Earth’s Moon

Grade 7Unit 1 : Lesson #5

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What you’ll learn

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Vocabulary Crater Moon phase Solar eclipse Lunar eclipse

Understand the Moon’s surface features and interiors.

Identify phases of the Moon. Explain why solar and lunar eclipses occur. Compare possible origins of the Moon. Why it’s important

The Moon is Earth’s closest neighbor in space.

The Moon’s Surface The surface of the moon when observed through telescopes reveal

dark-colored maria (a Latin word for “seas”) and lighter-colored highland areas.

Maria probable formed when lava flowed from the Moon’s interior flooded into large, bowl-like regions on the Moon’s surface. They reminded Galileo of oceans.

The rocks of the maria are 3.2 billion to 3.7 billion years old. So far, they are the youngest rocks to be found on the Moon. The oldest moon rocks analyzed so far dates to about 4.4 billion years old and were found in the lunar highlands.

The lunar highlands are areas of the lunar surface with an elevation that is several kilometers higher than the maria. Some lunar highlands are located in the south-central region of the moon.

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Try This!

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The surface of the moon when observed through telescopes reveal dark-colored _________, a Latin word for “seas”.

Answer: Maria

Craters There are many depressions on the Moon’s surface. These are

called craters.

Meteorites, asteroids, and comets striking the moon’s surface caused most of these craters. As meteorites struck the Moon, cracks could have formed in the Moon’s crust, allowing lava flows to fill in the large depressions.

Craters are useful for determining how old parts of a moon or a planet’s surface are compared to other parts. The more abundant the craters are in a region, the older the surface is.

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Try This!

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What are the depressions on the moon’s surface better known as?

Answer: Craters

The Moon’s Interior Astronauts left several seismographs on the Moon during the

Apollo space program. A seismograph is an instrument that detects tremors, or seismic vibrations. The seismographs are used to measure earthquake activity on Earth.

Based on the study of moonquakes using seismographs, a model of the Moon’s interior has been proposed. The Moon’s crust is about 60 km thick on the side facing the Earth and 150 km thick on the far side. The difference in thickness is probably the reason fewer lava flows occurred on the far side of the moon.

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The Moon’s Interior A solid layer called the mantle may extend 900 km to 950 km farther

down below the crust. A soft layer of mantle may continue another 500 km deeper. Below this, may be an iron-rich, solid core with a radius of about 300-450 km.

Like the Moon, Earth also has a dense, iron core. However, the Moon’s core is small compared to its total volume. Compared with Earth, the Moon is most like Earth’s outer two layers – the mantle and the crust – in density. This supports a hypothesis that the Moon may have been formed from material ejected from Earth’s mantle and crust.8

Motions of the Moon - Rotation and Revolution

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The Moon rotates on its axis and revolves around Earth at an average distance of about 384,000 km. It takes 27.3 days for the Moon to complete one orbit around the Earth.

The Moon also takes 27.3 days to rotate once on its own axis. Because the two motions of the Moon take the same amount of time, the same side of the Moon is always facing the Earth.

However, these two lunar motions, aren’texactly the same during the Moon’s 27.3 -day rotation-and-revolution period.Because the Moon’s orbit is an ellipse,it moves faster when it’s closer to theearth and slower when it’s farther away.During, one orbit, the observers are ableto see a little more of the eastern sideof the Moon and then a little more of thewestern side.

Try This! Why does the same side of the Moon always face Earth?

Answer: The Moon takes 27.3 days to complete one orbit around the Earth. The Moon also takes 27.3 days to rotate once on its own axis. Because the two motions of the Moon take the same amount of time, the same side of the Moon is always facing the Earth.

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Reflection of the Sun

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The Moon seems to shine because its surface reflects sunlight. As the moon revolves round the Earth, the Sun always lights one half of it.

However, we don’t always see the entire lighted part of the Moon. What we do see are phases, or different portions of the lighted part. Moon phases, are the changing views of the Moon as seen from the Earth.

New Moon

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The phase depends on the relative positions of the Moon, Earth, and the Sun.

A new moon occurs when the Moon is between the Earth and the Sun. During a new moon, the lighted half of the Moon is facing the Sun and the dark side faces Earth. The Moon is in the sky, but it cannot be seen.

The new moon rises and sets with the Sun.

Waxing Phases

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Shortly, after a new moon, more and more of its lighted side faces Earth and becomes visible. The phases are said to be waxing, or growing in size.

