equinox year, or tropical year. Because 97 out of 400 years are leap years, the mean length of the Gregorian calendar year is days; with a relative error below one ppm (8·10−7) relative to the current length of the mean tropical year ( days) and even closer to the current March equinox year of days that it aims to match. It is estimated that by the year 4000 CE, the northward equinox will fall back by one day in the Gregorian calendar, not because of this difference, but due to the slowing of the Earth's rotation and the associated lengthening of the day. Other calendars Historically, lunisolar calendars intercalated entire leap months on an observational basis. Lunisolar calendars have mostly fallen out of use except for liturgical reasons (Hebrew calendar, various Hindu calendars). A modern adaptation of the historical Jalali calendar, known as the Solar Hijri calendar (1925), is a purely solar calendar with an irregular pattern of leap days based on observation (or astronomical computation), aiming to place new year (Nowruz) on the day of vernal equinox (for the time zone of Tehran), as opposed to using an algorithmic system of leap years. Year numbering A calendar era assigns a cardinal number to each sequential year, using a reference event in the past (called the epoch) as the beginning of the era. The Gregorian calendar era is the world's most widely used civil calendar. Its epoch is a 6th century estimate of the date of birth of Jesus of Nazareth. Two notations are used to indicate year numbering in the Gregorian calendar: the Christian "Anno Domini" (meaning "in the year of the Lord"), abbreviated AD; and "Common Era", abbreviated CE, preferred by many of other faiths and none. Year numbers are based on inclusive counting, so that there is no "year zero". Years before the epoch are abbreviated BC for Before Christ or BCE for Before the Common Era. In Astronomical year numbering, positive numbers indicate years AD/CE, the number 0 designates 1 BC/BCE, −1 designates 2 BC/BCE, and so on. Other eras include that of Ancient Rome, ("from the foundation of the city), abbreviated AUC; ("year of the world"), used for the Hebrew calendar and abbreviated AM; and the Japanese emperor eras described above. The Islamic Hijri year, (year of the Hijrah, abbreviated AH), is a lunar calendar of twelve lunar months and thus is shorter than a