Four house systems, one birth moment, the gist

Four house systems, one birth moment

Four different rules for slicing a birth chart into its twelve sections are run on the very same birth moment, and the essay measures exactly how far apart their answers land.

This is the short way in. The essay has the working, the sources and the sky.

The four rules agree on one number and one only, the rising degree. After that they part company, and for this chart slightly more than half the zodiac is ground where they cannot all say which section a planet is in.

A birth chart divides the sky into twelve sections. In English they are houses; in Sanskrit, bhavas. Where does each section start? There is no single answer, only competing rules. Four are tested on one moment: a hypothetical birth at 9:12 on the morning of 22 July 2009 at Ujjain.

All four need the same two starting directions. One is the rising degree, the point coming up over the eastern horizon, worked out by hand in an earlier essay as 162 degrees 52 minutes 7 seconds. All four accept it unchanged. The other is the midheaven, the point of the sun's yearly track standing highest then, computed here in one line: 72 degrees 45 minutes 33 seconds.

Then they part. Equal houses is the simplest: start at the rising degree and add thirty degrees, eleven times. Sripati, named for an eleventh-century Indian astronomer, cuts each quarter of the circle into three, then treats those points as section middles rather than edges, which slides every boundary back about fifteen degrees. Placidus, from a seventeenth-century Italian monk, divides time instead of distance: it follows a point of sky from rising to highest and cuts that climb into thirds. That cannot be solved in one go, because each answer depends on itself, so it is guessed and corrected until the number stops moving. KP uses Placidus unchanged, and differs only in counting the zodiac from a starting point about six arcminutes from the one most Indian charts use.

The essay prints all four sets of twelve numbers side by side. At face value the biggest gap is nearly sixteen degrees, but almost all of that is Sripati counting middles while the others count edges. Match the conventions up and the real disagreement is smaller and uneven: nothing at the first boundary, six arcminutes at the fourth, a maximum of 1 degree 54 minutes at the second. One check runs the whole thing against Swiss Ephemeris, the library much chart software uses, which agrees to within fourteen arcseconds. The essay names no winner. That is a judgement, and the arithmetic does not have one.

Words decoded

house, or bhava (भाव): one of the twelve sections a birth chart divides the sky into
cusp: the boundary where one section ends and the next begins
house system: a rule for deciding where those twelve boundaries fall
rising degree, or ascendant, or lagna (लग्न): the point of the sun's yearly track coming up over the eastern horizon at a given moment
midheaven, written MC: the point of that same track standing highest at that moment
ecliptic: the track the sun follows through the sky across a year, along which the twelve signs are marked
degree, arcminute, arcsecond: a degree is a 360th of a circle, an arcminute a 60th of a degree, an arcsecond a 60th of an arcminute
declination: how far north or south of the sky's equator a point sits
semi-diurnal arc: the time a point of sky takes to travel from rising to standing highest
sidereal time: a clock reading that says which part of the sky is facing you
obliquity: the tilt of the earth's axis, about 23 degrees 26 minutes
ayanamsha (अयनांश): the gap between counting the zodiac from the spring equinox and counting it from the stars, now about 24 degrees
Sripati (श्रीपति): an Indian astronomer of the eleventh century, from what is now Maharashtra
KP, or Krishnamurti Paddhati: a twentieth-century Indian school of chart reading that borrowed the Placidus sections
Swiss Ephemeris: the calculation library that sits under a great deal of astrology software

Why anyone should careTwo people can hand you the same birth details and get charts that put the same planet in different houses, and nothing is broken. They picked different rules for drawing the boundaries. Most software does not announce which rule it used, and the difference is not trivial: for the chart in this essay, more than half the circle is ground where the four rules do not agree. Knowing that changes what a reading is worth arguing about. It also tells you a question to ask, which is simply: which house system is this, and why that one?

Read it properly, 8 min Four house systems, one birth moment