Also known as ‘decans’; divisions of the signs into
10° sections, each of which is governed by one of the traditional planets. These appear to be based upon the Egyptian division of
the year into 36 ten-day periods, each presided over by a particular stellar
deity. The 4th century astrologer Firmicus Maternus is among many who placed
great emphasis on their use, saying that a planet in its own decan is as good
as in its own sign. Like Manilius before him, he attributed decan rulership to
the signs of the zodiac, but most authors, including Ptolemy, gave them to the
planets in descending order towards the earth, (i.e., Saturn, Jupiter, Mars, Sun, Venus, Mercury, Moon). Their order reflects planetary
rulership over the days of the week.