page_title: Tetrabiblos
section_title: Introduction of principles
hierarchical_section_title: Tetrabiblos / Book I: principles and techniques / Introduction of principles
caption_attribution_description: English: Frontispiece illustration from Ptolemy's Almagest, Venice, 1496, depicting Ptolemy instructing Regiomontanus under an image of the zodiac encircling the celestial sphere
context_page_description: Tetrabiblos 'four books', also known in Greek as Apotelesmatiká "Effects", and in Latin as Quadripartitum "Four Parts", is a text on the philosophy and practice of astrology, written in the 2nd century AD by the Alexandrian scholar Claudius Ptolemy.
  Ptolemy's Almagest was an authoritative text on astronomy for more than a thousand years, and the Tetrabiblos, its companion volume, was equally influential in astrology, the study of the effects of astronomical cycles on earthly matters. But whilst the Almagest as an astronomical authority was superseded by acceptance of the heliocentric model of the Solar System, the Tetrabiblos remains an important theoretical work for astrology.
  Besides outlining the techniques of astrological practice, Ptolemy's philosophical defense of the subject as a natural, beneficial study helped secure theological tolerance towards astrology in Western Europe during the Medieval era. This allowed Ptolemaic teachings on astrology to be included in universities during the Renaissance, which brought an associated impact upon medical studies and literary works.
context_section_description: One of the unique features of the Tetrabiblos, amongst the astrological texts of its period, is the extent to which the first book not only introduces the basic astrological principles, but synthesises and explains the reasoning behind their reported associations in line with Aristotelian philosophy. Chapter four, for example, explains the "power of the planets" through their associations with the creative humoral qualities of warmth or moisture, or the reductive qualities of cold and dryness. Hence Mars is described as a destructive planet because its humoral association is excessive dryness, whilst Jupiter is defined as temperate and fertilising because its association is moderate warmth and humidity.  These associations are based on the arrangements of the planets with regard to the Sun, as perceived from the  geocentric perspective, by which their orbits are measured is if they are centred upon the Earth.
  Uniting these Aristotelian principles with a prevalent Greek philosophy employed by Zeno of Citium and the Pythagoreans, the next three chapters arrange the planets into pairs of opposites. They may be benefic (moderately warming or moistening) or malefic (excessively cooling or drying); masculine (drying) or feminine (moistening); active and diurnal (suited to the qualities of the day and aligned with the nature of the Sun) or passive and nocturnal  (suited to the qualities of the night and aligned with the nature of the Moon).  Since these humoral associations derive from configurations with the Sun, chapter eight describes how they are subtly modified according to the  phase of each planet's synodic cycle with the Sun.
  Chapter nine discusses the "power of the fixed stars". Here, rather than give direct humoral associations, Ptolemy describes their "temperatures" as being like that of the planets he has already defined. Hence Aldebaran ("called the Torch") is described as having "a temperature like that of Mars", whilst other stars in the Hyades are "like that of Saturn and moderately like that of Mercury". At the end of the chapter Ptolemy clarifies that these are not his proposals, but are drawn from historical sources, being "the observations of the effects of the stars themselves as made by our predecessors".
  Chapter ten returns to the humoral theme more explicitly, clarifying that the zodiac is aligned to the seasons and so expressive of the shifting emphasis through moisture, warmth, dryness and cold, (as brought about by spring, summer, autumn, and winter). Similarly,  the four angles of the chart present an humoral emphasis through association with the effects of the four cardinal winds that blow from their aligned directions. The remainder of book one (up to the final two chapters which concern the planetary phases and aspectual applications), presents the rulerships, divisions, and configurations of the zodiac signs, most of which are related to astronomical definitions, seasonal effects, physics and geometry. Geometrical principles are used to define the favourable or unfavourable quality of astrological aspects, based on the angular relationship of planets and signs with ecliptic degrees.
  In Ptolemy's era the boundaries of the zodiac signs were close to those of the visible constellations whose names they bear, but Ptolemy demonstrates the theoretical distinction between the two frames of reference in describing the starting point of the zodiac as fixed, not to the stars, but to the mathematically calculated vernal equinox.  This determines the seasonally based tropical zodiac which takes its name from the Greek word τροπικός tropikos: ‘of the turning’,  because it is set by the turn of the seasons and, being subject to precession, experiences a slow and gradual revolution through the visible constellations.  For the same reason, the signs that mark the Sun's summer and winter solstice points (Cancer and Capricorn) are described as the 'tropical signs', since these are the places where the Sun 'turns' its direction in celestial latitude, (thereby defining the terrestrial circles of latitude known as th
caption: Ptolemy instructing Regiomontanus under an image of the zodiac encircling the celestial spheres. Frontispiece from Ptolemy's Almagest, (Venice, 1496).
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