The fixing of 539 B.C.E. as the year when this historical event occurred is based on a stone document known as the Nabonidus (Nabunaid) Chronicle. This important find was discovered in ruins near the city of Baghdad in 1879, and it is now preserved in the British Museum. A translation of this finding was published by Sidney Smith in Babylonian Historical Texts Relating to the Capture and Downfall of Babylon, London, 1924, and reads in part: “In the month of Tashritu [Tishri, Hebrew 7th month], when Cyrus attacked the army of Akkad in Opis on the Tigris, the inhabitants of Akkad revolted, but he (Nabonidus) massacred the confused inhabitants. The 14th day, Sippar was seized without battle. Nabonidus fled. The 16th day [October 11-12, 539 B.C.E., Julian, or October 5-6, Gregorian] Gobryas (Ugbaru), the governor of Gutium and the army of Cyrus entered Babylon without battle. Afterwards Nabonidus was arrested in Babylon when he returned (there). . . . In the month of Arahshamnu [Heshvan, Hebrew 8th month], the 3rd day [October 28-29, Julian], Cyrus entered Babylon, green twigs were spread in front of him—the state of ‘Peace’ (Sulmu) was imposed upon the city.”—Ancient Near Eastern Texts Relating to the Old Testament (Princeton; 1955), James B. Pritchard, p. 306.
Please note, the Nabonidus Chronicle gives precise details as to the time when these events took place. This, in turn, enables modern scholars, with their knowledge of astronomy, to translate these dates into terms of the Julian or Gregorian calendars. Explaining why this Chronicle makes no particular reference to Belshazzar in connection with the capture of Babylon by Cyrus, and also confirming the date of 539, note what professor Jack Finegan says in Light from the Ancient Past (1959), pages 227-229:
“Nabunaid (Nabonidus) shared the kingship with his own oldest son Belshazzar. Belshazzar is named as the crown prince in Babylonian inscriptions. . . . Since, therefore, Belshazzar actually exercised the coregency at Babylon and may well have continued to do so unto the end, the book of Daniel (5:30) is not wrong in representing him as the last king of Babylon. In the seventeenth year of King Nabunaid, Babylon fell to Cyrus the Persian. The Nabunaid chronicle gives exact dates. In the month of Tashritu on the fourteenth day, October 10, 539 B.C., the Persian forces took Sippar; on the sixteenth day, October 12, ‘the army of Cyrus entered Babylon without battle’; and in the month of Arahsamnu, on the third day, October 29, Cyrus himself came into the city.”
Other investigators say this: “The Nabunaid Chronicle . . . states that Sippar fell to Persian forces VII/14/17 (Oct. 10, 539), that Babylon fell VII/16/17 (Oct. 12), and that Cyrus entered Babylon VIII/3/17 (Oct. 29). This fixes the end of Nabunaid’s reign and the beginning of the reign of Cyrus. Interestingly enough, the last tablet dated to Nabunaid from Uruk is dated the day after Babylon fell to Cyrus. News of its capture had not yet reached the southern city some 125 miles distant.”—Brown University Studies, Vol. XIX, Babylonian Chronology 626 B.C.—A.D. 75, Parker and Dubberstein, 1956, p. 13.
Recognized authorities of today accept 539 B.C.E. without any question as the year Babylon was overthrown by Cyrus the Great. In addition to the above quotations the following gives a small sampling from books of history representing a cross section of both general reference works and elementary textbooks. These brief quotations also show that this is not a date recently suggested, but one thoroughly investigated and generally accepted for the past sixty years.
“Cyrus entered Babylon in 539 B.C.” (Encyclopœdia Britannica, 1946, Vol. 2, p. 852) “When Cyrus defeated the army of Nabonidus, Babylon itself surrendered, in Oct. 539, to the Persian general Gobryas.”—Ibid., Vol. 6, p. 930.