Commonyear

Commonyear is a term used to denote a year within the Gregorian calendar system that is not a leap year. Leap years are introduced to keep the calendar year synchronized with the astronomical year or seasonal year. A common year consists of 365 days, whereas a leap year has 366 days, with the extra day added as February 29th.

The rules for determining a leap year are as follows: a year is a leap year if it is divisible by four, except for end-of-century years. An end-of-century year is a leap year only if it is divisible by 400. Therefore, years like 1900 and 2100 are common years, even though they are divisible by four, because they are not divisible by 400. Years like 2000 and 2400 are leap years as they are divisible by 400.

The distinction between common years and leap years ensures that the calendar's drift relative to the Earth's orbital period is minimized over long periods. Without this adjustment, the seasons would gradually shift over centuries. The concept of a common year is fundamental to the functioning of the Gregorian calendar, which is the most widely used civil calendar in the world today. The sequence of common years and leap years follows a predictable pattern.