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GEOG. 1710 EARTH SCIENCE
LECTURE 3. EARTH-SUN RELATIONS; SEASONS
Rotation and Revolution of the Earth
The earth rotates on its axis towards the east, completing
one rotation in 24 hours. Thus, heavenly bodies, including the
Sun, appear to rise in the east and set in the west; if viewed from
above the North Pole, the Earth would appear to rotate counterclockwise.
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The earth revolves around the Sun once in 365.25 days
(hence every 4 years we have a leap year with an extra day). The
orbit around the Sun, which lies along the plane of the
ECLIPTIC, is not a perfect circle, but an ellipse; the earth is
closer to the Sun at PERIHELION (91445000 miles) on January
3rd, and furthest from the Sun at APHELION (94555000 miles)
on July 4th. Therefore, during the northern hemisphere summer
(July) we are actually further from the Sun than in the winter the reasons for heat of our summer are more to do with the tilt of
the earth than distance from the Sun.
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The Earth's axis is not perpendicular with the plane of the
ecliptic. It is tilted 23.50 from the perpendicular; or 66.50 from the
plane of the ecliptic. It is a very important fact that the earth's
axis always points the same way as the earth revolves around the
sun - this is called the POLARITY or parallelism of the axis.
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The reason this is so important is that the tilt of the earth
determines the angle at which the sun's rays (incoming solar
radiation or insolation) strike the surface. Generally speaking, the
steeper the angle, the more the Sun's rays are concentrated and
the more heating occurs - this changes throughout the year and
is the cause of SEASONS.
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As shown, the northern hemisphere tilts towards the Sun in
June and gradually changes to tilting away from the Sun in
December (the opposite is true for the southern hemisphere) note that the angle of tilt stays the same as the Earth moves
around the Sun. The circle of illumination only passes directly
through the poles in March and September.
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Due to this tilt, the latitude where the noon rays of the Sun
are perpendicular to the surface changes throughout the year; the
northernmost latitude reached by the perpendicular rays of the
Sun is 23.50 north on about June 21st: this latitude is defined as
the TROPIC OF CANCER; the southernmost latitude reached by
the perpendicular rays of the Sun is 23.50 south on about
December 21st: this latitude is defined as the TROPIC OF
CAPRICORN. From the point of view of the northern
hemisphere, June 21st and December 21st are the summer and
winter SOLSTICE, respectively.
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It should also be noted that during summer solstice,
poleward of 66.50 north recieves continuous sunshine - so the day
is 24 hours long; poleward of 66.50 south recieves no sunshine - so
the night is 24 hours long. This is how the ARCTIC CIRCLE and
the ANTARCTIC CIRCLE are defined.
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Midnight sun
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As noted earlier, the circle of illumination only passes through the poles
midway between the solstices, in March and September. These times
are the EQUINOXES - Spring equinox on about March 20th,
Autumnal Equinox on about September 22nd. On these two days only,
the length of day and night everywhere is equal at 12 hours each. The
length of day is obviously also important in determining the amount of
heat received in a day; the northern hemisphere in the summer receives
insolation at a steeper angle and for a longer period than in the winter.
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