In the United States, the opening of spring and end of winter is marked by the vernal equinox occurs, a astrological term for when the sun sits directly above the equator.
As Earth revolves around the sun, the top half, called the Northern Hemisphere, becomes tilted more toward the sun as winter turns to spring. Meanwhile the bottom half, the Southern Hemisphere, becomes tilted more away from the sun. It is autumn for people in Australia and the southern parts of Africa and South America.
