today marks the beginning of spring, that is, when the sun crosses northward across the celestial equator at a point called the vernal equinox. the vernal and autumnal equinoxes mark the time of year that day and night are of the same length from any location on earth.
on the first day of spring and the last day of fall, the sun rises directly in the east and sets directly in the west (and only on these days)
the season's are caused by the tilt of the earth's axis of rotation (23 1/2 degrees), and not by changes in how far away we are from the sun. the earth-sun distance varies by only about 3 degrees over the course of a year.
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