March 20, 2015, 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, 2015, at precisely 6:45 P.M. EDT, 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 2015, it will be on Sept. 23 at 4:20 A.M. EDT. 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.
Modern astronomy aside, people have recognized the vernal equinox for thousands of years. There is no shortage of rituals and traditions surrounding the coming of spring. Many early peoples celebrated for the basic reason that their food supplies would soon be restored. The date is significant in Christianity because Easter always falls on the first Sunday after the first full moon after the vernal equinox. It is also probably no coincidence that early Egyptians built the Great Sphinx so that it points directly toward the rising Sun on the day of the vernal equinox.The first day of spring also marks the beginning of Nowruz, the Persian New Year. The celebration lasts 13 days and is rooted in the 3,000-year-old tradition of Zorastrianism.