About 24 hours after new moon, we can see a thin silver of the lighted side. This phase is called the waxing crescent. As the moon continues to trip around the Earth, we eventually can see half of the lighted side. This phase is first quarter and occurs about a week after new moon.

Waxing Phases

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The phase continues to wax. When more that one quarter is visible, it is called waxing gibbous. A full moon occurs when all of the moon’s surface facing Earth reflects light. At full moon, Earth is between the Sun and the Moon.

Try This! When the visible portion of the moon is increasing, the moon is

_________

A. Waxing

B. Full

C. Waning

D. Waning-crescent

Answer: A

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Waning Phases After a full moon, the phases are said to be waning (or

decreasing), as you see less of its illuminated half each night. Waning gibbous begins just after a full moon.

When you can see only half of the lighted side, it is the third-quarter phase. The Moon continues to appear to shrink. Waning crescent occurs just before another new moon. You see only a small slice of the Moon.

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Waning Phases It takes about 29.5 days for the Moon to complete its cycle of

phases. Recall that it takes about 27.3 days for the Moon to revolve around the Earth.

The discrepancy of two extra days is due to Earth’s revolution. The Moon takes these two days to keep up constantly with Earth as it orbits around the Sun.

A “day” and a “year” are the same on the Moon – 29.5 days!

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Try This! When only a small part of the moon is visible, the moon may be

in its _______

A. First-quarter phase

B. Waning-crescent phase

C. New moon phase

D. Last-quarter phase

Answer: B

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Eclipses The revolution of the Moon causes eclipses. Sometimes, during new

and full moon, shadows cast by one object will fall on another. When the Moon lines, up directly with the Sun, it can cast its shadow all the way to Earth. Earth can also cast a shadow onto the Moon during a full Moon. When shadows are cast these way, eclipses occur.

An eclipse can occur only when the Sun, the Moon, and Earth are lined up perfectly in the same plane.

Eclipses occur only a few times a year as the orbit of the Moon and Earth are not in the same plane.

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Solar eclipse During new moon, if Earth moves into

the Moon’s shadow, a solar eclipse occurs.

The Moon blocks sunlight from reaching a portion of Earth’s surface. Only areas on Earth in the Moon’s umbra, or the darkest part of its shadow, experience a total solar eclipse. Those areas in the penumbra, or lighter part of the shadow, experience a partial solar eclipse. Suring a total solar eclipse, the sky becomes dark and starts can be seen easily.

Because of the Earth’s rotation, a solar eclipse lasts only a few minutes in any one location.

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Solar eclipse Solar eclipses occur when the Sun, the Moon, and Earth are lined up

in a specific way.

Never look directly at the Sun during an eclipse. The light can permanently damage your eyes.

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Lunar eclipse A lunar eclipse occurs when

the Sun, Earth, and the Moon are lined up such that the full moon moves into Earth’s shadow.

Direct sunlight is blocked from reaching the Moon. When the Moon is in the darkest part of the Earth’s shadow, a total lunar eclipse occurs.

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Lunar eclipse During a total lunar eclipse, the full moon darkens. Because some

sunlight refracts through the Earth’s atmosphere, the Moon appears to be deep red. As the moon moves out of the umbra and into the penumbra, or lighter shadow, we can see the curved shadow of the Earth move across the Moon’s surface.

When the Moon passes partly through Earth’s umbra, a partial lunar eclipse occurs.

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Try This! Which of the following pictures shows the appearance of the Moon

when a solar eclipse occurs?

Answer: New Moon

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Waning Moon Quarter Moon Full Moon New Moon

The Origin of the MoonB

efore the Apollo space program, several early hypothesis were proposed to explain the origin of the moon. The co-formation hypothesis states that Earth and the Moon formed at the same time and out of the same material. The problem with this hypothesis is that the Earth and the Moon have somewhat different densities and compositions.

According to the capture hypothesis, Earth and the Moon formed at different locations in the solar system. Then, Earth’s gravity captured the Moon as it passed close to Earth. The fission hypothesis states that the Moon formed from material thrown off of a rapidly spinning Earth. A problem with the fission hypothesis lies in determining why the Earth would have been spinning so fast.

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How did the moon form?S

cientists have proposed several possible explanations, or hypothesis, to account for the formation of the Earth’s Moon. These include:

Co-Formation : Earth and the Moon form at the same time from a vast cloud of cosmic matter that condenses into the bodies of the solar system.

Capture: Earth’s gravity captures the moon into Earth’s orbit as the Moon passes close to the Earth.

Fission: A rapidly spinning molten Earth tears into two. The smaller blob of matter enters into orbit as the Moon.

