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Motions of the Earth
Our goals for learning:
What are the main motions of the Earth in space?
How do we see these motions on the ground? Howdoes it affect our lives?
How does the orientation of Earth's axis changewith time?
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What are the Main Motions of
the Earth in Space?
There are 3 main movements:
Rotation (daily)
Orbit or revolution (annual)
Precession (millennia)
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1. Daily Motion
Earth moves counterclockwise, so Sun
appears to move clockwise
Sun appears to rise in the East and set
in the west.
Speed average 15/hr for 360 in 24 hrs
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View of the Sun With LatitudeThanks to Earths rotation, the Sun appears to move in the
sky. But, its location depends on the viewer's latitude.
Here is the equinox,
when sunlight falls
evenly everywhereon the Earth.
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View of Daily motion from Ground
Sun path & latitude Hand out
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Suns Path in the Local Sky
http://path_of_the_sun.htm/7/25/2019 4. Earth Movements-student
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View of Daily motion
from Ground
Earth rotates west to east
(counterclockwise), so, like the
Sun, stars appear to circle fromeast to west. Stars will rise and
set, depending on your location
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View of Daily motion from Ground
You will always see !of the celestial sphere at one time.
However which part you see depends on your location onthe Earth.
This affects whichstars you see and
the angle at which
you see them.
Some stars will
rotate in and out of
view.
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View of Daily motion from Ground
If you are in the northern hemisphere then:
" Stars near the north celestial pole never set.
" We cannot see stars near the south celestial pole.
" All other stars (and Sun, Moon, planets) rise in east and set
in west.
Your Location Your View
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2. Orbital Motion
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Thought Question
TRUE OR FALSE? Earth is closer to the Sun in summer
and farther from the Sun in winter.
A. True
B. False
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How do we mark the seasons?We define four special points:
summer solstice (longest day)winter solstice
spring (vernal) equinox
fall (autumnal) equinox
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Summary: The Real Reason for Seasons
Earths axis points in the same direction(to Polaris) all year round, so itsorientation relative to the Sun changes asEarth orbits the Sun.
Seasons are caused by.
1) Constant Axial Tilt
2) Orbit around the Sun
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Why doesntdistance matter?
Variation of Earth-Sun distance is small about 3%; this small variation is overwhelmed
by the effects of axis tilt.
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View of Orbital Motion From Ground
Summer solstice: Highestpath, rise and set at most
extreme north of due east.
Equinoxes: Sun rises
precisely due east and sets
precisely due west.
Winter solstice: Lowest
path, rise and set at most
extreme south of due east.
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When the sun is high in the sky,
its light is more concentrated &
more effective in heating the Earth.
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Why does Temperature Change Seasonally?
Summer
High sun angle+ longer time in the sky
=> increased surface heating=> WARM
Winter
Low sun angle+ shorter time in the sky
=> decreased surface heating => COLD!
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Seasonal changes are more
extreme at high latitudes
summer solstice at the Arctic Circle-no sunset
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Thought Question
It is December 31 and you see 24 hours ofdaylight. Where are you standing?
A. North Pole
B. Equator
C. South Pole
D. San Mateo
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The sky varies as Earth orbits the Sun
As the Earth orbits the Sun, the Sun appears to move eastward
along the ecliptic. The result is that the sun blots out different stars in different
sesaons, while constellations opposite the Sun are visible
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The sky varies as Earth orbits the Sun
Different constellations swing into view at different seasons.
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How does the orientation
of Earths axis change
with time?
!Although the axis seems fixed on
human time scales, it actuallyprecesses over about 26,000 years.
!Earths axis precesses like the axis
of a spinning top
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The View of Precession
from the Ground
Polaris wont always be the
North Star.
Positions of equinoxes shift
around orbit; e.g., spring
equinox, once inAries, is now
in Pisces!