سنة كبيسة
The Julian calendar was instituted in the 1st century BCE. The Julian calendar year contained 365.25 days, with the 0.25 day provided for by a Leap year once every fourth year. However, the more precise length of a solar year is about 365.2422 days. By the 13th century, the less accurate 365.25 days had resulted in an accumulated error of about 10 days in the date of the vernal equinox. Sacrobosco made no proposal on how to get rid of the accumulated error. But looking to the future, he proposed to leave one day out of the calendar every 288 years to prevent further error.