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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