Chaldea, properly the southernmost portion of the valley of the Tigris and the Euphrates rivers. Sometimes it is extended to include Babylonia and thus comprises all S Mesopotamia, as in the Bible. The Chaldaeans were a Semitic people who first came into S Babylonia c.1000 B.C. With the death of Assurbanipal (626 B.C.), Nabopolassar seized the throne and established a new Babylonian or Chaldaean empire. The empire flourished under Nabopolassar's son Nebuchadnezzar II, but it declined rapidly thereafter and came to an end when Babylon fell to Cyrus the Great in 539 B.C. The study of astronomy and astrology was developed in this period, and "Chaldaean" came to mean simply "astrologer", as in the book of Daniel and among the Romans. The term is also understood in the Bible to mean Aramaean.