A finer grain of meaning

The decans give the astrologer a finer division than the sign, letting a planet in, say, the first ten degrees of Leo be read differently from one in the last ten. They were important in medieval astrological magic, where the decan images were carved as talismans, and they connect to the Egyptian star-calendar from which they sprang.

The atlas gives each decan its degrees, its rulers under the main schemes, and its traditional images. The decans sit among the finer tools of the chart alongside the fixed stars, the Sabian symbols and the Arabic parts.