What is Jyotish? The Vedic astrology of the sky
Jyotish is the Indian system of astronomy and astrology, and it is used here as a traditional system of interpretation.
What it is
Jyotish (jyotiṣa, 'the science of light') covers two parts that the texts treat together: the calculation of planetary positions and time, and the interpretation of those positions. Vedanga Jyotisha, one of the earliest surviving texts, deals with the calendar and timing of rituals.
How it is worked out or read
The calculation part uses the sidereal zodiac, the nakshatras (27 lunar mansions) and the tithis (lunar days). The interpretive part uses rules about houses, grahas, dashas and yogas, which the classical texts describe. This site separates the two: the tools compute positions, and the learn pages explain the rules as they are written.
Where it appears on this site
The tools compute the panchang, birth chart, dasha, muhurat and chart checks. The learn section explains each term.
Worked example
Take a question a beginner often asks: why does a newspaper's horoscope say one sign while a Jyotish chart says another? The newspaper usually uses the tropical zodiac, which begins Aries at the spring equinox. A Jyotish chart uses the sidereal zodiac, which is about 24 degrees behind it today. So the same birth moment gives two different sun signs. Neither is a calculation error. They measure from different starting points. To see this in practice, take a birth on a given day, note the Sun's sign in a tropical table, then subtract the ayanamsa from its longitude. The sign shifts back by roughly one sign in most cases. The birth chart tool does this subtraction for each graha and shows the result as a sidereal sign.
Common mistakes
Three mistakes are common. The first is treating Jyotish as a single prediction method. The tradition contains several schools of calculation and several schools of interpretation, and they do not always agree. The second is mixing the calculation with the interpretation. A position can be computed accurately while the meaning given to it is a matter of tradition and judgement. The third is forgetting that results depend on the ayanamsa. Lahiri, Raman and KP give slightly different signs for planets near a boundary. A careful reader states which one was used, as this site does on every tool page.
Who studies it, and how
Jyotish is studied in several ways. Some families pass it down through a teacher, learning the calculation by hand from an almanac and the interpretation by reading the classical texts with a guru. Others come to it through a university course in the history of science, which treats the calculations as an early form of positional astronomy. A third group uses the tools and the printed almanac to check the sky and then reads the texts for meaning. None of these routes is the only correct one. A reader who wants to learn seriously should start with the calculation, because it can be checked, and then move slowly to the interpretation, asking at each step which text or teacher a rule comes from.