Solution:The sunrays fall perpendicular at the equator twice in a year (on 21 March and on 23 September). On these dates, the duration of day and night becomes an equal i.e. 12-12 hour, in both of the hemispheres.As Earth revolves around the Sun, there are two moments each yearwhen the Sun is exactly above the equator. These moments - called equinoxes occur around March 19, 20 or 21 and September 22 or 23. Equinox literally means "equal night," since the length of day and night is nearly equal in all parts of the world during the equinoxes. The March equinox marks when the Northern Hemisphere starts to tilt toward the sun, which means longer, sunnier days. In the Northern Hemisphere, the March equinox is called the vernal equinox, because it signals the beginning of spring (vernal means fresh or new like the spring). The September equinox is called the autumnal equinox, because it marks the first day of fall (autumn). When the Northern Hemisphere starts to tilt toward the sun in spring, the Southern Hemisphere starts to tilt away from the sun, signaling the start of fall.
Thus, in the Southern Hemisphere, the March equinox is called the autumnal equinox, and the September equinox is called the vernal equinox. While the March equinox brings later sunrises, earlier sunsets, chillier winds and dry, falling leaves in the Southern Hemisphere, while the reverse happens in the Northern Hemisphere.
