The Earth and Sun
Shedding Some Light on the
System
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PSC 121
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Earth’s Rotation…

Earth’s rotation on its axis generates
periods of daylight and night.
http://www.onr.navy.mil/focus/spacescien
ces/observingsky/motion1.htm
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PSC 121
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Earth’s Rotation…

The amount of daylight/nighttime is also
determined by the amount of Earth’s tilt.
The Earth’s axis is tilted 23.5o from the
plane of the Earth’s orbit around the Sun.
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PSC 121
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Earth’s Rotation…

The amount of daylight/nighttime is also
determined by the amount of Earth’s tilt.
The Earth’s axis is tilted 23.5o from the
plane of the Earth’s orbit around the Sun.
http://janus.astro.umd.edu/astro/seasons/
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PSC 121
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Earth’s Revolution…
The motion of the
Earth in its orbit
around the Sun is an
ellipse, not a circle.
That means that at
some points in the
orbit the Earth is
closer to the Sun
than at others.
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Circle
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Ellipse
Earth’s Revolution…
When the Earth is closest it is at perihelion.
When it is farthest, it is at aphelion. The
difference in distance is about 3.5%...not
enough to account for the seasonal changes!
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PSC 121
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Earth’s Revolution…
So what causes the daily and seasonal
changes?
Earth’s curvature
And its tilt !
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PSC 121
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Earth’s Revolution…
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Earth’s Revolution…
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Earth’s revolution
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Earth’s Revolution
Since Earth’s tilt always points in the
same direction, at a given point on
the Earth the rays change in
directness and duration.
http://www.cs.sbcc.cc.ca.us/~physics/flash/
LengthofDay.swf
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PSC 121
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Earth’s Revolution
Since Earth’s tilt always points in the
same direction, points on the Earth
will experience seasonal changes.
http://www.astro.uiuc.edu/projects/data/Seasons/seasons.html
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PSC 121
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