student. All Primary Arcs which can be formed between the sensitive points in a Nativity
during an entire lifetime are formed during some 6 hours after birth, and are produced solely
by the rotation of the Earth on its axis: the planets retaining their radical places and thus
carried round the heavens to form aspects to the places of the significators. For its reliability
the method is dependent upon the correctness of the birth time to within a fraction of a
minute, since an error of 4 minutes in the birth time results in an error of a year in the timing
of an event.
As actually described by the ancients, the planets, by motion of the Primum Mobile (q.v.) are
gradually carried round the Earth past the cusps of the Houses, and are brought into sundry
successive mundane aspects one with another. The calculation of these aspects and their times
of formation is termed "directing"; the result is described as "the directions in force" for the
calculated time. The number of degrees and minutes of Right Ascension passing over the
meridian between the moment of birth and that when the aspect is complete, constitutes the
Arc of Direction, each degree equivalent to one year of life.
There are various systems of Primary Directions, their one object to determine the times of
events. Ptolemy's system of measurement employed arcs of direction based upon the apparent
motion of the heavens about the Earth by virtue of the rotation of the Earth on its axis, in
which the body of one planet is brought to the place of another in a proportion of its
ascensional or descensional time as measured by its semi-arc. Thus a planet will progress to
the Midheaven by degrees of Right Ascension, while one below the horizon will progress to
the Ascendant by degrees of Oblique Ascension, which takes cognizance of the latitude of the
place of birth. Since a planet must be directed under the Pole (elevation), due to its
proportional distance from the meridian, one on the Midheaven has no Pole, while one on the
Ascendant has the same Pole as the Ascendant, which is the latitude of the birthplace. All
others between the Midheaven and Ascendant, whether above or below the horizon, have a
Pole proportionate to their distance therefrom. Ptolemy confined his directions to aspects
between the bodies and the places of the planets.
Placidus de Titus added mundane aspects. In his system one third of the semi-arc of a planet
was equal to the space of one House. In both systems the motions of the planets are due to the
motion of the Earth on its axis after birth. The radical positions of the planets, taken in
connection with the planet to which direction is made, are held to determine the nature of the
event. The Significators - Sun, Moon, Midheaven and Ascendant - were directed to the points
where conjunctions or aspects would form to mundane and zodiacal positions.
Most Primary Directions can be worked to within 15' of arc, or 3 months' time, by means of
Tables of Houses, provided one knows the Poles of the various planets: the degree of
elevation in the Nativity in proportion to the latitude of the birthplace.