The ruler of the Tawantinsuyu was known as the Sapa Inca, the “son of the sun,” and it was from the term that Spaniards named the empire. In the early 16th century, the Sapa Inca Atahualpa presided over a far-flung civilization of perhaps 2 million people and a governing structure spread over mountainous, difficult terrain. The Inca calendar was built on the movements of the stars, with the time of day measured by the movement of the sun or the length of different tasks. In contrast to the Inca, the Aztec empire engaged in a tributary economy by sending goods to local markets as well as to Tenochtitlan as tribute to the Aztec ruler Moctezuma. Although none reached the size and scope of the Inca or Aztec, some groups controlled large territories, and some, such as the Mississippian Woodlands people, built large structures, including the mounds at present-day Cahokia.