1. Earth’s Motion and Seasons
Newton's First Law of Motion
Earth’s Motion and Seasons
Grade 7
Unit 1 : Lesson #4
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2. Newton's First Law of Motion
What you’ll learn
Vocabulary
Axis
Rotation
Revolution
Orbit
Solstice
Equinox
Identify Earth’s shape and other physical properties.
Compare and Contrast rotation and revolution of the Earth.
Understand the causes of Earth’s seasons.
Why it is important?
The Earth’s movement cause changes from day and night and from one season to another.
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3. Earth’s Physical Data While watching a sunset, we can notice that the
Newton's First Law of Motion
Earth’s Physical Data
While watching a sunset, we can notice that the
Sun sinks lower and lower in the western sky.
Eventually, as the Sun goes below the horizon,
the sky becomes darker.
In the past, many people thought that the Sun, the Moon, and other objects in space moved round the Earth each day. It is now known that some of the motions of these objects, as observed from the Earth, are caused by the movements of the Earth.
Many people also used to think that the Earth was flat.
They thought if they sailed far enough out to the sea,
they would eventually fall off which is not true. 3 3
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4. Newton's First Law of Motion
Try this!
What were the misconceptions that people had in the past regarding the Sun, Moon and the Earth?
Answer: In the past, many people thought that the Sun, the Moon, and other objects in space moved round the Earth each day. Many people also used to think that the Earth was flat.
Answer: In the past, many people thought that the Sun, the Moon, and other objects in space moved round the Earth each day. Many people also used to think that the Earth was flat. 4 4
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5. Spherical Earth and its evidence
Newton's First Law of Motion
Spherical Earth and its evidence
The pictures from space show that the Earth is
shaped like a ball, or a sphere. A sphere is a
three-dimensional object whose surface at all
points is the same distance from its centre.
While watching a sailboat arriving at a dock, we
can notice that the first thing we see is the top
of the boat’s sail. This occurs because the curved
shape of the Earth hides the rest of the boat
from view until it is closer to the dock. As it
comes closer to the dock, more and more of
its sail becomes visible. Finally, the entire boat
is visible. 5 5
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6. Newton's First Law of Motion
Try this!
What objects casts a shadow on the Moon during a lunar eclipse? Why?
Answer: The Earth casts a curved shadow
on the Moon during a lunar eclipse because
objects with curved surfaces cast curved
shadows.
Answer: The Earth casts a curved shadow on the Moon during a lunar eclipse because objects with curved surfaces cast curved shadows. 6 6
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7. Newton's First Law of Motion
Influence of Gravity
The spherical shape of the Earth and other planets is due to gravity. Gravity is a force that attracts all objects towards each other. The farther the objects, weaker is the pull of gravity. Larger the object, larger is its gravitational pull.
A large object in space is spherical because gravity attracted the particles towards its center while the object was in a liquid or gaseous state. The distance between particles in the object and its center decreases due to the spherical shape. In this way, the potential energy due to gravity is less, and a stable shape results.
Isn’t the Earth flat?
Even though the Earth is round, it may seem flat to us. This is because the surface of the Earth is very large compared to our size. 7 7
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8. Try this! Farther the object, _________ is the pull of gravity. Weaker
Newton's First Law of Motion
Try this!
Farther the object, _________ is the pull of gravity.
Weaker
Stronger
Gravity and distance are unrelated.
None of the above
Answer: A
Answer: A
Reason: The farther the objects, weaker is the pull of gravity. 8 8
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9. Physical Properties of Earth
Newton's First Law of Motion
Physical Properties of Earth
Although Earth is shaped like a sphere, it is not a perfect one. It bulges slightly at the equator and is somewhat flattened around the poles. This causes the Earth’s circumference at the equator to be a bit larger than the Earth’s circumference as measured through the north and south poles.
Axis 9 9
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10. Physical Properties of Earth
Newton's First Law of Motion
Physical Properties of Earth
Physical Properties of Earth
Diameter (pole to pole)
12,714 km
Diameter (equator)
12,756 km
Circumference (poles)
(distance around Earth through N and S poles)
40,008 km
Circumference (equator)
(distance around Earth through equator)
40,075 km
Mass
5.98 x 1024 kg
Average Density (average mass per unit volume)
5.52 g/cm3
Average distance from the Sun
149,600,000 km
Period of rotation relative to stars
Period of rotation relative to the Sun
23 h, 56 min
24 h
Period of revolution (1 year)
(path around the Sun)
365 d, 6 h, 9 min 10 10
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11. Newton's First Law of Motion
Motions of Earth
The Earth’s axis is the imaginary line drawn from the north geographic pole through Earth to the south geographic pole. The Earth’s geographic poles are located at the north and south ends of its axis. The Earth spins around this imaginary line.
The spinning of the Earth on its axis is called rotation.
This causes us to experience day and night.
Earth has another type of motion. As it rotates on its axis
each day, the Earth also moves along a path around the Sun.
