What a jathagam is, and what you need to cast one

A jathagam is the sky at the moment of birth, drawn as a chart. It records where the Sun, the Moon and the planets stood against the band of stars they all travel through, and which part of that band was rising on the eastern horizon. Everything a family astrologer reads, from the birth star to the running period, comes out of that one picture.

The three things you need

• The date of birth. It fixes where the slow grahas stood, and roughly where the Moon was.
• The time of birth, as exactly as it is known. The Moon moves about 13 degrees a day, so the time decides its star and pada, and the rising sign moves faster still.
• The place of birth. Its latitude and longitude, and the time zone in force that day, turn the clock time into the moment the sky is computed for.

The time matters more than most families expect. The lagna, the sign rising in the east, changes about every two hours, faster for some signs and slower for others, and four minutes of clock moves it a whole degree. A hospital record, a letter written that week or the old family notebook is worth looking for before anything is decided from a chart.

What the chart shows

The zodiac is divided into twelve rasis of 30 degrees each. In the South Indian chart every rasi keeps its own square: Meenam and Mesham sit at the top left, and the signs run clockwise around the edge. The planets are written into the square of the sign they were in, and the square of the lagna is marked. The North Indian chart draws the same facts as a diamond with the lagna always at the top, so the two look different while saying the same thing.

The positions are sidereal: they are measured against the stars rather than the seasons. The difference between the two, the ayanamsa, is about 24 degrees today, which is why a Western sun sign is usually one sign ahead of the Indian one. This site uses the Lahiri ayanamsa by default, as most Tamil almanacs do.

When the time is not known

Many older horoscopes carry only a date. The chart can still be cast for noon. The Moon moves slowly enough that its rasi is right on most days and its star on many, and the app says when the star or the rasi changed during that day. What cannot be trusted is the lagna and everything counted from it: the houses, the house lords and most of the yogas. Treat those as a placeholder until the time is found.

A jathagam is also cast once, not once a year. The birth chart stays the same for life; what changes is the running period and the transits of the planets over it, which later lessons explain.

Reading the sample

The chart beside this lesson is a real one: 25 April 1996, 08:30 in the morning, Villupuram. The lagna is Rishabam, so its square is shaded. The Moon is in Kadagam, in the star Poosam. The Sun and Sevvai share Mesham, Budhan and Sukran sit in the lagna itself, Guru is in Dhanusu, and Sani and Ketu share Meenam, with Rahu opposite in Kanni. The rest of the course uses this same chart, so every idea can be checked against something concrete.