Seasons are caused by the 23.4-degree tilt of Earth's axis, which alternately turns the North and South Poles toward the sun. Times when the sun's apparent path crosses the equator are known as equinoxes. Times when the sun's apparent path is at the greatest distance from the equator are known as solstices.
The lengths of the days are most extreme at each solstice. If Earth's axis were perpendicular to the plane of Earth's orbit around the sun, there would be no seasons, and the days always would be equal in length. Since Earth's axis is at an angle, the sun strikes Earth directly at the equator only twice a year: in March (vernal equinox) and September (autumnal equinox). In the Northern Hemisphere, spring begins at the vernal equinox, summer at the summer solstice, fall at the autumnal equinox, and winter at the winter solstice. The situation is reversed in the Southern Hemisphere.
