Tropical
Default in Western practice.
All traditions
Alexandria, c. 150 CE, Europe, 1647
Western Astrology
Babylonian record-keeping, Greek geometry and Renaissance printing, folded into one diagram of the sky at the moment you were born.
Where it comes from
The sky was catalogued long before it was interpreted. Babylonian scribes kept diaries of planetary positions for centuries, patient, unglamorous columns of observation, and by the fifth century BCE had divided the ecliptic into twelve equal thirty-degree signs. That division, not the constellations, is what a chart still uses.
Hellenistic Alexandria turned the catalogue into a technique. Someone, the tradition names Hermes, the evidence names nobody, combined the signs with the rising point of the eastern horizon, and the natal chart was born: not a general horoscope for a month, but a diagram for a single person at a single minute. Ptolemy's Tetrabiblos, written around 150 CE, gave it a physics and a reputation that survived a millennium.
The knowledge travelled by translation. Greek into Arabic in ninth-century Baghdad, where Abu Ma'shar systematised it; Arabic into Latin in twelfth-century Toledo and Sicily; Latin into vernacular print in the seventeenth century, when William Lilly's Christian Astrology (1647) put the craft in English for the first time and made it briefly a mass literacy. It was, throughout, a university subject sitting beside medicine and mathematics, and it was thrown out of the university by the same instruments that made its calculations exact.
What remains is a very old habit: standing under an indifferent sky and reading your own life in its arrangement.
The planets are calculated. What they mean is a two-thousand-year-old conversation, and you are joining it, not concluding it.
How a reading is built
- 01Time, then place
Your birth date and clock time are converted to universal time using the historical rules for your birthplace, including the war-time and daylight shifts that make so many mid-century charts wrong elsewhere. The place gives latitude and longitude; the moment gives everything else.
- 02Tropical or sidereal
Tropical zodiac starts at the spring equinox, so it follows the seasons; sidereal starts from the fixed stars. Because the Earth's axis drifts, the two have separated by about twenty-four degrees, which is why the same birth reads one sign earlier in sidereal. Neither is more accurate, they answer different questions. Western practice is tropical unless you follow a Vedic or star-based school.
- 03The Ascendant and Midheaven
The Ascendant is the degree rising on the eastern horizon at your birth; the Midheaven is the highest point of the ecliptic. Both move roughly one degree every four minutes, which is why birth time matters so much more than birth date for these. Without a time, they are omitted rather than guessed.
- 04House systems
Houses divide the chart into twelve areas of life. Placidus cuts by time of day, so houses stretch and compress as lived hours do, the common default. Whole Sign makes one sign one house, the oldest and plainest scheme. Equal House lays even thirty-degree slices from the Ascendant, which stays sane at far northern latitudes where Placidus distorts badly.
- 05Aspects
The angles between planets carry most of the meaning: conjunction fuses, opposition pulls apart, square grinds, trine flows, sextile offers. Each is allowed a small orb of tolerance, and a tight aspect is read louder than a loose one.
- 06Retrogrades and the Moon
A retrograde planet is not moving backwards; from Earth it appears to, and the tradition reads that as a turn inward. The Moon's phase and its position, which changes about half a degree an hour, give the chart its mood and its speed.
Choosing a school
What changes when you choose a school
These choices are named so you can decide deliberately. Some change the calculation and others change the interpretive frame; the reading records the choice you make.
Sidereal
Fixed-star alignment.
Placidus
Default. Time-based division.
Whole Sign
One sign, one house.
Equal House
Thirty degrees from the Ascendant.
What it will not tell you
- An unknown birth time removes the Ascendant, the Midheaven and every house placement. The reading says so instead of inventing them.
- A chart describes temperament and tension, not events, health outcomes or the character of other people.
To read further
- Ptolemy, Tetrabiblos (c. 150 CE)The foundation text, in Robbins' Loeb translation.
- William Lilly, Christian Astrology (1647)The craft in plain seventeenth-century English.
- Chris Brennan, Hellenistic Astrology (2017)Modern scholarship on the Alexandrian origins.
Western Astrology
Birth date, time, and place set the ground. We read temperament and tension, and name what an unknown birth time leaves open.