A civil unit of time equal to approximately 1/12 year, but varying from 28 to 31 days [3]. The Sun and the Moon are our traditional time keepers, but they are badly out of step with each other. A solar year equals approximately 12.368 lunar months. The large fraction in this number makes it difficult to design a calendar with a whole number of months in each year. There are at least three solutions to the problem: [i] Use leap months. In the traditional Chinese and Jewish calendars most years have 12 months, but some have a 13th month. In these luni-solar calendars the length of the year varies from 354 to 384 days. [ii] Define 12 synodic lunar months to be a year and don't worry about the length of the year. This is the solution of the Islamic calendar. Since the Islamic year has only 354 or 355 days, its length does not match the cycle of the seasons. [iii] Observe the solar year and let the months be 12 arbitrary periods; don't worry about the Moon. This is the solution adopted by Julius Caesar, who established the civil calendar we use today. In this calendar, all months have 30 or 31 days except the second month, February. February has 28 days in ordinary years and 29 in leap years. See also year [2]