One example of how the Incas kept time was detailed by the Spanish. They reported that overlooking Cuzco, in the northwest was a high mound called Cerro Picchu. On this hill were four marked pillars. When the sun went past the first pillar, it was time for farmers in the higher altitudes to prepare their fields. When the sun entered the two middle pillars, planting began in Cuzco. When the sun hit the middle of these two pillars, planting began in the valleys.
The Incas also used the dark nebula constellations of the Milky Way to tell time. When the constellation Tinamou appeared, they knew to watch their crops since the tinamous (birds resembling a partridges) would come. When Toad appeared, the Incas held their religious rituals for rain, because toads brought the rainy season.
In addition, the Incas constructed a lunar calendar to track their rituals. To keep this calendar in sync with the solar calendar, they tracked the sightings of the Pleiades. The first month began at the summer solstice, and featured the Great Feast of the Sun (Capac Inti-Rami). The following month was one of penance and fasting. In the third month, farmers harvested root crops and conducted ceremonies for growing corn. In the fourth month, farmers focused on their ripening crops. The fifth month honored the Inca Emperor at the Feast of the Incas. In the sixth month, the Incas harvested corn and gave thanks to their Gods. The seventh month was for the Inti-Raimi festival, when everyone had to go to Cuzco. The eighth month featured rituals for irrigation and water. In the ninth month, the Incas tilled their fields and sacrificed to all of their Gods for a successful growing season. The tenth month featured the Feast of the Moon. During this time, Cuzco was purified. The eleventh month was the dry season, when the Incas had ceremonies for rain. The last month was the Festival of the Dead.
My theory of why the Incas developed their solar-lunar calendar was to ensure social order and the welfare of their people. Living in the mountains, the Incas had to pay close attention to their environment to stay alive. To feed and clothe their people, the Inca Empire structured their ritual activities around the growing and harvest seasons. Moreover, these religious rituals not only honored their Gods, but also focused on keeping the empire intact. Since the official timekeeping was done at Cuzco, everyone was conscious of the Empire in their lives.
Aveni, Anthony, “People and the Sky,” Thames & Hudson: N.Y, 2008.
Gazzo, Bridget and Sarah Cahalan, “The Ancient Future: Mesoamerican and Andean Timekeeping,” Dumbarton Oaks, 2014, http://www.doaks.org/library-archives/library/library-exhibitions/the-ancient-future-mesoamerican-and-andean-timekeeping,
Jrcalhou, “The Inca Calendar,” WolfWikis NCSU, 2008, http://wikis.lib.ncsu.edu/index.php/The_Inca_Calendar,
---, “Inca Calendar,” Peru Travel Diary, 2009, http://www.machupicchu-inca.com/inca-calendar.html,
Magli, Giulio, “Mysteries and Discoveries of Archaeoastronomy,” Copernicus Books: N.Y., 2009.