Jyotish
Jyotish is Indian (Vedic) astrology, the "science of light." It differs from Western astrology above all in its zodiac: Jyotish uses the sidereal zodiac, tied to the stars, so its signs are shifted by about 24 degrees. Most people turn out to be one sign "earlier" in Jyotish than they're used to.
If the time is unknown, tick the box: the chart will have no Lagna or houses, and the Moon will be calculated for noon. Latitude, longitude and UTC offset are needed only if your city is not on the list; north latitude and east longitude are positive.
How It Differs from Western Astrology
The zodiac. Western astrology uses the tropical zodiac, tied to the vernal equinox. Jyotish uses the sidereal zodiac, tied to the fixed stars. The difference between them, the "ayanamsa," is currently about 24 degrees and grows by roughly one degree every 72 years because of the precession of Earth's axis.
The practical consequence: if you're an Aries in the Western system and were born in the first half of that period, in Jyotish you're a Pisces.
The Moon matters more than the Sun. In Jyotish, your sign is usually named by the Moon, not the Sun.
Nakshatras. 27 lunar mansions, a separate way of dividing the sky that doesn't exist in the Western tradition.
Dashas. Planetary periods instead of transits as the main predictive tool.
Rahu and Ketu are the lunar nodes, which in Jyotish count as full-fledged planets. Ketu stands for the experience of past lives — its sign and nakshatra are analysed by the past lives by date of birth calculator.
What Jyotish Includes
27 nakshatras: lunar mansions of 13°20′ each.
Dashas: a system of planetary periods that determine which theme is unfolding in your life right now.
Kundali Milan: a 36-point comparison of charts before marriage.
Panchang and Muhurta: the Indian calendar and the choice of an auspicious time.
Sade Sati — Shani's seven and a half years around the Moon, the best-known period in Jyotish, plus Shani's dhaiya.
Navamsa (D9) — the main divisional chart: the inner strength of the planets, marriage and the spouse.
Varshaphal — the Tajika annual horoscope: Muntha, the Lord of the Year and Mudda Dasha.
Beyond that there are vargas (divisional charts for specific topics), yogas (planetary combinations), and an extensive system of remedies: gemstones, mantras, rituals.
How the calculator works
The calculator builds a Janma Kundali, a birth chart in the sidereal zodiac with the Lahiri (Chitrapaksha) ayanamsa used in the official Indian ephemerides. Houses are whole signs from the Lagna, as in classical Jyotish. Rahu and Ketu default to the mean node, which most panchangs use; you can choose the true node in the form.
Positions are calculated right in your browser: the Moon with the theory from Jean Meeus's "Astronomical Algorithms", the Sun with the VSOP87 series, and the planets with JPL Keplerian elements corrected for the mutual perturbations of Jupiter and Saturn. Between 1900 and 2100 the difference from the professional Swiss Ephemeris is under 0.01° for the Moon and Sun and under 0.05° for the planets: this shows up neither in signs nor in nakshatras, except for a birth right on a boundary.
The chart is a foundation, not a ready-made reading. The calculator shows the Lagna, the signs and houses of the grahas, nakshatras, dignities and the current dasha, and briefly explains them according to tradition. It does not build vargas, yogas or predictions: in Jyotish interpretation depends on the whole chart, and the same yoga is read differently.
An Honest Look at the Method
Jyotish is an ancient system with an unbroken living tradition and enormous influence on Indian culture: it is used to choose wedding dates, children's names, and the time to start a venture.
It has as much testable predictive power as Western astrology, which is to say none. Indian scientists discuss this openly: in 2001, the decision to include Jyotish in university curricula drew protests from a large part of the scientific community.
The practical side deserves a separate word: recommendations to buy expensive gemstones and pay for rituals to "fix" a chart are the most commercialized part of the tradition, and they should be treated accordingly.