June 20-21 is a very important day for our planet and its relationship with the sun. 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. In 2012, the summer solstice occurs and summer begins in the Northern Hemisphere on June 20, at 7:09 p.m. EDT (23:09 UTC).The earth spins around its axis, an imaginary line going right through the planet between the north and south poles. The axis is tilted somewhat off the plane of the earth’s revolution around the sun. The tilt of the axis is 23.5 degrees; thanks to this tilt, we enjoy the four seasons. For several months of the year, one half of the earth receives more direct rays of the sun than the other half.
When the 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.
On June 21, there are 24 hours of daylight north of the Arctic Circle (66.5° north of the equator) and 24 hours of darkness south of the Antarctic Circle (66.5° south of the equator). The sun’s rays are directly overhead along the Tropic of Cancer (the latitude line at 23.5° north, passing through Mexico, Saharan Africa, and India) on June 21.
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. The earth is furthest from the sun about July 3; this point is known as the aphelion and the earth is 94,555,000 miles away from the sun. The perihelion takes place about January 4 when the earth is a mere 91,445,000 miles from the sun.
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.