Fall begins. The autumnal equinox is defined as the point at which the Sun appears to cross the celestial equator from north to south. The celestial equator is the circle in the celestial sphere halfway between the celestial poles. It can be thought of as the plane of Earth's equator projected out onto the sphere. Another definition of fall is nights of below-freezing temperatures combined with days of temperatures below 70 degrees Fahrenheit. The word equinox means "equal night"; night and day are about the same length of time. The spring equinox is in late March. In addition to the (approximately) equal hours of daylight and darkness, the equinoxes are times when the Sun's apparent motion undergoes the most rapid change. Around the time of the equinoxes, variations in the position on the horizon where the Sun rises and sets can be noticed from one day to the next by alert observers. See more about the first day of fall, photos, folklore, and more!