Vernal equinox
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The Vernal equinox (or more generally the Northward equinox) is the point in time within a solar year, at which the apparent position of the sun crosses the equator in a northward direction. In the northern hemisphere, this is regarded as the first day of spring, when using astronomical reckoning. In the southern hemisphere, this similarly defines the first day of autumn. This event is also commonly referred to as "The First Point of Aries" as it coincided with the time that the apparent position of the sun as seen from earth lay at the eastern extreme of the constellation of Aries in 130 BC. Due to the axial precession of the earth's rotation, the two events no longer coincide and currently differ by about 28 days, though the term "First Point of Aries" is still used to refer to vernal equinox.
The day (i.e. the period from sunset to sunset) on which the vernal equinox occurs, coincides with the festival of Naw-Ruz ordained by Bahá’u’lláh. The vernal equinox is also used to determine the new year in some other calendar systems, chiefly among them the Iranian calendar, though the rules for determining the day it falls on differ. Iran considers the event, which in this context is typically transcribed as Nowruz, to fall on the day on which the vernal equinox occurs before noon.
Timing[edit]
The timing of the vernal equinox differs from year to year, largely because a solar year is about 365.24 rather than exactly 365 days long, necessitating the insertion of leap years (which in the Badí' calendar are those years where Ayyám-i-Há is 5 days long), but also because the length of the year is not constant, as it is influenced by the gravitational pull of various objects in the solar system. Using sufficiently complex calculations, it is still possible to predict the time of the equinox to within seconds. It becomes necessary however, to define a reference point on the earth for the definition of the "day" on which the equinox falls, as the sun will already have set in some places at this time but not in others. The spot chosen by the Universal House of Justice is Tehran, the birthplace of Bahá'u'lláh.
