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. Alternatively, when the axis points away from the
sun from December to March, the southern hemisphere enjoys the direct
rays of the sun during their summer months.
June 21 is called the summer solstice in the Northern Hemisphere and
simultaneously the winter solstice in the Southern Hemisphere. Around
December 21 the solstices are reversed and winter begins in the northern
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.
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.
source. About.com geography
