Why is there a Julian calendar?
The Julian Calender (which became the Christian Calender with only a few holidays retained) is a solar calender, based on earth’s orbit of the sun. Previously the Romans used a lunar calender like the Chinese, based on the moon’s orbit of the earth. The solar calender was more efficient, and Julius Caeser was a man who liked efficiency, so he pushed for adoption of the calender which bears his name.
