June 20-21 is one of two solstices, days when the rays of the sun directly strike one of the two tropical latitude lines. June 21 marks the beginning of summer in the northern hemisphere and simultaneously heralds the beginning of winter in the southern hemisphere. When the earth's axis tilts towards the sun, as it does between June and September, it is summer in the northern hemisphere but winter in the southern hemisphere. In 2010, the solstice occurs and summer begins in the Northern Hemisphere summer begins early on June 21, at 7:28 a.m. June 21 is called the summer solstice in the Northern Hemisphere and simultaneously the winter solstice in the Southern Hemisphere.
Without the tilt of the earth's axis, we would have no seasons. The sun's rays would be directly overhead of the equator all year long. Only a slight change would occur as the earth makes its slightly elliptical orbit around the sun. When summer occurs in a hemisphere, it is due to that hemisphere receiving more direct rays of the sun than the opposite hemisphere where it is winter. In winter, the sun's energy hits the earth at oblique angles and is thus less concentrated.
During spring and fall, the earth's axis is pointing sideways so both hemispheres have moderate weather and the rays of the sun are directly overhead the equator. Between the Tropic of Cancer and the Tropic of Capricorn (23.5° latitude south) there really are no seasons as the sun is never very low in the sky so it stays warm and humid ("tropical") year-round. Only those people in the upper latitudes north and south of the tropics experience seasons.
