Last updated: August 2026
To generate a chart, we ask for your birth date, birth time, and birth place. Birth time is optional but recommended: without an accurate birth time, your Ascendant (rising sign), house placements, and dasha timing cannot be calculated precisely. If birth time is left blank or isn't in a recognized format, we use a default of 12:00 (noon) local time so we can still calculate your Sun, Moon, and planetary signs — but in that case, your Ascendant, houses, and dasha precision are marked as unavailable or reduced, rather than shown as if they were exact.
Your birth place (entered as free text, e.g. a city name) is matched against an offline database of world cities to resolve it to geographic coordinates (latitude/longitude) and an IANA timezone (e.g. Asia/Kolkata) — we do not call a live third-party geocoding API. If your place name matches more than one city, we select the most populous match, narrowed by any state, province, or country you included. Your local birth date and time are then converted to Coordinated Universal Time (UTC) using that timezone's full historical rules — including historical Daylight Saving Time changes — not just today's UTC offset.
Planetary positions for the Sun, Moon, Mercury, Venus, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto are calculated using astronomy-engine, an open-source astronomical calculation library, from your exact birth date, time (or the noon default), and UTC offset. We calculate each planet's geocentric apparent ecliptic longitude — its position as seen from Earth. A planet is marked retrograde when its calculated longitude one day later is behind its longitude at your birth moment.
AstroMystra calculates two parallel charts: a Western tropical chart, which measures zodiac signs from the spring equinox each year (the standard system used in Western astrology and Sun-sign astrology), and a Vedic (Jyotish) sidereal chart, which measures zodiac signs against the actual, fixed background of stars. The sidereal chart is derived from the tropical positions by subtracting the ayanamsa — the angular offset between the two zodiacs. We use the Lahiri (Chitrapaksha) ayanamsa, the reference used by India's official Rashtriya Panchang and the majority of Vedic astrology software, implemented as a linear approximation anchored to published historical Lahiri values (23.85333° on 1 January 2000, precessing at 0.0139289° per year) — accurate at the sign and nakshatra level, though it does not model the small periodic wobble present in the true value, so it is not intended for arcsecond-precision research use.
For the Western tropical chart, we calculate your Ascendant (rising sign), Midheaven, and house cusps using circular-natal-horoscope-js, a dedicated house-calculation library, with the Placidus house system — one of the most widely used systems in Western astrology. At extreme latitudes (66.5° or beyond), Placidus houses become mathematically undefined for parts of the year, so we automatically fall back to whole-sign houses in that case. For the Vedic sidereal chart, we use whole-sign houses — the standard system in traditional Vedic astrology — starting from the sign that contains your sidereal Ascendant. None of this is calculated, and your rising sign and house placements are not shown, when your birth time is unknown.
From your sidereal (Vedic) chart, we also calculate your Moon's Nakshatra and Pada (which of the 27 traditional lunar mansions it falls in, and which quarter of it); your Vimshottari Mahadasha and Antardasha, the current major and sub planetary period, using the standard 120-year, nine-planet Vimshottari cycle and the traditional balance-of-dasha formula based on your Moon's exact position within its birth nakshatra; and Mangal Dosha (Kuja Dosha, or 'Manglik' status) — whether Mars falls in one of the traditionally afflicted houses (1st, 2nd, 4th, 7th, 8th, or 12th) counted from your Ascendant or from your Moon. From your tropical chart, we also calculate major planetary aspects — Conjunction, Sextile, Square, Trine, and Opposition — wherever two planets fall within the traditional orb for that aspect.
The positions, houses, nakshatra, dasha, dosha, and aspect data described above are calculated first, by the deterministic calculation code described on this page — not by AI. That calculated data is then passed to an AI language model (Google Gemini for standard readings; GPT-5 Mini, via OpenRouter, for Oracle-tier premium readings), which writes the narrative, interpretive text you read, connecting your specific placements to traditional astrological meanings in natural language. The AI does not perform any astronomical or astrological calculation itself — it interprets calculations that have already been made.
AstroMystra's charts and readings are provided for entertainment, self-reflection, and spiritual or cultural exploration. Astrology and numerology are not scientifically validated predictive systems, and our content should not be used as a substitute for professional medical, legal, financial, or psychological advice. See our full Disclaimer for more detail.