Collision : A Mars-sized body collides with the primordial Earth. The colossal impact smashes off sufficient debris from Earth to form the Moon.

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The collision hypothesis, also known as the giant impact hypothesis is themost widely accepted today.

Collision hypothesis T

he collection and study of Moon rocks brought evidence to support one recent hypothesis, known as collision hypothesis. This hypothesis, involves a great collision. When Earth was about 100 million years old, a Mars- sized space object may have collided with the Earth. Such an object would have broken through Earth’s crust and plunged towards the core.

The collision would have thrown large amounts of gas and debris into orbit around Earth. Within about 1000 years, the gas and debris then could have condensed to form the Moon.

The collision hypothesis is strengthened by the fact that the Earth and the Moon have different densities. The Moon’s density is similar to material that would have been thrown off Earth’s mantle and crust when the object collided with Earth.

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Copyright © Ed2Net Learning, Inc.28

Summary

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The surface of the moon when observed through telescopes reveal dark-colored maria (a Latin word for “seas”) and lighter-colored highland areas.

There are many depressions on the Moon’s surface. These are called craters.

The Moon takes 27.3 days to complete one orbit around the Earth. The Moon also takes 27.3 days to rotate once on its own axis. Because the two motions of the Moon take the same amount of time, the same side of the Moon is always facing the Earth.

Copyright © Ed2Net Learning, Inc.29

Summary

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Moon phases, are the changing views of the Moon as seen from the Earth.

A new moon occurs when the Moon is between the Earth and the Sun.

A lunar eclipse occurs when the Earth is directly between the Sun and the Moon.

A solar eclipse occurs when the Moon is directly between the Sun and the Earth.

Copyright © Ed2Net Learning, Inc.30

Summary

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Scientists have proposed several possible explanations, or hypothesis, to account for the formation of the Earth’s Moon.

The different hypothesis are Co-formation, Capture, Fission and Collision.

The collision hypothesis, also known as the giant impact hypothesis is the most widely accepted today.

Critical thinking activityF

ill the concept map below to complete the moon phase cycle : full moon, waning gibbous, waning crescent, and first quarter.

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Waxing crescent

New Moon

Third quarter

Waxing gibbous

AssessmentW

hich moon surface feature probably formed when lava flows filled large basins?

A.Maria

B.Highlands

C.Craters

D.Volcanoes

Answer: A

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AssessmentM

eteorites that strike the Moon’s surface cause which surface feature?

A.Maria

B.Highlands

C.Craters

D.Volcanoes

Answer: C

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AssessmentH

ow long is the Moon’s period of revolution?

A.27.3 hours

B.27.3 days

C.29.5 hours

D.29.5 days

Answer: B

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AssessmentH

ow long does it take for the Moon to rotate once on its axis?

A.27.3 hours

B.27.3 days

C.29.5 hours

D.29.5 days

Answer: B

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AssessmentW

hat occurs when the Moon is directly between the Sun and the Earth?

A.Lunar eclipse

B.Full moon

C.Solar eclipse

D.Waxing crescent

Answer: C

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AssessmentW

hat occurs when the Earth is directly between the Sun and the Moon?

A.Lunar eclipse

B.Full moon

C.Solar eclipse

D.Waxing crescent

Answer: A

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AssessmentC

ompare and contrast the hypothesis surrounding the origin of the moon.

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Co-Formation Capture Fission Collision

Earth and the Moon form at the same time from a vast cloud of cosmic matter that condenses into the bodies of the solar system.

Earth’s gravity captures the moon into Earth’s orbit as the Moon passes close to the Earth.

A rapidly spinning molten Earth tears into two. The smaller blob of matter enters into orbit as the Moon.

A Mars-sized body collides with the primordial Earth. The colossal impact smashes off sufficient debris from Earth to form the Moon.

AssessmentD

escribe evidence that supports the collision hypothesis on how the Moon formed.

Answer: The collection and study of Moon rocks brought evidence to support one recent hypothesis, known as collision hypothesis.

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AssessmentD

escribe the arrangement of the Moon, the Sun, and the Earth during a solar eclipse. How is this different from the arrangement during a lunar eclipse?

Answer: A solar eclipse occurs when the Moon is directly between the Sun and the Earth. A lunar eclipse occurs when the Earth is directly between the Sun and the Moon.

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Assessment During what moon phase can a lunar eclipse occur?

A. Waxing gibbous

B. First quarter

C. New moon

D. Full moon

Answer: D

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Practice Explain why fewer lava flows occurred on the far side of the

Moon.

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Thank You!

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