Principle
All of the earth's weather results from the fact that the atmosphere
is unequally heated by the sun. The most significant cause
of unequal heating of the atmosphere is because the earth is
a sphere and, hence, sunlight does not strike the earth at
the same angle at every latitude. |
Figure 1 -
The Earth's Orbit Around the Sun
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The Earth-Sun
System
- The earth spins counterclockwise when viewed from above
the North Pole.
- The earth makes one revolution every 24 hours
- The earth averages 150 kilometers (93 million miles)
from the sun.
- The earth is closest to the sun in January and most
distant in June.
- The earth tilts 23.5 degrees off vertical.
- The Northern Hemisphere tilts toward the sun in summer
and away from the
sun in winter.
- The seasons are marked by the movement of the earth
around the sun and,
hence, which way the earth is tilted with respect to the sun.
Summer solstice =
June 22 = sun is directly above 23.5 degrees North Autumnal equinox =
Sept. 23 = sun is above equator (length of day length of night = after
length of day)
Winter solstice = Dec. 22 = sun above 23.5 degrees South
Vernal equinox = Mar. 21 = sun again above equator (days
= nights)
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Figure 2 -
Sunlight Reaching Earth at Equinox
The North Pole points up out of the plane of this image; the
Earth's axis is perpendicular to the sun.
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Equinoxes
- During the equinoxes (March
21 and September 21), a day lasts 12 hours and a night
lasts 12 hours at all latitudes.
- Sunlight strikes the earth most directly at the equator.
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Figure 3 -
Sunlight Reaching Earth at the Winter Solstice
The North Pole points away from the sun.
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Solstices
- During the winter solstice (pictured
above), the Northern Hemisphere day lasts fewer than 12
hours and the Southern Hemisphere day lasts more than 12
hours.
- During the winter solstice,
the North Pole has a 24-hour night and the South Pole has
a 24-hour day.
- During the winter solstice,
sunlight strikes the earth most directly at 23.5 degrees
South (the Tropic of Capricorn).
- During the summer solstice (not
pictured), the Northern Hemisphere day lasts more than
12 hours and the Southern Hemisphere day lasts fewer than
12 hours.
- During the summer solstice,
the North Pole has a 24-hour day and the South Pole has
a 24-hour night.
- During the summer solstice,
sunlight strikes the earth most directly at 23.5 degrees
North (the Tropic of Cancer).
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