One of the many objections urged against Ptolemy's system of astrology is that the signs are continually moving from their positions; but Ptolemy seems to have been aware of this motion of the signs, and has met this objection by what he says in the twenty-fifth chapter of the first book of the Tetra-biblos, where he makes it clear that the respective influences he ascribes to the twelve signs were considered by him to belong rather to the places they occupied in the ambient than to the stars of which they are composed; and he especially speaks of the ambient as producing the effects attributed to the respective signs of the zodiac when in the ascendant in a nativity; thus his astrology is just as applicable to modern astronomy as it was to his own.
The signs have been divided into four triplicities, thus: fiery
,
,
; earthy,
,