Causes of the Seasons
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Any college graduate should be able to answer the following three
questions (the picture is on p. 73 in the photocopied ClassNotes)
Once you think you know the answer
to the question above, click here
The perihelion is the point in the
earth's orbit at which it is closest to the sun. This occurs
around January 3. The aphelion is the point at which the earth is
farthest from the sun. This occurs around July 4.
The real cause of the seasons is the tilt of the earth relative to the
plane of its orbit around the sun.
The north pole is tilted away from
the sun on the northern hemisphere winter solstice (it is the summer
solstice in the southern hemisphere). The north pole is tilted
toward the sun on the summer solstice in the northern hemisphere.
If you were on the far side of this picture looking back toward the
sun. Here is what you would see
Can you identify the northern
hemisphere summer and winter solstices & the spring and fall
equinoxes in this picture?
Click here when you think you have the
answer.
Because the earth is sometimes tilted toward the sun, sometimes away
from the sun, the angle of the sun in the sky varies during the
year. This will partly determine how much incoming sunlight makes
it to the earth's surface and how effectively it can warm the ground.
In the summer when the sun reaches a high elevation angle
above the
horizon, an incoming beam of sunlight will shine on a small area of
ground. The ground will get hot. The two people sharing the
shaft of summer sunlight will get a sunburn.
In the winter the sun is
lower in the sky. The same beam of sunlight gets spread out over
a larger area. The energy is being used to try heat a larger
amount of ground. The result is the the ground won't get as
hot. 4 people are able to share the winter sunlight and won't get
burned as quickly.
As sunlight passes through the
atmosphere it can be absorbed
or
reflected. Both prevent energy from reaching and warming the
ground. On average (over the globe) only about 50% of the
sunlight arriving at
the top of the atmosphere actually makes it to the ground. A beam
of sunlight that travels through the atmosphere at a low angle (right
picture above) is less intense than beam that passes through the
atmosphere more directly (left picture). 4 out of the 6 arrows
taking the short path through the atmosphere make it to the ground in
the left figure (67%). In the right figure only 2 of the 6
survive the trip (33%).
There's something else that changes also. The T Th class,
especially, should be aware of this.
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when you think you have the answer.
