Setting aside any discussion of the merits of astrological beliefs, which vary from one astrologer to another, mathematics is indispensable for constructing the domification (from the Latin domus = house), a fundamental component of the birth chart, not to mention the calculations needed to establish ephemerides.
Since the ascendant and the sun sign do not distinguish between charts (they coincide, for instance, at the start of spring), astrologers sought to take the birthplace into account by dividing terrestrial space, rather than celestial space, into as many houses as there are signs, so as to preserve an analogy between houses/sky and signs/celestial sphere. These houses, numbered with Roman numerals, are supposed to compartmentalize human psychological functions. While astrologers have agreed on the meaning assigned to each house, the same cannot be said of their positions on the ecliptic (the plane of the sun's trajectory). Mathematicians have established many varieties of domification, proving, if proof were needed, astrology's charming inconsistency. Furthermore, of course, problems arise at the poles.
Domification systems: totally ineffective
In lunar astrology, there is one house for each day of the lunation—twenty-eight in all—spaced at intervals of 90/7° (roughly 12°51'26'') around the zodiac, while Tibetan astrologers divide the sky into eight. In ancient Greece, the earliest systems, known as Modus Aequalis, divided the ecliptic into equal 30° parts. Later, the system devised by the mathematician Campanus of Novara (c. 1220–1296), a translator of Euclid, attempted to divide space by partitioning the prime vertical equally—a circle passing through the zenith and the East and West cardinal points of the horizon—and then projecting the resulting division points onto the ecliptic using great circles passing through the South and North cardinal points of the horizon. The horizon is the circle cutting the celestial sphere into two parts, one visible, the other not. The system devised by Placidus (1603–1668), also a mathematician, is the most widespread, thanks to the wide circulation of his Tables of Houses. It divides time equally between the semidiurnal and seminocturnal arcs, which are then of unequal length. Refinements have been made to this method, but regardless, statistical studies carried out on more than twenty thousand charts over thirty years have proved the complete agreement of these domification systems… in their ineffectiveness!
