On Wednesday, March 20, 2012 at 11:02 UTC or 7:02 a.m. Eastern Daylight Time, the March Equinox (also known as the Spring Equinox or Vernal Equinox) occurs. On March 20, most consider that spring begins in the Northern Hemisphere and fall or autumn begins in the Southern Hemisphere. The sun is directly overhead the equator on the March Equinox at noon on March 20.
Equal Hours of Daylight on the EquinoxOn March 20, there are twelve hours of daylight and twelve hours of darkness at all points on the earth's surface. Sunrise is at 6 a.m. and sunset is at 6 p.m. local (solar) time for most points on the earth's surface. (This varies, of course, based on time zones, which are much broader regions than local solar time.)
At the North Pole the sun is on the horizon of the earth's surface on the March Equinox. The sun rises at the North Pole at noon to the horizon on the March Equinox and the North Pole remains light until the September Equinox.
At the South Pole on March 20, the sun sets at at noon after the South Pole having been light for the previous six months (since the September Equinox). The sun begins on the horizon in the morning and by the end of the day, the sun has set.
The Beginning of SpringIt has been tradition that spring "begins" on March 20, and the three other seasons begin on their solstices or equinox. However, there is no set scientific standard for the beginning of the seasons. Some consider the months of March, April, and May to be spring; this is especially true for those who study climate.
The beginning of spring often represents a mild transitional climate between the extremes of winter and summer. The areas between the Tropic of Cancer in the north and the Tropic of Capricorn in the south effectively do not have seasons because the sun is always high in the sky and thus there is a large amount of solar radiation received throughout the year. Seasonal changes primarily impact the higher latitudes (those above 23.5 degrees).