Kalenterivuosi

Kalenterivuosi, also known as a calendar year, is a period of 365 days, which is the time that Earth takes to orbit the Sun once. The calendar year is the basic unit of time for most societies, and its structure varies depending on the calendar system used. The most widely used calendar system is the Gregorian calendar, which was introduced in 1582 by Pope Gregory XIII. This calendar is a solar calendar, meaning it is based on the Earth's orbit around the Sun, and it has 12 months, each with a different number of days. The year begins on January 1st and ends on December 31st.

In the Gregorian calendar, a leap year occurs every four years to account for the fact that the Earth's orbit is not exactly 365 days long. A leap year has 366 days, with an extra day added to February, making it 29 days long instead of 28. The leap year rule is not perfect, as it does not account for the fact that the Earth's orbit is actually 365.2422 days long. To correct for this, the Gregorian calendar also has a rule that a year is not a leap year if it is divisible by 100 but not by 400. This means that the years 2100, 2200, and 2300 will not be leap years, even though they are divisible by 4.

The calendar year is divided into four seasons, each of which lasts approximately three months. The seasons are defined by the position of the Earth in its orbit around the Sun. In the Northern Hemisphere, the seasons are spring, summer, autumn, and winter, while in the Southern Hemisphere, they are autumn, winter, spring, and summer. The exact dates of the seasons vary from year to year, as they depend on the Earth's position in its orbit.

The calendar year is also used to measure time in many other contexts, such as in the naming of years, in the calculation of interest rates, and in the planning of events. It is an important unit of time for many societies, and its structure and organization have evolved over time to meet the needs of different cultures and societies.