For thousands of years, humans have marked specific times of year as especially important, whether for religious reasons, agricultural reasons, or simply out of curiosity. Ancient observers of the sky noticed a yearly cycle of very long, warm days to short, cooler days, followed by a return to longer days. Although the earliest farmers could not know the cause of this effect, they certainly understood its importance to the growth of their plants, as well as for the populations of the animal inhabitants of their land.
Today we know that the 23.5-degree tilt of Earth's rotational axis relative to the plane of its orbit, is responsible for the seasons and the gradual change in the number of sunlit hours each day. On or about December 21 of each year, the Sun appears farthest south in the sky; for northern hemisphere observers, this is the shortest day of the year and defines the onset of winter. Six months later, around June 21, the Sun appears highest in the sky, and summer begins. These two events are called solstices, meaning literally "sun stands," because they mark the day when the apparent northward or southward movement of the Sun reverses direction.
Halfway between the solstices, on or about March 21 and September 21, are the equinoxes. The word "equinox" is derived from two Latin words meaning "equal" and "night," and on the equinoxes there are exactly twelve hours of sunlight. The equinox in March is called the vernal, or spring equinox; the one in September is called the fall equinox.
Throughout history, humans have watched and marked the equinoxes with special interest. The spring equinox marked the beginning of the growing season in many regions, or at least provided a reference point from which the best times for sowing and reaping could be measured. From the megaliths of Stonehenge, to the ruins of the Mayan civilization in South America, to narrow slits in stone walls constructed by the ancient inhabitants of the American southwest, one can find stones or window aligned so that the Sun rises directly over them, or shines directly through them, on the equinoxes.
Today the equinoxes are not of such critical interest, although they still define the beginning of the spring and fall seasons. They still retain much of their symbolic value, however, even if many do not realize it. The holiday of Easter, which for practicers of the Christian religion represents the resurgence of life, always falls on the first Sunday following the first full Moon of spring--the first full Moon after the vernal equinox. We have refined our measurement systems considerably since the builders of Stonehenge put their enormous boulders in place, and we may no longer celebrate the specific days of equinoxes and solstices with the same fervor as the ancients, but the significance of the annual cycle of the seasons remains.
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