In J. R. R. Tolkien's legendarium, the history of Arda, also called the history of Middle-earth, began when the Ainur entered Arda, following the creation events in the Ainulindalë and long ages of labour throughout Eä, the fictional universe. Time from that point was measured using Valian Years, though the subsequent history of Arda was divided into three time periods using different years, known as the Years of the Lamps, the Years of the Trees and the Years of the Sun. A separate, overlapping chronology divides the history into 'Ages of the Children of Ilúvatar'. The first such Age began with the Awakening of the Elves during the Years of the Trees and continued for the first six centuries of the Years of the Sun. All the subsequent Ages took place during the Years of the Sun. Most Middle-earth stories take place in the first three Ages of the Children of Ilúvatar. Major themes of the history are the divine creation of the world, followed by the splintering of the created light as different wills come into conflict. Scholars have noted the biblical echoes of God, Satan, and the fall of man here, rooted in Tolkien's own Christian faith. Arda is, as critics have noted, "our own green and solid Earth at some quite remote epoch in the past." As such, it has not only an immediate story but a history, and the whole thing is an "imagined prehistory" of the Earth as it is now. Music of the Ainur The supreme deity of Tolkien's universe is Eru Ilúvatar. Ilúvatar created spirits named the Ainur from his thoughts, and some were considered brothers or sisters. Ilúvatar made divine music with them. Melkor, then the most powerful of the Ainur, broke the harmony of the music, until Ilúvatar began first a second theme, and then a third theme, which the Ainur could not comprehend since they were not the source of it. The essence of their song symbolized the history of the whole universe and the Children of Ilúvatar that were to dwell in it – Men and Elves. Then Ilúvatar created Eä, which means "to be," the universe itself, and formed within it Arda, the Earth, "globed within the void": the world together with the three airs is set apart from Avakúma, the "void" without. The first 15 of the Ainur that descended to Arda, and the most powerful ones,