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When is the first day of spring 2011?

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First Day of Spring is on March 20, 2011 Astronomical spring in the Northern Hemisphere as defined by the International Astronomical Union begins with the Vernal Equinox on March 20, 2011, at 7:21 p.m. EDT.

At the start of spring (spring equinox), day and night are approximately 12 hours long (at the equatorial plane) and the Sun is at the midpoint of the sky. Our north pole tilts towards the Sun.


NOTE-"the north pole tilts toward the sun." that is true on summer solstice, but in December the south pole tilts toward the sun. At the equinoxes the tilt is parallel to the sun.



First day of spring in the Northern Hemisphere
In general, the four seasons correspond to the relative position of the sun to the earth. Astronomical determination of spring is calculated according to when the sun passes through the equatorial plane. When going from winter to spring, the sun is moving north; as soon as the sun crosses the equator, we call it spring. (This applies to places north of the equator.)


First day of spring in the Southern Hemisphere
The "official" date of spring south of the equator (official is corresponding to the first day of fall in places north of the equator) would be around September 20/21, depending on when the sun crosses the equator.

Countries such as Australia and New Zealand, however, designate the first day of September as the official first day of spring (climatological counterpart). Preference between these two methods varies across Europe. Many east Asian countries use lunar dates to determine the beginning of spring.


In Addition
The climatological spring as defined by the World Meteorological Organization began on March 1, 2011.
The ecological spring begins locally with the beginning of the growing season. Usually in temperate climates when the mean daily temperature reaches 6 degrees C/42 degrees F. This can be as early as February in mild climates. and as late as April or May in cool climates.
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