legacy: programs such as Solar Fire and the free, open-source Astrolog are still actively used today, direct descendants of the chart engines hobbyists were typing in from cassette tape in the late 1970s.

The thread runs forward from there. In 1983 in Zürich, Alois Treindl founded Astrodienst, which would later release the Swiss Ephemeris — a high-precision astronomical library, developed principally by Dieter Koch, that compresses NASA JPL's planetary data into a form a chart engine can query directly. It is the direct ancestor of the calculation layer described in the next sections, and the engine StellarTies itself runs on.

As computing moved from printing chart reports to displaying them on screens, the symbols themselves needed a home in the digital character set. That came with Unicode, the standard that assigns every character a stable numeric code point.

Unicode 1.1, released in 1993, included the twelve zodiac signs as a contiguous run from U+2648 (Aries) to U+2653 (Pisces), inside the Miscellaneous Symbols block (U+2600–U+26FF), alongside symbols for the classical planets. Later releases broadened the set: the Alchemical Symbols block (U+1F700–U+1F77F), added in Unicode 6.0 (2010), encoded the historical marks for the four elements and many alchemical substances and processes. Symbols for modern bodies such as the dwarf planets Eris and Sedna were added later still — in Unicode 11.0 (2018), in the Miscellaneous Symbols and Arrows block, not the alchemical one.

There is a quiet durability here worth noting: code points are very rarely removed from Unicode once adopted, because removing one would break every document that already uses it. So these symbols, some of them thousands of years old, now have a stable and effectively permanent address in the world's text standard — the same standard that encodes every modern alphabet, emoji, and mathematical operator. A notation that began on clay and parchment was carried, intact, into the permanent vocabulary of digital text.

Look across this whole history and a pattern emerges: every machine, from the Antikythera gears to the IBM mainframe to the Commodore PET, was put to work on the same task — the raw arithmetic of where the planets are. Astroflash, Michelsen's ephemerides, and Erlewine's chart engines all attacked that calculation problem. It is the foundation, and it has to be exact.

StellarTies stands on that foundation. It uses the Swiss Ephemeris — Astrodienst's astronomical library, built on NASA JPL's Development Ephemeris (the DE431 and DE441 series that the Jet Propulsion Laboratory produces for spacecraft navigation) — to calculate every chart to sub-arcsecond accuracy. That precision is what makes a real birth chart possible at all, and it is non-negotiable: the interpretation is only as trustworthy as the numbers under it.

But raw positions are where the older systems stopped. What a modern platform adds is the layer above the arithmetic: structured interpretation. The harder, more interesting problem is not computing one chart — it is comparing two. StellarTies builds synastry on top of the precise calculations: it reads the geometry between two people's charts and generates a coherent narrative about the relationship — the shape of a connection between partners, friends, or family. Early computing automated the calculation. What StellarTies adds is the step the older systems never took: it uses a language model to synthesize the synastry geometry — the precise angles between two people's charts — into one coherent, readable narrative about the relationship, rather than leaving the reader to interpret a grid of aspects alone.