Primary directions
The primary directions are calculated according to the Choisnard method, exposed in "Langage astral" (french) or with the Gouchon method explained in "Moyens de pronostics en astrologie" (french) by Max Duval.
We call primary direction from a point A1 to a point A2 which follows it in
the order of the signs the angle that A2 must travel in its apparent movement to
reach the same position as A1.
The difference between Regiomontanus/Campanus and Placidus is what is meant by "same position".
Proastro allows you to calculate directions with or without latitude.
Directions without latitude:
If we choose not to take into account the latitude, we calculate the aspects at a
planet by simply adding the angle of the aspect to the longitude of the planet.
Thus, to calculate the position of the trine to the Moon, we add to the longitude of the
Moon 120°. The aspected point is taken without latitude (it is on the zodiac).
Directions with latitude:
If we choose to take the latitude into account, the problem is more complex.
Given a point P on the zodiac, of latitude L, there exists a
infinity of points PA in three-dimensional space which are at
given angular distance A to P.
For the conjunction, the point retained is the position of the planet, with its
latitude.
For any other aspect, we start from the formula (angular distance between P and PA):
cos(A) = sin latzP * sin latzPA+ cos latzP * cos latzPA* cos(lonzP - lonzPA)
we know A (60°, 90° 120° etc.)
we know latzP and lonzP.
We take latzPA = 0 and deduce lonzPA.
The aspected point is therefore (lonzPA, latzPA). it's a point
at angular distance A from the planet.
See also: