Final answer:
The Sumerian calendar was lunar, while the Egyptian calendar was solar, reflecting their societal needs and observations related to agriculture and astronomy.
Step-by-step explanation:
The main difference between the ancient Sumerian and Egyptian calendars is that the Sumerian calendar was lunar while the Egyptian calendar was solar. The Egyptian calendar, developed about 3000 years ago, was based on the 365-day year and was closely tied to the astronomical cycle of the bright star Sirius, which heralded the annual flooding of the Nile, an event that was crucial for their agricultural practices. In contrast, the Sumerians, like many other ancient cultures, used a lunar calendar, which was driven by the regular phases of the moon. These calendars reflected the needs and observations of their respective societies and had significant implications for agriculture, religious ceremonies, and social organization.