Download Chapter 17 Earth`s Cycles

CHAPTER 17
EARTH’S CYCLES
Lesson 1 – How does Earth move?
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Earth spins like a top as it circles around and
around the Sun. Earth is tipped to one side as it
moves. This tilt causes the changing seasons.
Why can’t we feel Earth moving?
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Everyone agrees that Earth doesn’t seem to be
moving.
You can’t sense its movement because you are
moving right along with it.
Another reason you don’t sense Earth’s motion is that
it moves steadily and smoothly.
What are some clues that Earth is
moving?
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1. To people on Earth, the Sun and the stars seem
to move across the sky. Why? Because the Earth is
constantly turning.
2. Another clue is that the seasons change during
the year for most places away from the equator.
The equator is the imaginary line that divides the
north and south halves of the Earth.
Earth’s Rotation
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Earth turns around its axis. The axis is the
imaginary line that goes through its center.
The spinning of Earth around its axis is called its
rotation.
How long does it take for Earth to spin once on its
axis?
23 hours and 56 minutes – 1 day
How does Earth’s rotation cause day
and night?
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As Earth spins or rotates on its axis, there are
certain times when areas of Earth are not facing the
Sun. The result is darkness or – night.
When Earth rotates, it turns from west to east. This
causes objects such as the Sun and stars to appear
to move from east to west in the sky.
Why do shadows change?
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Because of Earth’s rotation, shadows change their
positions and sizes during the day.
Think about it. In the morning and the afternoon
your shadow is much longer because the Sun is
shining on you at an angle. At 12:00 noon your
shadow is fairly short because the Sun is directly
over your head.
Earth’s Revolution
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We have just learned that Earth spins on its axis
and that is called rotation.
However, Earth is not just rotating. It is also moving
in a path around the Sun. This is called revolution.
It takes Earth an entire year to make one revolution
around the Sun.
The path that Earth takes around the Sun is called
its orbit.
Earth’s orbit is an ellipse. Think of an ellipse as a
stretched out circle.
What keeps Earth revolving around the
Sun?
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Why doesn’t Earth just float around in outer space?
Gravity!
Gravity is the force that pulls two objects toward
each other. There is gravity between the Earth and
the Sun.
Revolution
Seasons
What causes the seasons to change?
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The tilt of Earth on its axis.
During summers in the Northern Hemisphere, the
North Pole tilts toward the Sun and gets more direct
sunlight, while the South Pole tilts away from the Sun
and gets sunlight that is less direct.
United Streaming Video – Seasons (25 minutes)
Lesson 2 – What patterns can you see
in the sky?
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You see stars in different parts of the night sky as
Earth rotates on its axis and revolves around the
Sun. The Moon revolves around Earth. It also
rotates on its own axis. All these movements cause
changes in the patters that you see in the sky.
The Moon
The Moon
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The Moon looks as if it is shining with its own light.
However, the moon does not produce light. You can
see the Moon because it reflects sunlight off its
surface.
The Moon orbits around Earth in an elliptical orbit.
It takes the Moon about 27.3 days to complete one
revolution around Earth.
Like Earth, the Moon rotates on its axis.
As the Moon orbits Earth, the SAME SIDE of the
Moon always faces Earth.
Why does the Moon look different at
different times of the month?
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The changing of the Moon refers to the Moon’s
phases.
Different amounts of the lighted half of the Moon
face Earth at different times.
Phases of the Moon Video – 3 minutes
Eclipses
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An eclipse occurs when one object in space gets
between the Sun and another object, and casts its
shadow on the other object.
Lunar Eclipse
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A lunar eclipse occurs when the Moon passes
through Earth’s shadow.
Lunar Eclipse Video (1 minute)
Solar Eclipse
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A solar eclipse occurs when the moon comes
between the Earth and the Sun.
Solar Eclipse video (1 minute)
The Sun
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The Sun is a medium size star that is nearest Earth.
United Streaming Video – The Sun (1 minute)
Constellations
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A constellation is a star pattern in the sky.
For thousands of years, people have noticed that
the stars appear in shapes and patterns in the sky.
Constellations Video – 45 seconds