Rina Mohammed
The ancient Babylonians had a lunisolar calendar of 12 lunar months of 30 days each, and they added extra months when necessary. They defined a group of twelve constellations called the Zodiac. The ancient Egyptians were the first to replace the lunar calendar with a calendar based on a solar year, with the year beginning at the fall equinox. The Babylonian's calendar also started at the fall equinox. The Egyptians based this calendar on the flooding of the Nile which occured regularly soon after the last day that the bright star Sirius could be seen, just before sunrise. About 238 BC, King Ptolemy ordered that an extra day would be added to every fourth year, similar to the leap year. In ancient Greece, a lunisolar calendar was used, with a year of 354 days. The Greeks were the first to intercalate extra months into the calendar based on a scientific basis.