In Western Christendom, the March equinox is one of the factors used to determine the date of Easter. The effect accumulated from inception in 45 BC until the 16th century, when the northern vernal equinox fell on 10 or 11 March. This had the effect of adding about three quarters of an hour every four years.
It was based on a length for the year of 365 days and 6 hours (365.25 d), while the mean tropical year is about 11 minutes and 15 seconds less than that. The Julian calendar reform lengthened seven months and replaced the intercalary month with an intercalary day to be added every four years to February. The Indian national calendar starts the year on the day next to the vernal equinox on 22 March (21 March in leap years) with a 30-day month (31 days in leap years), then has 5 months of 31 days followed by 6 months of 30 days. The Persian calendar begins each year at the northward equinox, observationally determined at Tehran. The Babylonian calendar began with the first new moon after the March equinox, the day after the return of the Sumerian goddess Inanna (later known as Ishtar) from the underworld, in the Akitu ceremony, with parades through the Ishtar Gate to the Eanna temple and the ritual re-enactment of the marriage to Tammuz, or Sumerian Dummuzi. See also: September equinox § Culture Calendars At the north and south poles, the Sun appears to move steadily around the horizon, and just above the horizon, neither rising nor setting apart from a slight change in declination of about 0.39° per day as the equinox passes. This is most notable at the more extreme latitudes, where the Sun may be seen to travel sideways considerably during the morning and evening, drawing out the transition from day to night. These conditions produce differentials of actual durations of light and darkness at various locations on Earth during an equinox. Additionally, when seen from the Earth, the Sun is a bright disc in the sky and not just a point of light, thus sunrise and sunset can be said to start several minutes before the sun's geometric center even crosses the horizon, and extends equally long after. ĭue to refraction of light rays in the Earth's atmosphere, the Sun will be visible above the horizon even when its disc is completely below the limb of the Earth. The March equinox, like all equinoxes, is characterized by having an almost exactly equal amount of daylight and night across most latitudes on Earth. In its apparent motion on the day of an equinox, the Sun's disk crosses the Earth's horizon directly to the east at sunrise and again, some 12 hours later, directly to the west at sunset. See also: Equinox § Length of equinoctial day and night It also serves as a reference for calendars and celebrations in many cultures and religions. In astronomy, the March equinox is the zero point of sidereal time and, consequently, right ascension. The March equinox may be taken to mark the beginning of astronomical spring and the end of astronomical winter in the Northern Hemisphere but marks the beginning of astronomical autumn and the end of astronomical summer in the Southern Hemisphere. Balancing the increases of the common years against the losses of the leap years keeps the calendar date of the March equinox from drifting more than one day from 20 March each year. For a common year the computed time slippage is about 5 hours 49 minutes later than the previous year, and for a leap year about 18 hours 11 minutes earlier than the previous year. On the Gregorian calendar, the northward equinox can occur as early as 19 March or as late as 21 March at 0° longitude. The March equinox is known as the vernal equinox ( spring equinox) in the Northern Hemisphere and as the autumnal equinox in the Southern Hemisphere.
The March equinox or northward equinox is the equinox on the Earth when the subsolar point appears to leave the Southern Hemisphere and cross the celestial equator, heading northward as seen from Earth. Illumination of Earth by the Sun on the day of an equinox
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