are now around 86,400.002 SI seconds. An average Gregorian year may be said to be 365.2425 days (52.1775 weeks, and if an hour is defined as one twenty-fourth of a day, hours, minutes or seconds). Note however that in absolute time the average Gregorian year does not exist, because each period of 400 years is longer (by more than 1000 seconds) than the preceding one as the rotation of the earth slows. For this calendar, a common year is 365 days ( hours, minutes or seconds), and a leap year is 366 days ( hours, minutes or seconds). The 400-year civil cycle of the Gregorian calendar has days and hence exactly weeks. Greater astronomical years Equinoctial cycle The Great Year, or equinoctial cycle, corresponds to a complete revolution of the equinoxes around the ecliptic. Its length is about 25,700 years. Galactic year The Galactic year is the time it takes Earth's Solar System to revolve once around the Galactic Center. It comprises roughly 230 million Earth years. Seasonal year A seasonal year is the time between successive recurrences of a seasonal event such as the flooding of a river, the migration of a species of bird, the flowering of a species of plant, the first frost, or the first scheduled game of a certain sport. All of these events can have wide variations of more than a month from year to year. Symbols and abbreviations A common symbol for the year as a unit of time is "a", taken from the Latin word . For example, the U.S. National Institute of Standards and Technology (NIST) Guide for the Use of the International System of Units (SI) supports the symbol "a" as the unit of time for a year. In English, the abbreviations "y" or "yr" are more commonly used in non-scientific literature. In some Earth sciences branches (geology and paleontology), "kyr, myr, byr" (thousands, millions, and billions of years, respectively) and similar abbreviations are used to denote intervals of time remote from the present. In astronomy the abbreviations kyr, Myr and Gyr are in common use for kiloyears, megayears and gigayears. The Unified Code for Units of Measure (UCUM) disambiguates the varying symbologies of ISO 1000, ISO 2955 and ANSI X3.50 by using: at = days for the mean tropical year; aj = 365.25 days for the mean Julian year; ag = days for the mean Gregorian year; In the