This motion of the Earth around the Sun is called revolution. 11 11
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12. Try this! Earth’s spinning on its axis is which motion? Rotation
Newton's First Law of Motion
Try this!
Earth’s spinning on its axis is which motion?
Rotation
Revolution
Waxing
Waning
Answer: A
Answer: A
Reason: Earth’s spinning on its axis is known as rotation. 12 12
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13. Seasons
We saw how the Earth’s rotation causes day and night. The Earth also moves around the Sun, completing one revolution each year. Earth is a satellite of the Sun, moving around it along a curved path called an orbit.
The shape of the Earth’s orbit is an ellipse, which is rounded like a circle, but somewhat flattened. As Earth moves along in its orbit, the way in which the Sun’s light strikes the surface of the Earth changes.
The elliptical orbit of the Earth causes it to be closer to the Sun in January and farther from the Sun in July.
But, the total amount of energy which the Earth receives from the Sun changes little during a year. However, the amount of energy that specific places on Earth receives vary quite a lot.
For example: A new year has begun. As days and weeks pass, we notice that the Sun remains in the sky later and later each day. We look forward to spring when we will be able to stay outside longer in the evening because the days become longer and longer. 13
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14. Newton's First Law of Motion
Try this!
Which earth motion causes a point on Earth to experience different amounts of energy from the Sun at different times of the year?
Answer: Revolution
Answer: Revolution 14 14
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15. Newton's First Law of Motion
Earth’s tilt
We can observe one reason why the amount of energy from the Sun varies by moving a globe of earth around a light source. If we keep
the globe tilted in one direction, we will see that the top
half of the globe is tilted towards the light during part of
its orbit and tilted away from the light during another
part of its orbit.
Imagine a flat surface that contains the Earth’s orbit. Earth’s axis forms a 23.5 – degree angle with this imaginary surface, and always points to the North Star. Because of this, just as in our model, daylight hours are longer for the half of Earth, or hemisphere, tilted towards the Sun. Also, the Sun’s rays hit that hemisphere more directly, at a higher angle. In other words, the Sun is higher in the sky for longer periods of time. 15 15
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16. Newton's First Law of Motion
Try this!
During which season do we notice longer days and shorter nights?
Answer: Summer
Answer: Summer 16 16
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17. Newton's First Law of Motion
Solstices
The Sun’s position relative to the Earth’s equator changes because of the tilt of the Earth’s axis. Twice during an year, the Sun reaches its greatest distance north or south of the equator and is directly over the Tropic of Cancer or the Tropic of Capricorn. These times are known as the summer and winter solstices.
Summer solstice, which is about the longest day of the year, happens on June 21 or 22 for the northern hemisphere and on December 21 or 22 for the southern hemisphere.
Winter solstice, which is about the shortest day of the year, happens on December 21 or 22 for the northern hemisphere and on June 21 or 22 for the southern hemisphere. 17
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18. Newton's First Law of Motion
Try this!
What occurs when the Sun reaches its greatest distance north or south of the equator?
Orbit
Solstice
Equinox
Axis
Answer: B
Answer: B
Reason: Twice during an year, the Sun reaches its greatest distance north or south of the equator and is directly over the Tropic of Cancer or the Tropic of Capricorn. These times are known as the summer and winter solstices. 18 18
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19. Newton's First Law of Motion
Equinoxes
At equinox, when the Sun is directly above the Earth’s equator, the lengths of day and night are equal all over the world. During equinox, the Earth’s tilt is not towards or away from the Sun.
In the northern hemisphere, spring equinox is March 21 or 22 and fall equinox is September 21 or 22.
Why does the dates for solstices and equinoxes change slightly over time?
The time it takes for the Earth to revolve round the Sun is not a whole number of days. Because of this, the dates for solstices and equinoxes change slightly over time. 19
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20. Newton's First Law of Motion
Try this!
When are the lengths of day and night equal all over the world?
Orbit
Solstice
Equinox
Axis
Answer: C
Answer: C
Reason: At equinox, when the Sun is directly above the Earth’s equator, the lengths of day and night are equal all over the world. 20 20
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21. Newton's First Law of Motion
Summary
Earth’s shape is nearly spherical. The spherical shape of the Earth and other planets is due to gravity. Gravity is a force that attracts all objects towards each other.
Even though the Earth is round, it may seem flat to us. This is because the surface of the Earth is very large compared to our size.
The Earth’s circumference at the equator to be a bit larger than the Earth’s circumference as measured through the north and south poles. 21
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22. Newton's First Law of Motion
Summary
The Earth’s axis is the imaginary line drawn from the north geographic pole through Earth to the south geographic pole.
The spinning of the Earth on its axis is called rotation. This causes us to experience day and night.
As it rotates on its axis each day, the Earth also moves along a curved path around the Sun, known as an orbit. This motion of the Earth around the Sun is called revolution. 22 22
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23. Summary Earth’s tilt and revolution cause seasons to occur.
