Karkausvuonna

Karkausvuonna, also known as a leap year, is a year that contains one additional day added to keep the calendar year synchronized with the astronomical or seasonal year. The extra day is added in the form of February 29, which is called a leap day. The concept of a leap year is based on the fact that the Earth's orbit around the Sun takes approximately 365.2422 days, not the whole number 365 days. Without a leap year, the calendar would drift out of sync with the seasons over time.

The rule for determining whether a year is a leap year is as follows: a year is a leap year if it is divisible by 4, except for end-of-century years, which must be divisible by 400. This means that the year 2000 was a leap year, although 1900 was not. The next leap year after 2020 will be 2024.

Leap years have historical significance, as they were introduced by Julius Caesar in 46 BC as part of the Julian calendar. The Gregorian calendar, which is the calendar most widely used today, was introduced by Pope Gregory XIII in 1582 and also includes the concept of leap years. The extra day in a leap year helps to maintain the accuracy of the calendar over time.