Leap year

A leap year (or intercalary or bissextile year) is a year containing one extra day (or, in the case of lunisolar calendars, a month) in order to keep the calendar year synchronized with the astronomical or seasonal year.

A tropical year (also known as a solar year), for general purposes, is the length of time that the Sun takes to return to the same position in the cycle of seasons, as seen from Earth; for example, the time from vernal equinox to vernal equinox, or from summer solstice to summer solstice. Because of the precession of the equinoxes, the seasonal cycle does not remain exactly synchronised with the position of the Earth in its orbit around the Sun. As a consequence, the tropical year is about 20 minutes shorter than the time it takes Earth to complete one full orbit around the Sun as measured with respect to the fixed stars (the sidereal year).

Havent know the next   =0

Bessel Year (named after the German astronomer Bessel F. XIX century) - a tropicalyear, which is taken as the beginning point in time when the mean longitude of the sun minus the constant coefficient of aberration (20.496 arcsec) is exactly equal to 280 °. Start a Bessel year falls on the same time for any item of the Earth. Length isthe length of Bessel, the rain in a day and can be expressed by T0 = 365,24219879 - 0,00000614 T, where T - the number of centuries that have passed since 1900.