Hello larryk,
The seasons happen because the earth’s axis (the line that runs
through the earth from the North to the South Pole) is tilted. This
means that as the earth travels around the sun, it reaches one point
where the northern half is most tilted towards the sun, while the
southern half is most tilted away from the sun. At that point, it is
summer in the northern hemisphere and winter in the southern
hemisphere. Six months later, the earth has travelled 180 degrees
round its orbit. The southern half is most tilted towards the sun,
while the northern half is most tilted away from the sun, and so it is
winter in the northern hemisphere and summer in the southern
hemisphere.
“Near June 21st, the summer solstice, the Earth is tilted such that
the Sun is positioned directly over the Tropic of Cancer at 23.5
degrees north latitude. This situates the northern hemisphere in a
more direct path of the Sun's energy. What this means is less
sunlight gets scattered before reaching the ground because it has less
distance to travel through the atmosphere. In addition, the high sun
angle produces long days. The opposite is true in the southern
hemisphere, where the low sun angle produces short days. Furthermore,
a large amount of the Sun's energy is scattered before reaching the
ground because the energy has to travel through more of the
atmosphere. Therefore near June 21st, the southern hemisphere is
having its winter solstice because it "leans" away from the Sun.”
http://www.crh.noaa.gov/fsd/astro/season.htm (web site of the National
Weather Service Forecast Office, Sioux Falls)
Over the equator, the tilt of the earth makes very little difference
because it is less pronounced in the middle than at the ends of the
axis (If you take a pencil, hold it in the middle and tilt it from
side to side, you will see that the ends move more than the middle).
The angle at which the sun’s rays strike stays more or less the same,
so the length of the day and night are more or less equal right
through the year. Also, there is little difference in the amount by
which the sunlight gets scattered, so there is little change in
temperature throughout the year.
The URL given above has a diagram to show the effect of the earth’s
tilt. However, you can also view an animation produced by NASA
showing the earth moving around the sun, while a narrator explains how
the seasons happen.:
http://kids.msfc.nasa.gov/Earth/Seasons/Seasons.htm
On the other hand, the tropics on either side of the equator, while
not having the four seasons as we know them, do have another type of
seasons: dry season and rainy season: “Temperature is fairly constant
near the equator, but rainfall varies greatly. Tropical areas do not
have "winter" and "summer": they have "dry season" and "wet season".
As warm air rises near the equator, it produces heavy rainfall. This
zone of rising motion and heavy rain is called the intertropical
convergence zone, or ITCZ. The rising motion occurs where the Earth is
being heated most strongly, so the ITCZ lies north of the equator in
June-August, and south of the equator in December-February. As the
ITCZ moves north and south of the equator, the tropics receive more or
less rainfall. This difference can be huge! For example, the Amazon
rain forest (just south of the equator) receives on average 1 meter of
rainfall during December-February, and just 5 centimeters during
June-August. The Congo (just north of the equator) is the other way
around: it gets 1 meters of rain during June-August, and 5 centimeters
during Dec-Feb.” http://www.mit.edu/~goodmanj/madsci/965844185.Es.r.html
(MadSci Network: Earth Sciences) |
