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 the Tropic of Cancer and the Tropic of Capricorn). Whereas other ancient astrological writers gave their emphasis to the astrological interpretation of such definitions (for example, in describing how tropical signs are indicative of quickly changing situations), Ptolemy's focus is notably different; given to the astronomical and philosophical factors that underlie the definitions rather than their astrological meaning in practice. Ptolemy explains that the definitions of the zodiac are not his own, but present "the natural characters of the zodiacal signs, as they have been handed down by tradition". His approach finds elegant expression where he is demonstrating the logic