Pat Shingleton: Winter, Spring, Summer and Now.
At 9:29 tonight, the Sun slides across the equator, initiating the autumnal equinox. Before the designation of seasons, cultures recognized seasons as either rainy or dry. Others recognized them as growing, harvesting and winter, while others have marked ten or more seasons. The designation of four seasons has a definitive beginning and end with key moments when the Earth moves around the Sun. Equinox is Latin for "equal night." This isn't the case at the exact moment of the autumnal equinox for two reasons, sunrise and sunset occur when the Sun's top edge crosses the horizon. Earth's atmosphere changes the Sun's apparent position when it is low. Fall and spring equinoxes are the only times when the Sun rises due east and sets due west.