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25.2 The Earth-Moon System
Apollo 14
commander Alan
Shepard planted
an American flag
on the moon’s
surface in 1971.
25.2 The Earth-Moon System
Earth’s Moon
Why does the moon’s temperature vary
widely?
The lack of an atmosphere allows the
moon’s surface temperature to vary
tremendously.
25.2 The Earth-Moon System
Earth’s Moon
The moon’s gravity is too weak to hold onto
gas molecules, which simply float away into
space.
In direct sunlight, the average surface
temperature of the moon is about 130°C. At
night the average surface temperature
plummets to –180°C.
25.2 The Earth-Moon System
Earth’s Moon
Because there is no atmosphere, any liquid
water would have long ago evaporated into
space.
Any water remaining on the moon must be
frozen.
Evidence of ice near the moon’s north and
south poles was detected by spacecraft
observations.
25.2 The Earth-Moon System
Surface Features
What features are found on the moon’s
surface?
The major surface features of the moon are
maria, highlands, and craters.
25.2 The Earth-Moon System
Surface Features
Maria and Highlands
Maria are low, flat plains formed by ancient lunar
lava flows. They cover about 15 percent of the
moon’s surface.
Light-colored areas called lunar highlands
surround the maria. The lunar highlands, which
cover most of the moon's surface, are rough,
mountainous regions.
25.2 The Earth-Moon System
Surface Features
These photos show
features of the moon. The
large, dark areas in the top
photo are maria. The lighter
areas surrounding the
maria are lunar highlands.
The moon is covered with
craters that formed from
the impacts of smaller
objects.
25.2 The Earth-Moon System
Surface Features
Craters
Much of the moon’s surface is covered with
impact craters. These craters are round
depressions in the surface caused by the impact
of high-speed meteoroids.
Meteoroids are chunks of rock that move through
the solar system.
Although meteoroid collisions still occur, most
craters formed more than 3.8 billion years ago.
25.2 The Earth-Moon System
Surface Features
On Earth, most old craters disappeared as
the surface was altered by erosion or
collisions between tectonic plates.
On the moon, however, there is little erosion
and no plate motion to alter craters.
The lunar highlands are much more heavily
cratered than the maria. Older craters within
the maria were covered by more recent lava
flows.
25.2 The Earth-Moon System
Formation of the Moon
How did the moon form?
Scientists hypothesize that the moon formed
after an enormous collision early in Earth’s
history.
25.2 The Earth-Moon System
Formation of the Moon
About 4.6 billion years ago, the solar system
was full of pieces of rocky debris, some of
which were the size of small planets.
• While Earth was still forming, a Mars-sized
object probably collided with it off-center.
• This collision ejected a large amount of material
into space.
• This material orbited Earth and eventually
formed the moon.
25.2 The Earth-Moon System
Formation of the Moon
The moon is thought to have formed in a
spectacular collision between the early Earth
and a Mars-sized object.
25.2 The Earth-Moon System
Phases of the Moon
What causes the phases of the moon?
The moon’s phases are caused by changes
in the relative positions of the moon, sun,
and Earth as the moon revolves around
Earth.
25.2 The Earth-Moon System
Phases of the Moon
The moon doesn’t produce its own light. It
reflects light from the sun.
• When you look at the moon, it sometimes
appears full and bright, and sometimes appears
as only a sliver.
• The different shapes of the moon visible from
Earth are called phases.
25.2 The Earth-Moon System
Phases of the Moon
Sunlight illuminates half of the moon. The
phase of the moon depends on how much of
the sunlit portion of the moon is facing Earth.
• A full moon occurs when the moon is on the
opposite side of Earth from the sun.
• A new moon occurs when the moon is directly
between the sun and Earth. A new moon
cannot be seen from Earth because the sunlit
side of the moon is facing away from us.
25.2 The Earth-Moon System
Phases of the Moon
Two different views of the phases of the
moon are shown in this diagram.
25.2 The Earth-Moon System
Phases of the Moon
It takes about 29.5 days for the moon to
complete a full cycle of phases. This period of
time is known as a lunar month.
The same side of the moon always faces
Earth.
This is because the moon makes one full
rotation around its axis while making one full
revolution around Earth.
25.2 The Earth-Moon System
Eclipses
What causes solar and lunar eclipses?
A solar eclipse occurs when the moon casts
a shadow on a portion of Earth’s surface.
A lunar eclipse occurs when Earth casts a
shadow on the moon.
25.2 The Earth-Moon System
Eclipses
An eclipse occurs when the shadow of one
body in space, such as a planet or moon,
falls on another.
• For an eclipse to occur on Earth, the sun,
moon, and Earth must all lie along a straight
line.
25.2 The Earth-Moon System
Eclipses
• Eclipses do not occur every month because
the plane of the moon’s orbit is tilted about
5 degrees with respect to the ecliptic.
