What is a year?

A year is a certain unit of time,it is equal to the period of rotation of the planet around the Sun. The average duration of the year is 12 months (52 weeks plus a day or two). What is a year, know almost everything. But many do not know that there are several kinds of years: astronomical years, which, in turn, are divided into tropical, sidereal and anomalistic years; as well as calendar years.

The Gregorian calendar (365 days - a non-leap year, 366 days - a leap year).
The Julian calendar (365 days - a non-leap year, 366 days - a leap year).
The Islamic calendar (365 days - a non-leap year, 366 days - a leap year).
The Jewish calendar (353, 354, 355 days - a non-leap year, 383, 384, 385 days - a leap year).

Duration of the year (average):

366.62 days is a draconic year. During this time, the Sun returns to the same site of the Moon's orbit.
353, 354, 355 days are non-leap years in the lunisolar calendar.
354.37 days - the duration of the year in the lunar calendar.
365,2 day is a non-leap year in the solar calendar.
365.2 days - a tropical year.
365.2 days New Calendar
365.2 days - the Iranian calendar (developed by Omar Khayamam).
365.2424 days - the interval between the vernal equinoxes.
365,243 days - a year in the Gregorian calendar.
365.25 days - a year in the Julian calendar.
365.2564 days - the star (sidereal) year.
365,257 days in the Gaussian year.
365.26 days - in the anomalistic year.
366 days - a leap year in solar calendars.
383, 384, 385 days - leap years in the lunisolar calendar.
383.9 days - leap years in other types of luni-solar calendars.

Earth days, that is, one day, are 86,400 seconds. They are defined in this way: 24 * 60 * 60.

Why in the year 12 months?

In 46 BC. Julius Caesar decided to reform the calendar, which then consisted of 10 months. The first month was March. But according to the advice of the Egyptian astronomer Sozigen, another two months were added (January and February). The beginning of the year Julius Caesar also changed for January.