How Sunsign Daily makes its readings

What the app computes, what it takes from tradition, and where the two are not the same thing. This page carries the evidence disclosure and every source note that used to sit under the readings themselves.

Published by Infoash UG (haftungsbeschränkt), Germany. Questions: hello@infoash.de (hello​@infoash​.de).

The disclosure, first

Sunsign Daily is for entertainment and self-reflection. There is no scientific evidence that astrology predicts events. Nothing in the app is advice.

That sentence is not a formality bolted on at the end. Describing something as a tradition is the framing that requires the disclosure — the US Federal Trade Commission's Health Products Compliance Guidance is explicit that a traditional- or historic-use claim has to clearly communicate the lack of scientific evidence — so the app was built with the sentence in it from the first screen, and it is repeated here in the same words.

What the app actually computes

One thing, and it computes it properly. Your sign is worked out from an ephemeris for the year you were born, not from a fixed date table. The Sun does not enter Aries at the same clock time every year: a tropical year runs about 365.2422 days against a civil year of 365, so each ingress slides roughly six hours later each year and then jumps back most of a day when February gets a twenty-ninth. For most people a date table gets the right answer. For people born within a day of a boundary it does not, and a table cannot tell you which of the two you are.

Everything else in the app — the daily reading, the colour, the pairings — is written content selected by that sign and the date. It is not calculated from anything, and it is not generated as you read it.

Where the readings come from

They are written in advance, by people, and reviewed before release. The app holds a bank of readings and picks from it; no text is produced by a language model while you read, and nothing is written individually for you. When a reading fits your day, that is selection working, not prediction.

The library, and what is ours rather than the tradition's

The app's library section is history and astronomy. Nothing in it is a reading. These are the notes that used to appear beneath each entry.

• Cardinal, fixed and mutable are modern labels. Ptolemy's own words for the three groups are solstitial and equinoctial, solid, and bicorporeal, and his reasoning is about the seasons rather than about temperament.
• The four elements are not Ptolemy's grouping. He does group the same three signs together, but he argues it from winds and compass directions, not from fire, earth, air and water. The elemental reading is a real tradition — it is just not the one the Tetrabiblos makes.
• Modern rulerships have discovery dates. Uranus was found in 1781, Neptune in 1846, Pluto in 1930. A rulership involving one of them cannot be older than that date, so the app gives the year and also gives the traditional ruler it replaced.
• Decan rulerships are a later addition. Decans are the three ten-degree thirds of a sign. Two rulership schemes are in circulation and they disagree, so the app names the one it is showing. Ptolemy has no ten-degree decan rulership scheme at all, so none is attributed to him.
• Colours are partly our choice. The planet-to-metal correspondences are documented. Planet-to-colour ones are not — no source was found for them that was not itself an astrology site. Where a colour is our choice rather than the tradition's, the app says so.

Where the zodiac came from

Twelve equal signs are an invention, and a fairly late one

Before there were twelve signs, the Babylonian MUL.APIN tablets listed around seventeen constellations standing in the path of the Moon — the same band the Sun and planets travel — and they were different sizes, because constellations are pictures and pictures are whatever size the stars make them. Somewhere in the fifth century BCE Babylonian astronomers threw that away and replaced it with twelve equal arcs of thirty degrees each: a coordinate system rather than a set of drawings. The oldest star list that uses the signs dates to around 410 BCE. Everything the West has done with the zodiac since rests on that one swap, from a picture to a number, and it was made for measurement.

The thirty-six decans arrived from a calendar, not from astrology

Egypt ran a civil year of twelve thirty-day months plus five days outside any month, and marked each of the thirty-six ten-day weeks by a star that rose just before dawn as it began. The scheme is drawn on the undersides of coffin lids from around the Eleventh Dynasty, late in the third millennium BCE. When Greek astrology later needed to subdivide a thirty-degree sign, thirty-six was already the number lying around: three neighbouring Egyptian decans went to each sign and became its three ten-degree thirds. A timekeeping habit turned into a coordinate, which is roughly how most of this subject was assembled.

Hipparchus noticed the sky had slipped

Around 128 BCE he compared his own measurements of Spica against records a century and a half old and found the star had moved about two degrees relative to the equinox. Not the star: the equinox.

