Leap year refers to a year that has an extra day or month. Leap year is also known as a bissextile or an intercalary year. A leap year such as in the lunisolar calendar has an additional month because it needs to be synchronized with the seasonal or astronomical year. Since astronomical events and seasons are not repeated through a number of days, calendars containing a uniform number of days every year tend to drift over time with regard to events that are to be tracked throughout the year. Inserting an extra month or day into the year helps correct this drift. Years that do not have an extra day or month are referred to as common years. For instance, the Gregorian calendar has 366 days every leap year as opposed to the normal 365 days. This calendar extends the month of February by one day to make it 29 days instead of the ordinary 28 days. In a similar way, the Hebrew lunisolar calendar, Adar Aleph contains an additional lunar month. This extra month is included in the calendar seven times after every 19 years to prevent the calendar year from shifting through seasons.
How Leap Years Occur
The year is determined by the time that the Earth takes to rotate round the sun just once. Normally, the Earth takes around 365 and a quarter days to complete one orbit round the sun. A day us defined as the duration that the Earth takes to rotate on its axis. Adding one more day after each set of four years means that the Earth is on the exact same spot on its orbit at the exact same time of the calendar each year. Prior to the development of a leap year calendar, seasons changed throughout the calendar as the drift is around a quarter day each year.