Earth and the Moon

Clocks and Calendars

What were the origins of our modern annual calendar?

The original model for our calendar was created by the ancient Romans and Greeks as far back as the eighth century B.C.E. With the help of the Roman astronomer Sosigenes, Julius Caesar created what is known as the Julian calendar in 46 B.C.E. This was the first calendar with a leap year, and a day was added every fourth year. This meant that each year was 365.25 days long. The Julian calendar was off by only eleven minutes and fourteen seconds each year when compared to the actual orbit of Earth around the Sun. That is pretty impressive, but over the centuries it added up until, by the sixteenth century, the calendar was off by nearly eleven days.


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