The rate, so you can do the arithmetic yourself

Earth's axis wobbles like a slow top, one full turn in roughly 25,800 years. That works out at about fifty arcseconds a year, or one degree every seventy-two years — imperceptible in a lifetime and unmissable across a civilisation. Run it forward from Hipparchus and the equinox point drifts a full sign in about 2,150 years. Which is exactly what happened next.

Ptolemy knew, and chose the equinox anyway

This is the part that gets left out. By the time the Tetrabiblos was written in the second century CE, precession was known — Ptolemy inherited it from Hipparchus and wrote about it himself — and the signs had already come loose from the star groups they were named after. He does not hide it or hedge it. He states that a circle has no natural beginning, that Aries is placed at the vernal equinox by agreement, and that the beginnings of the signs are reasonably reckoned from the equinoxes and the solstices rather than from the constellations, because it is the turning points that his system claims to be about. So the mismatch everybody brings up as a debunking is not an error that crept in. It is a documented decision, made in the open, roughly eighteen centuries ago, by the man whose book made Western astrology what it is. You can think the decision was wrong. You cannot say nobody noticed.

Two zodiacs, one of which stayed with the stars

The Western tradition followed Ptolemy and kept its zero point at the March equinox: that is the tropical zodiac, and it is the one this app uses. Indian astrology kept its zero point tied to the fixed stars instead: that is the sidereal zodiac. For a while it made no practical difference, because the two nearly coincided — somewhere around the third century CE, though which century exactly depends on which fixed star you calibrate against, and there is no agreement on that. Precession has been pulling them apart ever since at that same degree per seventy-two years. The gap today is around two dozen degrees, close to a whole sign, which is why the same birthday gets one sign from a Western app and often the previous one from an Indian one. Neither has miscalculated. They are answering different questions and both say so.

Where the zero point actually is right now

The March equinox point — the origin of the whole coordinate system, the thing 0° Aries means — sits in the constellation Pisces, and has done since about the year 1. Under the constellation boundaries the International Astronomical Union settled in 1930, it crosses into Aquarius in 2597. That is the entire content of the Age of Aquarius: a line on a map, drawn in the twentieth century, that a slowly moving point will cross in the twenty-sixth.

The dates move for a much duller reason than precession

Worth separating, because popular explanations run them together constantly. Precession is the twenty-six-thousand-year wobble that moved 0° Aries out of the constellation Aries; it changes nothing about your birthday. The reason the Sun enters Aries on the 20th in one year and the 21st in another is the calendar, as described above. That is the drift this app computes, from an ephemeris, for the year you were actually born.

Three rulerships, and all three have a discovery date

Uranus was found in 1781, by an astronomer who first took it for a comet. Neptune was found in 1846, at a position worked out on paper before anyone looked. Pluto was found in 1930, named by an eleven-year-old, and demoted to dwarf planet by the International Astronomical Union in 2006. Their rulerships of Aquarius, Pisces and Scorpio therefore cannot be older than those dates — you cannot assign a planet nobody has seen. The signs' traditional rulers are Saturn, Jupiter and Mars, and they are in Ptolemy. The app shows both, with the dates, because a rulership presented without one is a claim you have no way to weigh.

The twelve-boxes-by-birthday format is about a hundred years old

Horoscopic astrology going back two thousand years worked from a whole chart — a moment, a place, seven moving bodies. Sorting the entire public into twelve groups by birthday alone is a newspaper format, and it dates to 1930, when a British Sunday paper ran a piece on a newly born princess, found it sold, and commissioned a weekly column that needed to address every reader at once. Twelve boxes was the answer to a production problem. That is not an argument against reading one — it is an argument for knowing what you are reading, which is a column format that outlived its newspaper.

What is deliberately not here

• No ayanamsha figure. The value depends on which convention you adopt and the conventions disagree about the starting date. The only sources found for a number were astrology sites, so the disagreement is stated instead of a figure.
• No claim that either zodiac is the correct one. That is not a factual question.
• No name attached to the 1930 newspaper column. Three popular accounts agree and none is a primary source, so the format gets a date and nobody gets a credit.

Sources

Ptolemy, Tetrabiblos, in the Robbins translation (Loeb) for the sign groupings, the elemental argument and the equinox decision; standard histories of Babylonian and Egyptian astronomy for the twelve signs and the decans; the International Astronomical Union for constellation boundaries and Pluto's 2006 reclassification; NASA for planetary discovery dates; a modern ephemeris for the ingress computation. The full list with section references is kept alongside the app's content and is available on request at hello@infoash.de.