And of the fixed stars, the Pleiades are seven. And the Bears, by the help of which agriculture and navigation are carried through, consist of seven stars. And in periods of seven days the moon undergoes its changes. In the first week she becomes half moon; in the second, full moon; and in the third, in her wane, again half moon; and in the fourth she disappears. Further, as Seleucus the mathematician lays down, she has seven phases. First, from being invisible she becomes crescent-shaped, then half moon, then gibbous and full; and in her wane again gibbous, and in like manner half moon and crescent-shaped.