Ancient Aztec astronomy has always been a part of Aztec culture. The stars, the planets, and the movements of heavenly bodies in the universe was a great part of the traditions of the Aztec people and it had a lot to do with their everyday movements and the decisions that they made on a daily basis.
The Aztec people carefully tracked the movements of celestial bodies such as the sun, moon, stars, and planets, much like all ancient Mesoamerican people. Most of the observations and calculations that were made were done by nobles and priests. This can be seen through the Codex Mendoza, in which a priest is depicted observing the stars in order to keep track of the schedule for nightly rituals. It was even a theological idea that a ritual sacrifice of blood was needed in order to keep the sun in its course and the earth bountiful.
Aztec astronomers had their own reasons for sky watching. The Aztecs saw in the heavens the sustainers of life--the gods they sought to repay, with the blood of sacrifice, for bringing favorable rains, for keeping the earth from quaking, for spurring them on in battle. Among the gods was Black Tezcatlipoca, who ruled the night from his abode in the north, with its wheel (the Big Dipper). He presided over the cosmic ball court (Gemini) where the gods played a game to set the fate of humankind.
He lit the fire sticks (Orion's belt) that brought warmth to the hearth. And at the end of every fifty-two-year calendrical cycle, Black Tezcatlipoca timed the rattlesnake's tail (the Pleiades) so that it passed overhead at midnight--a guarantee that the world would not come to an end but that humanity would be granted another epoch of life. The priests in Tenochtitlan, the Aztec capital, climbed to the top of their sky watchers' temple on the Hill of the Star to witness this auspicious sign. The Aztecs lived their sky, knowing that everything that happened on earth was the outcome of destiny. And they wrote it all down in their books.
Aside from the tracking of celestial bodies for building alignment purposes, the Aztecs utilized astronomy in other ways. The celestial cycles allowed the Aztecs to track the length of the solar year, lunar month, and the revolution of Venus (found to be 584 days). The discoveries they made enabled for the calibration of calendars and signalling of special ceremonial days. For example, the New Fire ceremony was a ceremony that was held to celebrate the start of the new 52-year calendar round, and it was signalled by the passage of the constellation Pleiades across the zenith of the midnight sky. The astronomical observations made by the Aztecs also included the prediction of solar and lunar eclipses and observation of comets and shooting stars. These observations and the attention the Aztec people paid to astronomy were mostly done for divination and ritual as the celestial heavenly bodies were thought to have religious significance.