1. Chapter Preview Questions
What is the source of most of Earth’s heat and natural light?
a. the moon
b. Earth itself
c. the sun
d. stars other than the sun

2. Chapter Preview Questions
What is the source of most of Earth’s heat and natural light?
a. the moon
b. Earth itself
c. the sun
d. stars other than the sun

3. Chapter Preview Questions
2. What force keeps Earth in motion around the sun?
a. friction between Earth and the planets
b. gravity between Earth and the moon
c. friction between Earth and the sun
d. gravity between Earth and the sun

4. Chapter Preview Questions
2. What force keeps Earth in motion around the sun?
a. friction between Earth and the planets
b. gravity between Earth and the moon
c. friction between Earth and the sun
d. gravity between Earth and the sun

5. Chapter Preview Questions
3. How many natural satellites does Earth have?
a. one
b. two
c. six
d. dozens

6. Chapter Preview Questions
3. How many natural satellites does Earth have?
a. one
b. two
c. six
d. dozens

7. Chapter Preview Questions
4. What causes day and night?
a. the tilt of Earth’s axis
b. the sun moving behind the moon
c. Earth’s movement around the sun
d. Earth’s rotation on its axis

8. Chapter Preview Questions
4. What causes day and night?
a. the tilt of Earth’s axis
b. the sun moving behind the moon
c. Earth’s movement around the sun
d. Earth’s rotation on its axis

9. What events are caused by the motion of Earth and the moon?
Imagine that you are looking up at a full moon in the sky. Why is the moon so bright? Does the moon produce its own light, like a flashlight, or does its light come from somewhere else? Explain your reasoning.

10. Section 1: Earth in Space
Standard 8.4.e: Students know the appearance, general composition, relative position and size, and motion of objects in the solar system, including planets, planetary satellites, comets, and asteroids.

11. How Earth Moves How doe Earth move in space? Rotation Revolution
Earth moves through space in two major ways: rotation and revolution. Earth rotates on its axis every 24 hours, causing day and night. Earth revolves around the Sun in a cycle of about days. The modern calendar compensates for the one-fourth day by adding a day every four years. We call this a “leap year.”

12. Sunlight Striking Earth’s Surface
Near the equator, sunlight strikes Earth’s surface more directly and is less spread out than near the poles.

13. Earth’s Tilted Axis What causes the cycle of seasons on Earth?
Earth has seasons because its axis is tilted as it revolves around the sun. Earth’s axis is tilted 23.5 degrees relative to the Sun. The globe has imaginary lines delineated 23.5 degrees from the Equator to identify the areas of most direct sunlight in the Northern and Southern Hemispheres.

14. Earth’s Tilted Axis Tropic of Cancer Tropic of Capricorn
Located at 23.5 degrees north latitude.
Summer occurs in the Northern Hemisphere when this imaginary line is in direct alignment to the Sun in late June.
Located 23.5 degrees south latitude.
Summer occurs in the Southern Hemisphere when this imaginary line is in direct alignment to the Sun in late December.

15. Seasons on Earth
Earth has seasons because its axis is tilted as it revolves around the sun.

16. Seasons on Earth
The height of the sun above the horizon varies with the season.

17. Section 2: Gravity and Motion
Standard 8.2.f: Students know the role of gravity in forming and maintaining the shapes of planets, stars, and the solar system.

18. Gravity, Mass, and Distance
The strength of the force of gravity between two objects depends on two factors: the masses of the objects and the distance between them.

19. Gravity Versus Distance
As a rocket leaves a planet’s surface, the force of gravity between the rocket and the planet changes. Use the graph to answer the following questions.

20. Gravity Versus Distance
Reading Graphs:
What is the force of gravity on the rocket at the planet’s surface?
Four million newtons

21. Gravity Versus Distance
Reading Graphs:
What is the force of gravity on the rocket at a distance of two units (twice the planet’s radius from its center)?
One million newtons

22. Gravity Versus Distance
Making Generalizations:
According to the graph, is the relationship between gravity and distance linear or nonlinear? Explain.
Nonlinear. The graph is a curve.

23. Gravity Versus Distance
Drawing Conclusions:
In general, how does the force of gravity pulling on the rocket change as the distance between it and the planet increases?
It decreases.

24. Gravity Versus Distance
Predicting:
What would the force of gravity on the rocket be at a distance of five units?
0.16 million newtons, or 160,000 newtons

25. Gravity and Inertia
Newton concluded that two factors–inertia and gravity–combine to keep Earth in orbit around the sun and the moon in orbit around Earth.

26. Section 3: Phases, Eclipses, and Tides
Standard 8.2.g Students know the role of gravity in forming and maintaining the shapes of planets, stars, and the solar system. Standard 8.4.d Students know that stars are the source of light for all bright objects in outer space and that the Moon and planets shine by reflected sunlight, not by their own light.

27. Motions of the Moon
The changing relative positions of the moon, Earth, and sun cause the phases of the moon, eclipses, and tides.

28. Phases of the Moon
The phase of the moon you see depends on how much of the sunlit side of the moon faces Earth.

29. The Moon’s Orbit
The moon’s orbit is tilted about 5 degrees relative to Earth’s orbit around the sun.

30. Solar Eclipse
A solar eclipse occurs when the moon passes directly between Earth and the sun, blocking sunlight from Earth.

31. Lunar Eclipse
During a lunar eclipse, Earth blocks sunlight from reaching the moon’s surface.

32. Tides
Tides occur mainly because of differences in the force of gravity between the moon and different parts of Earth.

33. Spring and Neap Tides
When Earth, the sun, and the moon are in a straight line, a spring tide occurs. When the moon is at a right angle to the sun, a neap tide occurs.

34. Section 4: Earth’s Moon
Standard 8.4.e Students know the appearance, general composition, relative position and size, and motion of objects in the solar system, including planets, planetary satellites, comets, and asteroids.

35. The Moon’s Surface What features are found on the moon’s surface?
The moon’s surface has dark, flat areas, which Galileo called maria.
These “seas” are actually hardened rock from huge lava flows that occurred between 3 and 4 billion years ago.
The moon’s surface contains large round pits called craters.
Craters can be hundreds of kilometers across.
They were caused by impacts of meteoroids early in its history.
A third major feature of the moon surface is the presence of highlands, or mountains.
What features are found on the moon’s surface?

36. The Moon’s Surface
Features on the moon’s surface include maria, craters, and highlands.

37. Characteristics of the Moon
The moon is 3,476 km in diameter, a little less than the distance across the contiguous United States.
It is about one-fourth the diameter of Earth.
The moon has only one-eightieth the mass of Earth.
Temperatures range from 130°C in direct sunlight to -180°C at night.
Temperatures vary so much because it has no atmosphere.
The moon has no liquid water.

38. The Origin of the Moon
Scientists theorize that a planet-sized object collided with earth to form the moon.