March 20, 2014, is a date that most of us recognize as symbolic of changing seasons. As we welcome spring, people south of the equator are actually gearing up for the cooler temperatures of autumn.
Far from being an arbitrary indicator of the changing seasons, March 20 (March 21 in some years) is significant for astronomical reasons. On March 20, 2014, at precisely 11:57 A.M. CDT, the Sun will cross directly over the Earth's equator. This moment is known as the vernal equinox in the Northern Hemisphere. For the Southern Hemisphere, this is the moment of the autumnal equinox.
Translated literally, equinox means "equal night." Because the Sun is positioned above the equator, day and night are about equal in length all over the world during the equinoxes. A second equinox occurs each year on Sept. 22 or 23; in 2014, it will be on Sept. 22 at 9:29 P.M. CDT. This date will mark the autumnal equinox in the Northern Hemisphere and the vernal equinox in the Southern (vernal denotes "spring").
These brief but monumental moments owe their significance to the 23.4 degree tilt of the Earth's axis. Because of the tilt, we receive the Sun's rays most directly in the summer. In the winter, when we are tilted away from the Sun, the rays pass through the atmosphere at a greater slant, bringing lower temperatures. If the Earth rotated on an axis perpendicular to the plane of the Earth's orbit around the Sun, there would be no variation in day lengths or temperatures throughout the year, and we would not have seasons.