Motions of the Earth

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Motions of the Earth
http//www.fpsoftlab.com/images/screenshots/earth-
640x480-1.jpg
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There are two motions Rotation Revolution
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Rotation The Earth rotates on its axis from west
to east. It makes a complete 360 turn in 24
hours at a speed of approximately 1600 km/h at
the equator. Most places on earth
experience a period of daylight and one of
darkness during this 24 hour period.
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http//upload.wikimedia.org/wikipedia/commons/6/62
/Rotating_earth_(slower).gif
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The most important effects of rotation are
related to changes in light, heat and humidity
that occur between night and day. By imposing a
daily rhythm of day and night on many things,
this motion has had a profound effect the
development of all forms of plants and
animals. The earths rotation also has an
important influence on the movement of air and
water on the earths surface.
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Revolution Earth revolves around the sun once in
365.242 days. (Leap Year add one day to
February every four years to make up the four .25
days).
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http//www.pbs.org/wgbh/nova/galileo/mistake.html
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The path or orbit made by the earth as it
revolves around the sun is elliptical in shape
and not circular.
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A circle has one focus.
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An ellipse has two foci.
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The sun is at one of the two foci of the
elliptical orbit of Earth. Therefore, as Earth
revolves around the sun, its distance from the
sun varies.
http//www.uwsp.edu/gEo/faculty/ritter/images/atmo
sphere/energy/elliptical_orbit.jpg
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The distance at aphelion (farthest) is 152
million km (July 4) and at perihelion (closest)
the distance is 147 million km (Jan. 3). Note
This variation has no detectable effect on the
temperature of the earth.
http//www.uwsp.edu/gEo/faculty/ritter/images/atmo
sphere/energy/elliptical_orbit.jpg
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The path followed by the earth around the sun can
be thought of as forming a flat surface or plane.
This is known as the plane of orbit or the plane
of the ecliptic. The earths axis is tilted 23.5
from the perpendicular to the plane of orbit, in
other words the angle between the axis and the
plane of orbit is 66.5 (90 - 23.5 66.5).
http//divulgence.net/AxialTilt.jpg
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Furthermore, Earths axis always maintains this
degree of tilt, remaining parallel to all its
previous positions. It points in the same
direction all the time toward Polaris or the
North Star. This is known as the parallelism of
the axis.
http//www.hort.purdue.edu/newcrop/tropical/lectur
e_02/lec_02.html
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Parallelism of Earths axis
The axis is always parallel to itself.
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http//www.aapscience7.net/Chapter20Work/radec_ea
rth_orbit.gif
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Vernal Equinox
Winter Solstice
Summer Solstice
Autumnal Equinox
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http//www.roebuckclasses.com/maps/physicmap/earth
sun/earthrevolution.gif
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• The revolution of the earth about the sun on an
  inclined axis has two very important effects
• seasonal temperature differences (called the
  intensity of solar radiation), and
• variations in the length of day and night (called
  the duration of solar radiation).
• This is what accounts for seasons.
• Neither would happen if the axis was not inclined.

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Four important dates should be noted and are
related to five important lines of latitude. 1.
June 21st
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Summer Solstice
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1. June 21st The suns direct rays are on the
Tropic of Cancer. (23.5N) This day is called
Summer Solstice in the Northern Hemisphere. The
days are longer because on this day the Northern
Hemisphere is inclined towards the sun. During
one complete rotation, all places in the Northern
Hemisphere spend a greater proportion of the 24
hours in daylight than they do in darkness. As
you travel further north the day becomes longer.
Any point north of the Arctic Circle receives 24
hours of daylight and any point south of the
Antarctic Circle receives 24 hours of night.
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Summer Solstice
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2. Sept. 21st The suns direct rays are over
the Equator. This day is called Fall (Autumnal)
Equinox. The days are very close to being 12
hours day and 12 hours night.
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Vernal Equinox
Winter Solstice
Summer Solstice
Autumnal Equinox
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3. Dec. 21st The suns direct rays are on the
Tropic of Capricorn. This time is called Winter
Solstice in the Northern Hemisphere. Days are
shorter. Nights are longer.
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Vernal Equinox
Winter Solstice
Summer Solstice
Autumnal Equinox
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4. March 21st The suns direct rays are over
the Equator again. This day is called Spring
(Vernal) Equinox. The days are very close to
being 12 hours day and 12 hours night.
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Vernal Equinox
Winter Solstice
Summer Solstice
Autumnal Equinox
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Portion of Earth that is in sunlight.
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Equinox (Vernal or Autumnal)
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Solstices
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The Winter Solstice Northern Hemisphere
Above the Arctic Circle (66.5), the sun never
gets above the horizon.
NP - 90
23.5
23.5
On the equator the sun is 23.5 from the vertical.
Equator - 0
Tropic of Capricorn (23.5S) sun is directly
overhead
Above the Antarctic Circle (66.5) the sun never
sets. 24 hour daylight.
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There are two principal causes for the seasonal
differences in temperature created by the tilt of
the earth and the curvature of the earth 1. the
INTENSITY of solar radiation received 2. the
DURATION of solar radiation received Intensity Var
iations in intensity are caused by the difference
in the angle at which the suns rays strike the
earths surface. Duration The longer the length
of daylight a place experiences (that is, the
greater the duration), the greater the amount of
energy that place will receive.
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Duration and Intensity of Solar Radiation

• How do they affect climate and create the seasons?

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Intensity
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The same amount of energy is received on the
surface from the beam of light in A as in B, but
in B it is spread over a larger area. Therefore,
less energy is absorbed of each unit of surface
and the temperatures are less.
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Tropic of Cancer, 23.5N
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Tropic of Cancer, 23.5N
Tropic of Capricorn, 23.5S
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Duration
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Sunlight on this day is directly over Tropic of
Cancer. The length of time the Tropic of Cancer
is exposed to daylight on this date is greater
than the length of time it is in darkness. The
yellow line (day) is longer than the red line
(night). This means more solar radiation is
received and temperatures are higher. This is the
duration of solar radiation received.
Sunlight
June 21/22 - Summer Solstice in Northern
Hemisphere
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The opposite is true at the Tropic of
Capricorn. The length of time the Tropic of
Capricorn spends in the daylight is shorter than
in darkness. The red line (night) is longer than
the yellow line (day). Therefore, less solar
energy is received here, and temperatures are
cooler. It is winter in the Southern Hemisphere.
Sunlight
June 21/22 - Winter Solstice in Southern
Hemisphere
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Notice that above the Arctic Circle, 66.5oN, the
sunlight is constant. The sun never sets. It is
called the Land of the Midnight Sun.
South of the Antarctic Circle the opposite is
true 24 hours of no (direct) sunlight.
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Rotation and Revolution