The Natal Chart and the House Systems

What a natal chart is

A natal chart is not a picture of the sky as it looks. It is a measurement: where each body stood along the ecliptic, the circle the Sun appears to travel through the year, at the moment and from the place a person was born.

Three numbers make it. The date and time fix the positions of the bodies. The longitude of the birthplace fixes which degree of the zodiac was rising in the east at that instant, the Ascendant. The latitude decides how steeply the zodiac meets the horizon there, which is why two people born at the same second in Oslo and Nairobi have different rising signs.

Everything an astrologer reads afterwards is built from those measurements: a body’s sign, the house it falls in, the angular distances between bodies, and the dignity it holds where it stands. The chart itself makes no claims. The reading does.

It helps to keep the two layers apart. The positions of the planets are astronomy, and any competent calculation of them will agree with any other to a small fraction of a degree. The houses, the dignities and the meanings are astrology, and that is where traditions, schools and individual astrologers part company. Much of what follows is about the first point at which they part: how the circle is divided into houses.

The two points every system agrees on

Every house system in use agrees on two points, because those two are measurement rather than convention. The Ascendant is the degree of the ecliptic rising on the eastern horizon. The Midheaven (Medium Coeli, MC) is the degree culminating on the meridian, the highest point the ecliptic reaches over the birthplace at that moment. Opposite them lie the Descendant, setting in the west, and the Imum Coeli (IC), directly underfoot.

Everything a house system does after that is a decision about how to fill the space between those points. Whichever system is chosen, the planetary longitudes, the Ascendant and the Midheaven are the same numbers: house systems disagree about the boundaries between the angles, never about the angles themselves.

Whole-sign houses

A house system answers one question: how do you cut the circle into twelve fields of life? The whole-sign answer is the simplest and the earliest attested in Hellenistic practice. The sign that holds the Ascendant is the first house, entire, and each following sign is the next house, entire.

The consequence is that houses and signs coincide. If the Ascendant falls at 28 degrees of Leo, the whole of Leo is the first house, the whole of Virgo the second, and a planet at 2 degrees of Leo is in the first house even though it has already risen above the horizon.

Whole sign does not try to fit the Midheaven to a house cusp at all. The rising sign is the first house, and the Midheaven falls wherever it falls, which may be the tenth house, the ninth or the eleventh. Hellenistic astrologers treated the Midheaven degree as a sensitive point in its own right, read alongside the tenth sign rather than as its boundary. Much of the classical timing material, including annual profections and zodiacal releasing, is built on whole signs and only makes sense in them.

Equal houses

Equal houses keep the Ascendant’s exact degree and step 30 degrees at a time from it. With the Ascendant at 28 Leo, the second house begins at 28 Virgo, the third at 28 Libra, and so on round the circle. Like whole sign, the equal system gives twelve houses of the same size, and like whole sign it is older than the attempt to make houses unequal. Its practical difference from whole sign is that a planet early in the rising sign, above the horizon, falls in the twelfth house rather than the first.

The quadrant systems

The quadrant systems all begin from the opposite instinct: that the Midheaven should be the cusp of the tenth house and the IC the cusp of the fourth. That divides the circle into four quadrants of usually unequal size, and each system then has to decide how to cut each quadrant into three. They differ in what they divide.

Porphyry

Porphyry simply trisects the arc of ecliptic in each quadrant. If there are 84 degrees of zodiac between the Ascendant and the IC, each of the three houses in that quadrant receives 28. It is the simplest of the quadrant methods and is named after the Neoplatonist philosopher, though the method itself is older.

Regiomontanus

Regiomontanus divides the celestial equator into twelve equal parts and projects the result onto the ecliptic, through great circles that pass through the north and south points of the horizon. It is the standard of late medieval and Renaissance work, and horary astrology is usually cast in it, because the classical horary texts were written with its cusps in mind.

Campanus

Campanus divides the prime vertical, the circle running through the east point, the west point and the point directly overhead. Its houses are therefore a division of the sky around the observer rather than of a coordinate such as the ecliptic or the equator.

Placidus

Placidus divides something else again: time. Each degree of the ecliptic takes a certain span to travel from the horizon to the meridian, and Placidus cuts that span into three. It is the most widely used system today, largely because the tables of houses printed in the nineteenth and twentieth centuries were Placidian.

It is also the only common system that can fail outright. Inside the polar circles some degrees of the ecliptic never rise at all. They have no arc above the horizon to divide, and the construction has no answer for them. Software that offers Placidus has to fall back to another system at those latitudes, Porphyry being a common choice, rather than print a cusp it cannot justify.

One curiosity is worth knowing. At the equator there is no pole to tilt the house circles by, and Regiomontanus, Campanus and Placidus collapse onto exactly the same cusps.

Why the choice matters

The systems disagree most for a body sitting near a cusp, and they can disagree by a whole house. In quadrant systems at higher latitudes a house can be fifty degrees wide or fifteen, so a planet’s house genuinely changes from one system to another. A planet at 2 degrees of Leo with 28 Leo rising is in the first house by whole sign and in the twelfth by Placidus or Porphyry.

Neither answer is a mistake. They are different doctrines about what a house is: a sign, a thirty-degree stretch from the rising degree, or a portion of the daily turning of the sky. That is worth knowing before comparing two readings of the same chart, because an argument about which house Mars is in is very often an argument about which system was used.

A sensible practice is to know which tradition a technique comes from and to use the house division that tradition used. Hellenistic material reads naturally in whole signs; the medieval and Renaissance horary literature assumes Regiomontanus; much of twentieth-century psychological astrology was written with Placidus.

Why the birth time matters most

The birth time moves a chart faster than anything else in it. The Ascendant passes through the whole zodiac in a day, roughly a degree every four minutes, so a single minute of clock error moves it by about a quarter of a degree. An hour out shifts the rising sign, and with it every house and everything measured from them, while the planets barely move at all. The Moon advances about half a degree in that hour; the outer planets, almost nothing. Next to that uncertainty, the small differences between good astronomical calculations are invisible.

So if the rising sign of a chart looks wrong but the Sun and Moon look right, the time is the thing to check.

Summer time and the clock as it read

The commonest way to get a chart wrong is to help. A birth time should be taken exactly as the clock at the birthplace read it, and then converted using what that place was actually doing at that date. Someone who knows the clocks were forward and takes an hour off before the conversion is made will see the hour come off twice.

Whether summer time was in force on a given date cannot be worked out from the date alone. Countries adopted it, dropped it and adopted it again; wartime rules differed from peacetime rules; some regions kept a different practice from their capitals. What was actually in force has to be looked up in the historical record for that place and year, which is what the time-zone databases used by modern calculation do. An offset is a number that was true on one date. A zone is a place with a history, and only the history can answer for a birth in 1943, or in a country that changed its mind twice in a decade.

A birth time recorded only to the nearest hour, or remembered by a parent as “around lunchtime”, should be read with that uncertainty in mind. The planets in signs, and the aspects between them, will usually stand. The Ascendant, the houses and anything timed from them are suggestions until the time is better known.