Without clocks or modern tools, ancient Mexicans watched the sun to maintain a farming calendar that precisely tracked seasons and even adjusted for leap years.

In the early 16th century, the largest city in Spain had a population of less than 50,000, whereby in contrast the land the Spanish were on the verge of conquering could sustain millions.

By this time, the people living in the Mexican Basin had been using a calendar so precise, that it allowed them to time the planting of their crops to avoid the dangers of hot dry springs and summer monsoons, and detect both equinoxes, as well as both solstices, and leap years.

Research now compiled by the University of California, Riverside, shows how the Aztec, or Mexica as they called themselves, were able to achieve such accuracy in timing the seasons and the weather.