• An eclipse occurs only when the moon
crosses Earth’s ecliptic plane at the time of
a new moon or full moon.
25.2 The Earth-Moon System
Eclipses
Solar Eclipse
During a new moon, when the moon is between
the sun and Earth, Earth may pass through the
shadow of the moon.
The small cone-shaped umbra is the darkest part
of the moon’s shadow.
• In the umbra, the moon completely blocks sunlight.
• Only people in a very narrow path across Earth’s
surface see a total solar eclipse.
25.2 The Earth-Moon System
Eclipses
Surrounding the umbra is the larger
penumbra, a region of shadow that is less
dark than the umbra.
People within the penumbra see a partial
solar eclipse.
25.2 The Earth-Moon System
Eclipses
Lunar Eclipse
During a full moon, when Earth is between the sun
and moon, the moon may pass through Earth’s
shadow.
• Earth’s shadow has an umbra and a penumbra.
• If the moon passes completely into Earth’s umbra,
you see a total lunar eclipse.
• If the moon passes partially through the umbra, you
see a partial lunar eclipse.
25.2 The Earth-Moon System
Eclipses
Eclipses occur only at the new moon or the
full moon.
25.2 The Earth-Moon System
Tides on Earth
What causes the tides?
Tides are the regular rise and fall of ocean
waters.
Tides are caused mainly by differences in
the moon’s gravitational pull on Earth.
25.2 The Earth-Moon System
Tides on Earth
The gravitational force of the moon pulls
more strongly on the side of Earth that faces
it, and less strongly on the side that is away
from it.
This difference causes the entire Earth to be
stretched. The effect of the moon’s gravity on
the oceans is much greater than on the solid
Earth.
25.2 The Earth-Moon System
Tides on Earth
The gravitational pull of the moon causes the
oceans to bulge at the locations closest to
and farthest from it.
• As Earth rotates on its axis, high tides occur
when an area moves through either of these
two bulges.
• Most coastal areas experience two high tides
and two low tides each day.
25.2 The Earth-Moon System
Tides on Earth
The sun also has a gravitational effect on the
oceans, but its tidal effects are only about
half those of the moon.
During a new or full moon, the sun, moon,
and Earth are in a straight line.
The combined forces produce a spring tide,
where the change between daily high and
low tides is the greatest.
25.2 The Earth-Moon System
Tides on Earth
When the moon is in its first or third quarter,
the moon and the sun are pulling the Earth at
right angles.
The result is a neap tide, where there is the
least change between daily high and low
tides.
25.2 The Earth-Moon System
Tides on Earth
A. During spring tides, the pulls of the moon
and sun add together.
25.2 The Earth-Moon System
Tides on Earth
A. During spring tides, the pulls of the moon
and sun add together.
B. During neap tides, the moon and sun pull
Earth and its oceans at right angles.
25.2 The Earth-Moon System
Assessment Questions
1. Why do temperatures on the moon’s surface vary
greatly?
a. There is no gravity on the moon.
b. The moon orbits about Earth.
c. The uneven, rocky surface of the moon creates large
shadow regions.
d. There is no atmosphere on the moon.
25.2 The Earth-Moon System
Assessment Questions
1. Why do temperatures on the moon’s surface vary
greatly?
a. There is no gravity on the moon.
b. The moon orbits about Earth.
c. The uneven, rocky surface of the moon creates large
shadow regions.
d. There is no atmosphere on the moon.
ANS: D
25.2 The Earth-Moon System
Assessment Questions
2. What are the round impact depressions that cover
much of the moon’s surface?
a.
b.
c.
d.
maria
craters
lakebeds
meteoroids
25.2 The Earth-Moon System
Assessment Questions
2. What are the round impact depressions that cover
much of the moon’s surface?
a.
b.
c.
d.
maria
craters
lakebeds
meteoroids
ANS: B
25.2 The Earth-Moon System
Assessment Questions
3. What hypothesis about the formation of the moon is
supported by much of the data currently available?
a. The moon was a large asteroid captured by Earth’s gravity.
b. The moon formed when a planet-sized body collided with Earth.
c. Internal stresses caused the early Earth to split into the moon and
Earth.
d. Earth and the moon formed together from a planetary nebula.
25.2 The Earth-Moon System
Assessment Questions
3. What hypothesis about the formation of the moon is
supported by much of the data currently available?
a. The moon was a large asteroid captured by Earth’s gravity.
b. The moon formed when a planet-sized body collided with Earth.
c. Internal stresses caused the early Earth to split into the moon and
Earth.
d. Earth and the moon formed together from a planetary nebula.
ANS: B
25.2 The Earth-Moon System
Assessment Questions
1. A lunar eclipse can occur when the moon is
between the sun and Earth.
True
False
25.2 The Earth-Moon System
Assessment Questions
1. A lunar eclipse can occur when the moon is
between the sun and Earth.
True
False
ANS:
F, solar eclipse