Placidus houses and cusps: reading your report's table

A cusp is the exact degree where a house begins: that is the definition given in your report's glossary. It is located like a planet, by a degree, a minute and a sign. The house then extends up to the next cusp.

Four cusps have names of their own, because they coincide with the angles of the chart: the cusp of house 1 is the Ascendant, that of house 4 the Imum Coeli, that of house 7 the Descendant, that of house 10 the Midheaven. The other eight are intermediate cusps, and it is on them that house systems diverge the most.

A cusp serves two purposes. It says which house each planet falls in, which fills the House column of the positions table. And its sign designates the house ruler, the planet through which that area is set in motion.

The astrological tradition has produced many ways of dividing the sky, and none has prevailed everywhere. Four come up most often:

• Whole signs: the sign rising at the Ascendant forms house 1 in its entirety, the next sign house 2, and so on. Many historians see it as the main method of Hellenistic astrology, although the question is still debated, and it has been enjoying a revival since the translations of ancient texts begun in the 1990s.
• Equal houses: twelve houses of 30° each, counted from the degree of the Ascendant. The Midheaven no longer necessarily coincides with the cusp of house 10.
• Placidus: a system named after the Italian monk Placidus de Titis (1603-1668), who championed it in the 17th century, although the method is older. It divides into equal parts the time each degree of the zodiac takes to travel from the horizon to the meridian. The houses are unequal, and the four angles remain the cusps of houses 1, 4, 7 and 10. It is today the most widespread system in Western astrology.
• Koch: a 20th-century system, also based on time and with unequal houses. It bears the name of the German astrologer Walter Koch (1895-1970), but it was worked out by Friedrich Zanzinger and Heinz Specht.

Each rests on a coherent conception of space and time. The astrologers who use them draw solid readings from them, and the debate between them remains open.

AstralPro uses Placidus. The engine explicitly enforces it in every calculation instead of relying on the ephemeris software's default setting: if that setting changed one day, every chart would shift without anything signaling it. The same system serves the natal chart, the solar return and the progressions, so that the houses correspond from one report to another. Your report's method note mentions it: Placidus houses.

The twelve cusps come from a single calculation, and no house is recalculated anywhere else. This is a rule of consistency: two systems in the same report would produce invisible contradictions, a table placing a planet in house 12 and a sentence discussing it in house 1.

This choice does not say the other systems are wrong. It guarantees that everything you read, tables, prose and life domains, speaks of the same houses. If you compare with a site that works in whole signs, a difference in house is to be expected and does not signal a calculation error.

In Chapter I, just below the positions table, your report gives a second table titled “House cusps”. It has four columns: House, Position, Sign, Ruler. The degrees there come from the same calculation as the rest of the report, so that you can check every sentence of the kind “your house 10 has its cusp in Virgo”.

For Camille Morel, AstralPro's sample chart (fictional data), it reads as follows:

• House 1: 15°59' Scorpio, ruler Pluto (this is her Ascendant)
• House 2: 15°40' Sagittarius, ruler Jupiter
• House 3: 22°06' Capricorn, ruler Saturn
• House 4: 0°15' Pisces, ruler Neptune (her Imum Coeli)
• Houses 5 and 6: 2°12' and 26°43' Aries, ruler Mars for both
• House 7: 15°59' Taurus, ruler Venus (her Descendant)
• House 8: 15°40' Gemini, ruler Mercury
• House 9: 22°06' Cancer, ruler the Moon
• House 10: 0°15' Virgo, ruler Mercury (her Midheaven)
• Houses 11 and 12: 2°12' and 26°43' Libra, ruler Venus for both

Opposite cusps share the same degree in opposite signs: 15°59' Scorpio and 15°59' Taurus, 0°15' Pisces and 0°15' Virgo, 22°06' Capricorn and 22°06' Cancer. This symmetry is not specific to Placidus, all the usual systems respect it, and you can check it on your own table.

Unequal houses have a visible consequence here. Camille has six planets in Scorpio, but they are not all in house 1. Her Ascendant is at 15°59' Scorpio: Jupiter (0°50'), Venus (6°30') and Mercury (6°37') lie before this degree, and so still in house 12, which begins at 26°43' Libra. The Sun (21°55'), Pluto (25°17') and the Moon (27°09'), placed after the Ascendant, are in house 1.

In whole signs, all of Scorpio would form house 1, and the six planets would be gathered there. Her report, calculated in Placidus, instead reads Mercury, Venus and Jupiter in the register of house 12, the house of what is prepared away from view. A single sign can thus be shared between two houses, and it is the cusp that decides.

Another effect of unequal houses: two cusps can fall in the same sign (Aries carries houses 5 and 6, Libra houses 11 and 12), while a whole sign can be lodged inside a house without carrying any cusp. In Camille's chart, this is the case for Aquarius, contained in house 3, and for Leo, contained in house 9.