What a birth chart actually computes

Give a chart one instant of time and one place on the earth. It reports where nine grahas sit on a 360 degree ring, plus the lagna, the degree of that ring rising in the east. Ten numbers, only nine independent, since one graha is always exactly opposite another.

Everything else is sorting. A sign is that number divided by thirty. A nakshatra is the same number divided by 13 degrees and 20 minutes of arc. Houses, or bhavas, are the same numbers measured from the lagna instead of from zero. Nothing is added between the numbers and the diagram.

Producing them takes about seven steps. The first, and the one that spoils most charts, is the clock. Legal time is a political arrangement, not physics. India has had one national offset since 1906, but the old city clocks did not give up at once. Calcutta kept its own, twenty-three minutes ahead, until 1948; Bombay its own, thirty-nine minutes behind, until 1955. "Quarter past nine, Bombay, 1951" is therefore two instants thirty-nine minutes apart, and the register rarely says which clock.

Place matters too. The earth turns fifteen degrees an hour, so one degree of longitude is four minutes of clock. Mumbai sits nine and a half degrees west of the line Indian time is set by. Its sky runs some thirty-eight and a half minutes behind the wall clock.

Four steps remain: the clock the sky itself keeps, which gains about four minutes a day on the wall clock; the planetary positions; two coordinate conversions; and the lagna, the chart's fastest-moving number.

The essay's real work is sorting those steps into three bins. Some involve no choice: two correctly written programs must agree, and if they do not, one is broken and can be found. Others are conventions, defensible more than one way: which ayanamsha, which house system, whether a dasha year runs 365.25 days or 360. Then comes interpretation, which begins at the exact word "means".

The essay has no quarrel with interpretation, only with a chart that lets the first bin's certainty leak into the third.