++ [ originally posted by Shadowfax ] ++
In the northern hemisphere March 20th or 21st is the first day of spring. The cold winds of winter are fading, and wild flowers are starting to bloom.
The reason for these changes has to do with the earth's yearly trip around the sun. For part of the year the earth's north pole points away from the sun and part of the time towards it. When the north pole points toward the sun, the sun's rays hit the northern half of the world more directly and it is summer. But when the north pole is pointed toward the sun, the south pole is pointed away. So the sun's light hits the earth at a less direct angle, spreading the warmth over a larger area, and it is winter.
In between winter and summer on March 20th or 21st, day and night are each 12 hours long. This is called the vernal equinox . It is the first day of spring north of the equator, and the first day of autumn in the southern half of the world.
In between summer and winter there is another equinox, called the autumnal equinox. Just like the vernal equinox, day and night are the same length. This is the first day of autumn north of the equator, and the start of spring to the south.