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How we calculate

Astrology is interpretation, but the sky it interprets is measurable. Here is exactly what we calculate, from which data, and where the limits are.

Planet positions and birth charts

Positions of the Sun, Moon and planets are computed with the open-source Skyfield library from the NASA JPL DE421 planetary ephemeris. We use apparent geocentric ecliptic longitudes. Houses use the whole-sign system, and the Rising sign comes from your birth time and place.

Birth time and time zones

Birth times are converted to UTC using an approximation based on longitude (15 degrees per hour), not a historical time-zone or daylight-saving lookup. For most births this is accurate to within a house, but a birth near a sign or house boundary may differ from a chart made with a full time-zone database.

Moon phases, retrogrades and calendars

Moon phase times come from Skyfield's almanac routines and are given in UTC. Retrograde windows run from station to station: the exact moment (found at hourly resolution) when a planet's apparent ecliptic longitude stops increasing and starts decreasing, and back again. Dates are UTC and match published ephemeris tables, though the calendar date can differ by a day in other time zones. The 2026-2027 tables are published as open data at /open-data.

Panchang (tithi and nakshatra)

Tithi is the Moon-Sun elongation in 12-degree steps. Nakshatras use sidereal Moon longitude with the Lahiri ayanamsa. Values are calculated for early morning in India (about sunrise). Exact muhurat times shift by location and time of day, so confirm with a local panchang.

Compatibility, personality and Chinese zodiac

Sun-sign compatibility compares classical element and modality. Chinese zodiac animals follow the Chinese lunar calendar (Chinese New Year dates from a lunar-calendar library), and yearly outlooks apply the traditional San He, Liu He, Liu Chong and Liu Hai relationships.

How horoscopes and answers are written

Daily, weekly and monthly horoscopes are assembled from a curated library of original sentences, chosen by a seeded selection for each sign and date, and paired with the real Moon phase and Mercury status for that day. They are general reflections, not individual predictions. "Ask an Astrologer" answers are built by a rule-based engine from real planet positions (solar houses, or transits to natal planets when you provide birth details); an open-source language model may optionally phrase them but is limited to the calculated facts.

Sources and open-source tools

• NASA JPL planetary and lunar ephemerides (DE421)
• Skyfield, the open-source astronomy library used for all positions
• International Astronomical Union for astronomical terminology
• Chinese calendar conversion via the open-source lunardate library

Our own tables are published as open data at /open-data.

Editorial standards and limits

• Astrology content is for reflection and entertainment. It is not medical, legal, financial or professional advice.
• We do not make guarantees or claim that astrology predicts outcomes.
• We do not publish fake testimonials, invented statistics or paid reviews.
• Birth details you enter are used only to calculate your result. Community questions are stored anonymously and never include birth details.
• Found an error? Email hello@simplyastro.com and we will correct it.