The Chaldean priests, whose interest in astrology so greatly added to the fund of Babylonian astronomical knowledge that the word "Chaldean" came to mean astronomer, continually undermined the monarchy. Finally, in 539 BC, they opened the gates of Babylon to Cyrus the Persian, thus fulfilling Daniel's message of doom upon the notorious Belshazzar, the last Chaldean ruler: "You have been weighed in the balances and found wanting" (Dan.