Newton's First Law of Motion
Summary
Earth’s tilt and revolution cause seasons to occur.
Twice during an year, the Sun reaches its greatest distance north or south of the equator and is directly over the Tropic of Cancer or the Tropic of Capricorn. These times are known as the summer and winter solstices.
At equinox, when the Sun is directly above the Earth’s equator, the lengths of day and night are equal all over the world. During equinox, the Earth’s tilt is not towards or away from the Sun. 23 23
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24. Critical thinking activity
Newton's First Law of Motion
Critical thinking activity
Describe how Earth’s shape is influenced by gravity.
Notes: A large object in space like the Earth is spherical because gravity attracted the particles towards its center while the object was in a liquid or gaseous state. The distance between particles in the object and its center decreases due to the spherical shape. In this way, the potential energy due to gravity is less, and a stable shape results. 24 24
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25. Newton's First Law of Motion
Assessment
What is a sphere?
Answer: A sphere is a three-dimensional object whose surface at all points is the same distance from its centre.
Answer: A sphere is a three-dimensional object whose surface at all points is the same distance from its centre. 25
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26. Newton's First Law of Motion
Assessment
Why do we first see only the top of a boat’s sail while watching it arrive at a dock?
Answer: While watching a sailboat arriving at a dock, the first thing we see is the top of the boat’s sail. This occurs because the curved shape of the Earth hides the rest of the boat from view until it is closer to the dock.
Answer: While watching a sailboat arriving at a dock, the first thing we see is the top of the boat’s sail. This occurs because the curved shape of the Earth hides the rest of the boat from view until it is closer to the dock. 26
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27. Newton's First Law of Motion
Assessment
The spherical shape of the Earth and other planets is due to ________
Answer: Gravity
Answer: The spherical shape of the Earth and other planets is due to gravity. 27
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28. Newton's First Law of Motion
Assessment
Larger the object, ________ is its gravitational pull.
Answer: larger
Answer: Larger the object, larger is its gravitational pull. 28
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29. Newton's First Law of Motion
Assessment
Compare and contrast Rotation and Revolution.
Rotation
Revolution
The spinning of the Earth on its axis is called rotation.
This motion of the Earth around the Sun along an orbit is called revolution.
Period of rotation relative to the Sun is 24 h
Period of revolution around the Sun is 365 d, 6 h, 9 min 29
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30. Assessment The shape of the Earth’s orbit is an _________ Circle
Newton's First Law of Motion
Assessment
The shape of the Earth’s orbit is an _________
Circle
Ellipse
Sphere
None of the above
Answer: B
Answer: B
Reason: The shape of the Earth’s orbit is an ellipse, which is rounded like a circle, but somewhat flattened. 30 30
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31. Assessment The Earth’s axis has a _______ degree angle of tilt.
Newton's First Law of Motion
Assessment
The Earth’s axis has a _______ degree angle of tilt.
Answer: 23.5
Answer: 23.5 31 31
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32. Assessment What are solstices?
Answer: The Sun’s position relative to the Earth’s equator changes because of the tilt of the Earth’s axis. Twice during an year, the Sun reaches its greatest distance north or south of the equator and is directly over the Tropic of Cancer or the Tropic of Capricorn. These times are known as the summer and winter solstices.
Answer: The Sun’s position relative to the Earth’s equator changes because of the tilt of the Earth’s axis. Twice during an year, the Sun reaches its greatest distance north or south of the equator and is directly over the Tropic of Cancer or the Tropic of Capricorn. These times are known as the summer and winter solstices. 32 32

33. Assessment What are equinoxes?
Answer: At equinox, when the Sun is directly above the Earth’s equator, the lengths of day and night are equal all over the world. During equinox, the Earth’s tilt is not towards or away from the Sun.
Answer: At equinox, when the Sun is directly above the Earth’s equator, the lengths of day and night are equal all over the world. During equinox, the Earth’s tilt is not towards or away from the Sun. 33 33

34. Assessment Compare and contrast summer and winter solstices.
Answer: Summer solstice, which is about the longest day of the year, happens on June 21 or 22 for the northern hemisphere and on December 21 or 22 for the southern hemisphere. Winter solstice, which is about the shortest day of the year, happens on December 21 or 22 for the northern hemisphere and on June 21 or 22 for the southern hemisphere.
Answer: Summer solstice, which is about the longest day of the year, happens on June 21 or 22 for the northern hemisphere and on December 21 or 22 for the southern hemisphere. Winter solstice, which is about the shortest day of the year, happens on December 21 or 22 for the northern hemisphere and on June 21 or 22 for the southern hemisphere. 34 34

35. Practice
Use a globe and an unshaded light source to illustrate how the tilt of the Earth on its axis, as it rotates and revolves around the Sun., causes changes in the lengths of a day.
Notes: Build a model. 35 35

36. Thank You! 36
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