Ancient cultures used astronomy for agriculture, religion, and timekeeping. Calendars were based on the lunar cycles and used to track months and seasons. In Mesopotamia, calendars were recorded using cuneiform writing and depicted images from daily life. Sites like Stonehenge used the rising and setting of the sun and moon to mark the passage of time. By determining azimuths and celestial alignments at different locations, ancient people could closely track the seasons and days of the year, crucial for their civilizations. The most influential astronomical model was Ptolemy's geocentric system from the 2nd century AD, which explained planetary retrograde motion and remained the dominant view of the universe for over 1500 years.